NORMAL RESPONSES OF SHEEP T0
ACUTE THERMAL STRESS

Thesis for the Degree of Ph. D.
MICHTGAN STATE UNIVERSITY
DAVID R; AMES
' 1968

TH 5818

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This is to certify that the
thesis entitled

NORMAL RESPONSES OF SHEEP TO
ACUTE THERMAL STRESS

presented by

David B. Ames

has been accepted towards fulfillment
of the requirements for
Ph.D. dqya3h1Animal Husbandry and
Physiology

 

Major profts 1'

Date 9

0-169

 

 

ABSTRACT

NORMAL RESPONSES OF SHEEP TO
ACUTE THERMAL STRESS

by David Robert Ames

Biothermal responses of sheep to a wide range of thermal
environments were studied to: (1) evaluate the influence of
the environment on the efficiency of animal production; (2)
establish standards of environmental control for semi or
completely closed animal management; and (3) extend the study
of basic environmental physiology. This study describes
physiological responses of defined sheep (i.e., in terms of
diet, fleece, size, age, breed and geographic location) ex-
posed to ambient temperatures ranging from 10 to #50 co
Rectal temperature, skin temperatures (ear, face. forelimb,
trunk, stifle), metabolic heat production, respiratory rate,
respiratory evaporative water loss and cutaneous evaporative
water loss were determined. The "normal limits" of rectal
temperature in sheep were relatively wide resulting in a
substantial Q10 effect on physiological reactions. Thermal

circulatory indices and changes in skin surface temperatures
indicated that the ears, forelimbs and face (characterized

by freedom from fleece, high surface area to mass ratio, and

overlying areas of little fat or muscle tissue) were capable

David Robert Ames

of vasomotion and were able to modify sensible heat exchange.
Conversely, on these same basis the trunk and stifle areas
(characterized by the presence of fleece, low surface area
to mass ratio and overlying subcutaneous fat and muscle
tissue) were probably incapable of altering skin blood flow
and consequently unable to modify sensible heat exchange.
Respiratory rate increased logarithmically from critical
temperature II (hypathermal rise) until when ambient tempera-
ture equalled rectal temperature. These data suggested that
changes in respiratory rate were valid indicators of thermo-
regulatory mechanisms. Evaluation of panting indices
suggested that at rates exceeding 240 breaths per minute,
panting became a thermal liability. Comparison of obligatory
and observed cutaneous evaporative water loss indicated the
presence of active sweating in sheep. It was hypothesized
that cutaneous and respiratory evaporative water loss were
both important avenues of heat loss in shorn sheep during
heat stress. It appeared that respiratory evaporative water
loss was relatively more important during mild heat stress
(350 C.) but that sweating was relatively more important
during severe heat stress (40 and 450 C.). Increases of
heat production during cold stress was due to shivering.

Increases during heat stress were attributed to respiratory

David Robert Ames

activity of panting but more importantly to the Q10 effect.

The 5° C. intervals of ambient temperature used did not

allow for the accurate determination of critical temperatures.
The data suggest that critical temperature I is equal to or
greater than 20° C. and that critical temperature II is equal
to or less than 30° C. On the basis of physiological responses
indicative of heat and cold stress, the lower limit of the
thermoneutral zone for sheep (as defined in this experiment)

is equal to or greater than 200 C. while the upper limit is
equal to or less than 30° C. The use of skin surface

temperature to estimate the thermoneutral zone is questioned.

NORMAL RESPONSES OF SHEEP TO
ACUTE THERMAL STRESS

By

David R. Ames

A Thesis

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Departments of Animal Husbandry and Physiology

1968

ACKNOWLEDGMENTS

The author wishes to express his appreciation to the
members of the Departments of Animal Husbandry and Physiology
for their contributions leading to the completion of this
study. In particular I would like to express my gratitude
to Dr. John E. Nellor and Dr. Thomas Adams, my academic and
research advisers, respectively, for their assistance
throughout the author's doctorate program. Appreciation is
extended to Dr. Gail Riegle, Dr. Hugh Henderson, Dr. Harold
Henneman, and Dr. Elwin Miller for their counsel.

The author wishes to thank the staff of the Endocrine
Research Unit for their 000peration throughout the experimental
studies.

The writer is grateful to his wife, Marsha, for her

continual support.

11

TABLE OF CONTENTS

I. Introduction

II. Literature Review
2.A Basic Thermoregulation in the Sheep

2.A.1 Thermoneutral Zone

2.A.2 Critical Temperatures

2.A.3 Rectal Temperature

2.A.4 Skin Temperature

2.A.5 Other Temperature Measurements
T2.A.6 Respiratory Frequency

2.B Heat Exposure

2.B.1 Sensible Heat Exchange
2.B.2 Evaporative Water Loss
20B03 Sweating in Sheep

2.C Cold Exposure

.1 Behavioral Adjustments
.2 Sensible Heat Loss

.3 Evaporative Water Loss
.4 Metabolic Rate

III. Materials and Methods

Animals

Training

Annex Room
Environmental Chamber
Respiration Rate

Heat Production
Temperature Measurements
Evaporative Water Loss
Whole Body Insulation
Thermal Circulatory Index
Panting Index

Mean Body Temperature
Body Heat Content

Mean Skin Temperature
Experimental Procedures
Statistical Analysis

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IV.

V.
VI.
VII.

VIII.

Results

0
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Rectal Temperature

.2 Skin Temperature

.3 Body Heat Content

.h Whole Body Insulation

.5 Thermal Circulatory Index
.6 Respiratory Rate

.7 Respiratory Water Loss

.8 Cutaneous Water Loss

4.9 Heat Production

##¢P¢##¢

Discussion
Conclusions
Appendix

Bibliography

iv

Table

LIST OF TABLES

Rectal, Skin, and Body Temperatures
Recorded at Ambient Temperatures From
10 to 45° C.

Physiological Parameters Recorded at
Different Ambient Temperatures

Ambient Temperature Interval and Magnitude
of Greatest Change of Rectal and Skin
Temperatures

Thermal Circulatory Indices at Different
Ambient Temperatures

Partition of Q10 and Panting Effects on
Metabolic Heat Production and Calculation
of Panting Index

59

60

61

Figure

10

11

12

13

14

15

LIST OF FIGURES

Picture of Temperature - Humidity Control
Chamber

Schematic Drawing of Thermocouple
Attachments

Schematic Drawing of Evaporative Water
Loss Air Sampling Systems

Schematic Drawing of Air Flow System for
Determination of Evaporative Water Loss

Rectal Temperature as a Function of Ambient
Temperature

Ear Skin Temperature as a Function of
Ambient Temperature

Face Skin Temperature as a Function of
Ambient Temperature

Forelimb Skin Temperature as a Function of
Ambient Temperature

Trunk Skin Temperature as a Function of
Ambient Temperature

Stifle Skin Temperature as a Function of
Ambient Temperature

Skin Temperatures as a Function of Ambient
Temperature

Whole Body Tissue Insulation as a Function
of Ambient Temperature

Thermal Circulatory Index of the Ear as a
Function of Ambient Temperature

Thermal Circulatory Index of the Face as a
Function of Ambient Temperature

Thermal Circulatory Index of the Forelimb
as a Function of Ambient Temperature

vi

64

65

66

67

68

69

7O

71

72

73

74

75

76

Figure
16

17

18

19

20

21

22

23

2L»

25

26

Thermal Circulatory Index of the Trunk as
a Function of Ambient Temperature

Thermal Circulatory Index of the Stifle as
a Function of Ambient Temperature

Whole Body Heat Content as a Function of
Ambient Temperature

Respiratory Rate as a Function of Ambient
Temperature

Log of Respiratory Rate as a Function of
Ambient Temperature

Respiratory Evaporative Water Loss as a
Function of Ambient Temperature

Respiratory Evaporative Water Loss Per
Respiratory Rate as a Function of Ambient
Temperature

Panting Index as a Function of Respiratory
Rate

Cutaneous Evaporative water Loss as a
Function of Ambient Temperature

Comparison of vapor Pressure and Cutaneous
Evaporative Water Loss as a Function of
Trunk Skin Temperature

Metabolic Heat Production as a Function of
Ambient Temperature

vii

79

8O

81

82

83

84

85

86

87

Appendix
I

II
III

LIST OF APPENDICES

Frequently Used Symbols
Correlation Coefficients

Regression Equation

viii

 

I. INTRODUCTION

Homeotherms are those animals which through complex
physiological mechanisms maintain body temperature within
narrow limits, even when exposed to varying and stressing
thermal environments. Previous investigations describing
thermoregulatory mechanisms primarily focused upon man and
conventional laboratory animals. A growing interest is
developing in the biothermal responses of large domesticated
animals. This impetus is attributed to: (1) the importance
of the environmental influences on the efficiency of animal
production; (2) the impending increase in semi or com-
pletely closed animal management that will require standards
for environmental control; and (3) the logical extension of
basic environmental physiological studies to a greater range
of animals.

This study describes the normal biothermal responses of
Isheep acutely exposed to a range of thermal environments.
'Bhe sheep was chosen as an experimental animal because: it
is (1) an economically important animal; (2) well suited to
Experimental procedures; (3) exhibits unusual thermal

balance problems in the fleeced and shorn condition; and

 

(a) mechanisms of thermal control in sheep are inadequately
described. The experiments in this study were designed to
determine the biothermal responses of sheep exposed to a
range of ambient temperatures, to establish thermoneutral
zones, and critical temperatures for a "well-defined" (i.e..
size, breed, sex, fleece, diet,-and geographic location)

animal exposed to controlled environmental conditions.

 

II. LITERATURE REVIEW

Mechanisms of thermal control in homeotherms have been
widely reviewed (Strom, i960; vonEuler, 1961; Hardy,,1961;
Hammel, 1968). It is tacitly assumed that basic biothermal
control in sheep is qualitatively similar to that described
for other mammals, although the relative importance of
specific avenues of heat exchange and the manner by which
body temperature is controlled in sheep is unique to the

species.
2.A. Basic Thermoregulation in the Sheep

2.A.1 Thermoneutral Zone

The thermoneutral zone has been defined as the ambient
temperature below which heat production and above which heat
loss must be increased if a constant internal body tempera-
ture is to prevail (Graham. 33 al.. 1959). Dukes (1956)
described the thermoneutral zone for domestic animals
(swine, cattle), excluding the sheep because of complexities
inherent with the fleece. Diet (Graham 33 21., 1959), tissue
insulation (Slee, 1966) and other variables also have been

shown to alter the thermoneutral zone in sheep. These

4

changes have not been qualitatively related to specific
environmental conditions.

Thermoneutral temperature for shorn, adult, Down Cross
wethers (Blaxter §t_§l.. 1959a and Armstronggt 51., 1960),
adult, Scottish Blackface sheep (Slee and Sykes, 1968) and
young lambs (Alexander and Brook, 1960) is reportedly 30° C.
(86° F.). Hemingway and Hemingway (1966), using water
immersed sheep, suggested 33 to 35° C. (91.9 to 950 F.) as

the thermoneutral zone of fasted adult sheep.

2.A.2 Critical Temperatures

The environmental temperature below which an animal
must increase its metabolic heat production to avoid hypo-
thermia has been defined as critical temperature (Graham
gt_gl., 1959). Blaxter (1962) considers this temperature to
be coincident with the environmental temperature where
vasoconstriction in response to cold is complete. Graham
gt El° (1959) reported that critical temperatures of closely
clipped sheep on a "medium energy level ration" (1200 gm.
dried grass cubes) was 33° C. (860 F.). Further investiga-
tion by Graham revealed that the critical temperature.
changes with diet; rising to 39 to 40° c. (100.2 to 104° F.)
with low energy rations (600 gm. dried grass cubes) and

falling to 2A to 27° c. (75.2 to 80.60 F.) with high level

rations (1800 gm. dried grass cubes).

The effect of fleece length on the critical temperature
of sheep was reported by Armstrong §£_§l. (1960). The
critical-temperature in shorn adult sheep was 30° C. (86° F.).
but values as low as 0° C. (320 F.) were observed in sheep
with 10 cm. of fleece. Blaxter (1962) implied that tissue
insulation also effects critical temperature, although,
specific critical temperatures relating to explicit degrees
of insulation were not disclosed. Slee and Sykes (1968)
reported that the critical temperature-was reduced in cold

acclimatized adult sheep.

2.A.3 Rectal Temperature

Numerous mean rectal temperatures have been reported
for sheep: Brody (1945), 100 to 103° F. (37.9 to 39.5° 0.);
Kammlade (1947), 100.9 to 103.80 F. (38.2 to 39.80 0.); Lee
(1950), 101.5 to 103.00 F. (38.5 to 39.50 0.); MacFarlane
(1964), 37 to 39° C. (98.6 to 102.2° F.). Differences in
these values may be due to individual and breed variability;
variations in fleece; the ability to produce and conserve
heat in cold environments; the efficiency of heat loss in
warm environments; and existing environmental conditions.

Rectal temperature is reported to be positively correlated
with ambient temperature. Experiments by Knapp and Robinson

(1954) showed that the rectal temperature of a Corriedale ewe

increased from 102 to 106° F. (38.9 to 40.20 C.) as ambient
temperature rose from 86 to 104° F. (30 to 40° C.).
Similarly, Webster (1966) reported a decline in the rectal
temperature of sheep when ambient temperature fell below the
thermoneutral zone. Webster (1966) reported that rectal
temperature rose above thermoneutral zone levels when
ambient temperature changed from 5 to 8° C. (41 to 46.4° F.).
He explained this as an "overshoot" of metabolic response to
cold. Rectal temperature of Cheviot wethers with six weeks
growth of wool declined when the ambient temperature de-
creased from 20 to -11° C. (68 to 12.2° F.) (Bailey, 1964).
This supports the findings of Webster (1966) as an "over-
shoot" of rectal temperature in response to thermal stress.
A characteristic rise in rectal temperature following
initial cold exposure has been observed in adult sheep (Joyce
and Blaxter, 1964 and Slee, 1966) and in lambs (Alexander,
1961). Slee (1966) suggested that this increase results
from an overcompensating rise of metabolic rate coupled with
vasomotor changes. Slee and Sykes (1967) noted that rectal
temperatures of sheep previously acclimatized at subscritical
temperatures of 80 C. (46.6° F.) were higher than non-cold
acclimatized sheep at a thermoneutral temperature (300 C.)
(86° F.). variations in physiological responses were ob-

served among sheep of the same sex, breed, age, and weight

 

 

to acute cold exposure. Eyal (1963a) compared rectal
temperatures of shorn and unshorn Awassi sheep in the shade
versus direct sunlight. When ambient temperature was less
than 30° C. (86° F.). deep body temperature was lower in
shorn than in unshorn animals. Rectal temperatures rose at
a greater rate in fleeced sheep. The deep body temperature
of shorn sheep was equal to or greater than fleeced sheep
when ambient temperatures were above 30° C. (860 F.). Web-
ster (1966) reported that when environmental temperature was
above the critical temperature, rectal temperature of
Cheviot and Suffolk sheep increased by 0.5° C./cm. of fleece
depth. Blaxter 23 al. (1959b) stated that fleece.length
influenced body heat content of sheep.

The effect of relative humidity on rectal temperatures
of Awassi sheep was described by Eyal (1963d). The least
variation in rectal temperatures between shorn and fleeced
individuals occurred at lower humidities.

Blaxter §t_§l. (1965) reported that increased air
velocity lowered the insulatory properties of the fleece.
Eyal (1963d) reported that wind increases the differences
(shorn lower than fleeced) in rectal temperature when ambient
temperature is between 24° C. (75.20 F.) and 29° C. (84.20F3).
In direct sunlight, increased wind velocity decreased the

effect of direct radiation on the shorn sheep.

 

2.A.4 Skin Temperature

Skin temperatures in adult sheep vary with the measure-
ment site and depend on the rate of cutaneous blood flow
(Webster and Johnson, 1964). Webster and Blaxter (1966)
reported that the trunk surface temperature decreased when
ambient temperature was reduced from 5 to ~10° C. (41 to
14° F.) in Suffolk and Cheviot sheep with varying amounts of
fleece. A greater decrease was apparent when ambient
temperature was reduced from 30 to 0 to 5° C. (86.to 32 to
41° F.) in shorn sheep, shank surface temperature decreased
at a greater rate than did trunk surface temperature (Suffolk
and Cheviot sheep do not have wool on the forelimb, ears, or
face). The rate of shank surface temperature decline was
reduced at sub-freezing temperature; surface temperatures
did not fall below 00 c. (32° F.).

Cold induced vasodilation, defined as fluctuating skin
temperatures during severe cold stress, has been reported to
occur in the limbs of sheep by Webster and Blaxter (1966).
The ear exhibited greater cold induced vasodilation than the
shank or trunk. The car had the "hunting reaction" described
by Lewis (1930). Temperature changes were so large that a
mean ear skin temperature in the cold (below 0° C.) (32° F.)
was misleading in view of the wide range of temperatures.

The importance of this phenomenon in preventing tissue injury

9

in cold environments is not clear in sheep.

Blaxter 33 al. (1959a) related skin surface temperatures
to ambient temperatures above 22° C. (69.8° F.). Skin
temperatures increased as ambient temperatures increased from
22 to 38° C. (69.8 to 100.4° F.). These workers emphasized
the differences between skin temperature responses of areas
with insignificant muscle layer (i.e., legs and ears) as com-
pared with the trunk. Ear temperatures remained about 1 to
2° C. (1.8 to 3.60 F.) above environmental temperature until
25 to 27° C. (77 to 80.6° F.). When environmental tempera-
ture rose above 25 to 27° C. (77 to 80.6° F.). ear skin
temperature increased to 36 to 38° 0. (96.8 to 100.4° F.).
Further increases in environmental temperature resulted in
smaller increases in ear skin temperature. This reportedly
reflected opening of artero-venous anastomoses in the ears,
and to increased blood flow. These workers concluded that
there is increased blood supply to areas without fasical
tissue above the thermoneutral zone. Eyal (1963c) reported
that as ambient temperatures ranged between 10 to 42° C. (50
to 105.8° F.). skin temperatures varied from 10 to 40° C. (50
to 104° F.) in shorn and from 29 to 40° C. (82.4 to 104° F.)
in unshorn adult sheep. During exposure to the sun, skin

surface temperatures of shorn sheep reached 47° C. (116.6°FL).

10

2.A.5 Other Temperature Measurements

Rectal temperature is commonly used as an index of
"deep" body temperature. Various temperature measurements
in the sheep have been utilized to establish more reliable
estimates of "deep" body temperature. Mendal and Raghaven
(1964) compared rumen, rectal, skin (at various locations),
jugular and subcutaneous tissue temperatures to that of the
internal carotid artery. Jugular temperature was more
positively correlated with carotid blood temperature than
was rectal temperature. Rectal temperature used as an index
of mean deep body temperature was criticized since it did
not reflect rapid fluctuation in blood temperature. However,
since "deep" body temperature is not the same measurement as
carotid blood temperatures, it may be a reliable indicator
of mean "deep" body temperature, in that it "sums" the
effects of minor regional temperature changes in the main

body mass.

2.A.6 Respiratory Frequency

Numerous authors (Lee and Robinson, 1941; Miller and
Monge, 1946; Riek 22.2l-v 1950; Lee, 1950; Knapp and Robina
son, 1954; and others) have reported marked increases in
respiratory rate in heat exposed sheep. Values of 200
breaths per minute were reported (Lee, 1950; Blaxter gt al..

1959a; and Hales and Webster, 1967).

 

11

Hales and Webster (1967) defined two phases of respiratory
response in sheep. The first phase is characterized by an
increased respiratory minute volume due to a rise in respira-
tory rate even though a decrease in tidal volume. Following
prolonged heat exposure (90 min.), a second phase is charac-
terized by increased respiratory minute volume resulting
from increased tidal volume; respiratory rate is lower than

in the first phase but above that observed at thermoneutral

 

values. Respiratory frequency in sheep subjected to ambient
temperatures below the thermoneutral zone is relatively con-
stant until the ambient temperature falls below 0° c. (32° F.).
according to Slee and Sykes (1968). Below this temperature

the increase in respiratory frequency was attributed to in-
creased oxygen demands of the cold stressed animal.

Blaxter 32 51.. (1959a) reported that diet, fleece and
exercise are related to changes in respiratory frequency at
given ambient temperatures, but asserted that these variables
shift the thermoneutral zone and do not directly influence

respiratory rate.

2.B. Heat Exposure

2.B.1 Sensible Heat Exchange

Heat exchange by radiation, conduction and convection is

12

termed sensible heat exchange. The initial response of homeo-
therms exposed to heat stress is an increased sensible heat
loss by vasodilation (Hardy, 1961; Adams, 1968; and Hammel,
1968). MacFarlane (1964) noted that regions of the body sur-
face are characterized by different degrees of vasomotion.
This infers that variations occur in the rates of sensible
heat loss.

The insulating properties of the fleece were investigated
by Scholander 22 a1. (1950), who reported that the pelt of
Dell sheep had an insulating value of 2.7 clo [bne 010 is
defined by Adams, 1960, as the amount of insulative clothing
required to maintain a resting-sitting man with a metabolic
rate of one met indefinitely comfortable at an environmental
temperature of 21° C. (70° F.)]. Soholander's data were
obtained on the pelt and may not be applicable to the fleece
of the living animal. Blaxter £3 al. (1959a) reported that
within a narrow range above the thermoneutral zone sensible
heat loss from the skin of shorn sheep could be increased
three-fold by increasing blood flow through the peripheral
layers of the extremities. In fleeced sheep, Blaxter 32 al.
(1959b) reported that the rate of sensible heat loss is
inversely related to fleece length. Armstrong gt 31. (1960)
supported these findings by stating that sensible heat loss

divided by skin conductance of fleece covered areas is

 

13
linearly related to the logarithm of the fleece length.
Studies by Eyal (1963b) and by Rick §t_§l. (1950) indi»
cated that pulse rate is positively correlated with ambient
temperature above the thermoneutral zone (30° C.) (86° F.).
Riek proposed that high ambient temperatures and humidity re-
sulted in increased pulse rates in sheep. Neither Eyal nor

Riek attributed changes of pulse rate entirely to vasomotor

 

changes but suggested that such a relationship may exist.

2.B.2 Evaporative Water Loss

Heat losses in sheep exposed to heat stresses are pri-
marily a function of increased skin and respiratory
evaporative water loss (Brockway §£.§l°' 1965; Brook and
Short,1960; Alexander and Brook, 1960; Wodzicka, 1959: Knapp

and Robinson, 1954; and Rick 23 51., 1950).

2.B.3 Sweating in Sheep

The question of the sheep's ability to sweat is dew
batable. Early workers did not find moisture in the fleece
following a period of high climatic temperature or excessive
exercise (Lennox, 1938, and Freney, 1940) and concluded that
sheep probably do not sweat. Ironically, the description of
an apocrine gland in close association with each primary wool
follicle (Duerden and Ritchie, 1924) was published at

approximately the same time when experiments suggested that

14

sheep apparently didn't sweat. Carter and Clarke (1957) later
determined, in a wide variety of breeds of sheep, that the
mean number of sweat glands in the sheep is 2.9/mm2 of skin.
More recent reports using desiccating capsules and calori-
metric chambers (Cragg and Davies, 1947; Rick gt 51., 1950;
Wodzicka, 1959; and Brook and Short, 1960) proposed that
sheep do sweat in response to a heat stress. Disagreement
emerges, however, as to sweating rates of sheep at different
environmental temperatures. Values ranging from 3 gm./m2/hr.
(Brockway _e_i_;_ 31., 1965) to 210 gm./m2/hr. (Wodzicka, 1959)
have been reported.

Variables other than temperature which may alter
sweating rate in sheep have been reported. Fleece does not
appreciably affect sweating rate according to Blaxter gt al.
(1959b). They compared cutaneous evaporative heat loss at
various ambient temperatures to fleece lengths and concluded
that fleece does not affect cutaneous water loss. Eyal
(1963d) stated that the sweating rates at high ambient
temperatures may be the same in shorn and fleeced sheep but
that efficiency of heat loss may favor the shorn animal
since the heat of vaporization at the fleece surface may be
influenced more by the environment than the animal. Dukes
(1956) and Eyal (1963d) reported that the rate of sweating

in sheep was inversely related to relative humidity. Dukes

 

15

supported this by citing one example of a sheep able to with-
stand ambient temperatures of 100° F. (37.1° C.) but only
when the relative humidity was less than 65%.

The relative importance of cutaneous (sweating) and
respiratory water loss in sheep is not clear. Riek gt a1.
(1950) concluded that at ambient temperatures from 30 to 40°

C. (86 to 104° F.). cutaneous evaporative water loss is

 

greater than respiratory water loss. Knapp and Robinson
(1954) supported.this contention by concluding that twoe
thirds of the total water loss of the sheep is cutaneous
water. Conversely, Brook and Short (1960), in the adult
sheep, and Alexander and Williams (1962), in the lamb, re-
ported that respiration is responsible for the major portion
of evaporative water loss when ambient temperature is above
30° 0. (86°.F.).

Brockway 23 a1. (1965) specifically studied the basis
for a controversy between the relative importance of respira-
tory and cutaneous water loss. Only respiratory evaporative
water loss varied with ambient temperature and they concluded
that sheep rely primarily upon this avenue of heat loss to
maintain thermal balance during heat stress. This study was
conducted at ambient temperatures of 12, 20, and 30° C.
(53.6, 68, and 86° F.) and did not include temperatures above

the thermoneutral zone where evaporative water loss mechanisms

16
are most important. Conversely, Brook and Short (1960) found
increases in sweating rates of sheep when ambient temperatures
increased from 20 to 40° C. (68 to 104° F.). .Knapp and
Robinson (1954) reported sweating rates of 76 gm./m2/hr.. an
amount which represented two-thirds of total evaporative
water loss during heat stress. It remains that the relative
importance of respiratory and cutaneous evaporative water

loss in the sheep has not been determined.

2.C. Cold Exposure

2.0.1 Behavioral Adjustments

Behavioral adjustments to severe cold exposure in sheep
consist of postural changes such as tucking the legs and
burying the nose in the shoulder of the fleece, seeking
shelter, and huddling. The importance of these voluntary
adjustments in reducing heat loss under natural conditions
is difficult to assess. Munro (1962) stated that increased
wind velocity induces sheep to seek shelter, whereas Powell
(1963) reported that sheltering is related to the chill
index of the environment. Brockway gt a1. (1965) observed
a reduction in oxygen consumption in sheep when lying as
opposed to standing, however, they did not relate these

findings to thermoregulation.

17

2.C.2 Sensible EEEEILOSS

Vasoconstriction at environmental temperatures below the
thermoneutral zone in sheep reduces sensible heat loss by
increasing tissue insulation. Blaxter (1964) indicated that
following this initial adjustment to cold, additional cooling
has little effect on tissue insulation, concluding that vaso-
constriction is an all-or-none phenomenon, and that tissue
insulation cannot be increased by a further reduction in

ambient temperature. Webster and Blaxter (1966) found that

 

tissue insulation at -10° C. (14° F.) was less than at
either 5° C. (41° F.) or 0° C. (32° F.). Increased sensible
heat loss below thermoneutral zone temperatures occurs
regardless of vasoconstriction. Blaxter gt al. (1959b) ca1~
culated that at subcritical temperatures, a constant value
of 85 cal./m2/24 hr./°C. will be lost via sensible heat loss.
External insulation affects the conservation of heat by
reducing sensible heat loss. The sheep is of particular
interest compared to bare skinned mammals because of the ex—
ternal insulation of the fleece. Armstrong gt a1. (1960)
reported a threeufold increase in insulation due to fleece
when compared to shorn animals. Webster and Blaxter (1966)
suggested that fleece type has little effect on the insula»
tion provided by a given thickness of fleece in still air.
The effect of wind on sensible heat loss was described

by Joyce and Blaxter (1964). These studies indicated that

18
wind does not affect tissue insulation regardless of the
amound of fleece. Differences in tissue insulation existed
between breeds of sheep. Joyce and Blaxter (1964)
originally reported that this difference was due to blood
vessel patterns observed previously by Ryder (1955). However,
Blaxter (1959a, 1959b) in more recent articles discounts this
hypothesis stating that "the amount of fat present in differ-
ent breeds permits increased tissue insulation". Conversely,
Armstrong gt a1. (1960) suggested that breeds do not differ
in the insulation provided by the skin and subcutaneous
tissue.

The effect of wind on external insulation was investie
gated by Joyce and Blaxter (1964). These workers found that
at a wind speed of 0.6 mph, the external insulation was 0.46
units per mm. fleece length, whereas, when wind speed was
9.6 mph. the insulation was only 0.27 units/mm.

Webster and Blaxter (1966) reported that sensible heat
loss from the extremities is low when skin surface temperature
is near that of air, but as air temperature falls below 0° C.
(32° F.) sensible heat loss increases markedly.

Webster and Blaxter (1966) reported that at air tempera-
tures below 0° C. (32° F.). cold induced vasodilation (CIVD)
occurred frequently in the ears but rarely in the shanks. At

air temperatures above 0° C. (32° F.). CIVD was rarely seen.

 

 

19

2.0.3 Evaporative Wgter Loss

Evaporative water loss is reduced at temperatures below
the thermoneutral zone (Knapp and Robinson, 1954; Blaxter gt
gt.. 1959a, 1959b; Alexander, 1962; and Brockway gt gt..
1965. Blaxter gt gt. (1959b) suggested that total evapora-
tive water loss during cold stressing temperature is constant
at approximately 310.cal./m2/24 hr. in adult sheep, providing

that temperatures do not fall to values which alter respira-

 

tion rates. When increases in metabolic rate are necessary
to increase heat production, increased respiratory frequency
lead to increased evaporative water loss (Blaxter gt g;,,
1959b). Brockway gt_gt. (1965) maintained, as mentioned pre-
viously, that sweating is constant in sheep;,however, the
majority of authors (Brook and Short, 1960; Eyal,.1963d; and
others) report reduction of sweating at ambient temperatures

below thermoneutral temperatures.

2.0.4 Metabolic Rate

Sheep exposed to temperatures below the critical
temperature continue to lose heat regardless of increased
tissue insulation. To increase heat production, the sheep
must increase metabolic rate.

The basal metabolism of sheep in a thermoneutral environa
ment was measured by Blaxter (1962) under the following

conditions:

20

fasting 850 kcal./m2/24.hrs.
maintenance 1250 kcal./m2/24 hrs.
full feed 1600 kcal./m2/24 hrs.

These data emphasize the role of Plane of nutrition in basal
metabolic heat production.

During cold stress sheep must increase heat production
to maintain a constant body temperature. Kleiber (1961) come

pared the rate of increase of metabolic rate in sheep with

 

varying amounts.of fleece at environmental temperatures below
critical temperatures, and reported that the greater the 1
length of fleece the slower the rate of increase of metabolic
rate. Similarly, Webster (1966) noted that sheep with less
than 15 mm. of fleece length exhibited oxygen consumption
increases of 71% of final equilibrium value in 10 minutes
during sudden exposure to -8.9° C. (15.8° F.). Sheep with 15
to 25 mm. fleece length only increased to 13% of the final
equilibrium value in 10 minutes. Graham gt gt. (1959) re:
ported that heat production in shorn sheep increased at a
constant rate (AHm/t/m2/°C.) irrespective of the level of
feeding, and that heat production at 80 C. (46.4° F.) was
more than double the minimal metabolism determined at the
medium level of feeding. The rate of increase was, in this
case, 115 cal./m2/24,hrs./°0. ambient temperature.

The immediate increase in metabolic heat production in
sheep exposed to cold temperatures [23° C. (71.6° F.) in

shorn sheep] results from shiVering (Blaxter, 1962). Hardy

21

(1961) indicated that shivering is capable of increasing
energy expenditure by a factor of about four. Alexander
(1962) reported that lambs are capable of increasing heat
production by a factor of five by shivering.

Although non-shivering thermogenesis in adult sheep ex-
posed to cold has not been reported, Alexander (1961) stated

that heat production in the newborn lamb can be increased two

 

to three times by increased oxidation of fats. Dawes and
Mott (1959) also observed non-shivering increases in oxygen - I

consumption in lambs at birth.

III. MATERIALS AND METHODS

3.1 emails
Three yearling, non—pregnant, purebred Suffolk ewes

(77.5 to 88.0 kg. in body weight) were utilized in this

study. Animals were selected with similar genetic back-

ground in order to reduce intersubject variability (Blaxter

gt gt., 1966 and Miller and Mange, 1946). Suffolk sheep

were selected since they readily adapt to new environments

(Webster and Blaxter, 1966) and this reduced the training

periods. Each animal was maintained shorn (less than 0.3

cm. of fleece) by weekly trimming, and the sheep were group

housed in a 6 x 6 meter pen. The diet contained 1500 kca1./day

and water was fed gg libitum.

3.2 Training

The effect of training on the responses of sheep in cli—
mate chambers has been reported (Webster and Blaxter, 1966;
and Slee and Halliday, 1968). The sheep were trained for
three weeks before the beginning of trials in the environ—

mental chamber.

3.3 Annex Room

Sheep were housed in a heated, insulated annex adjacent

22

23

to the laboratory containing the environmental chamber. The

temperature of the annex was maintained between 17 and 27° C.

3.4 Environmental Chamber

A temperature and humidity controlled chamber (Figure
1) was constructed of plywood with 3.75 cm. of fiber glass
insulation and had an effective range of 5 to 50° C. 1 5° 0.
The heating system consisted of a three gallon insulated
water bath containing two immersible heating coils (Chill
Chaser Co. Matheson Cat. No. 28420) which maintained the bath
at 80 to 90° C. The cooling system was a 15 gallon insulated
bath which contained 50% methanol solution cooled by an
immersed coil. A thermistor temperature controller (Yellow
Springs, Model 71) regulated the pumping of either heated or
cooled liquid through convective heat exchangers inside the
chamber. Fans were positioned behind each to increase air
flow across the heat exchangers.

Chamber humidity was controlled within 5 percent relae
tive humidity at a given temperature by a humidifier (Kenmore
Model No. 758.72912) and a dehumidifier (Coldspot Model
106.637140) cycled by a humidity controller (Humistat Model
15-3205, Hygrodynamics Inc.). Both systems were located
outside the chamber and connected to the unit by 10 inch
diameter flexible air ducts.

Air was circulated inside the chamber by an oscillating,

 

24

20 cm. fan. Two plexiglass sliding windows (66 x 91 cm.)
allowed observation of the animal and adjustment of apparatus

during the experiment.

3.5 Respiration Rate

Respiratory rate was measured with a mercury in rubber
strain gauge transducer matched to a Model 270 plethysmoe
graph and monitored on one channel of a dual strip chart
recorder, (Sanborn Model 322 Dual Channel DC Amphifer-
recorder). The strain gauge was attached to an elastic band
(2.5 cm. wide) placed around the animal's trunk posterior to

the last rib.

3.6 Heat Production

Heat production was calculated by open circuit, indirect
calorimetry techniques (Figure 3). A ventilated cylindrical
hood 45 cm. in diameter and 61 cm. long was placed over the
head (including ears of the animal). Air was drawn through
the hood at a rate of 60 liters/min. Oxygen consumption
(Oxford Instrument Co.) was recorded (STPD) and converted to
kcal./kg.3/° as suggested by Kleiber (1961). For temperatures
above the thermoneutral zone (25° C. and above) metabolic heat
production was calculated by the following equation (Joyce
and Blaxter, 1964):

Hm = k(02 consumed)

25

where: Hm = heat production (kcal.)
02 consumed = oxygen consumption (liters/hr.)

k = 5, no. of kilocalories per liter of
oxygen utilized.

The following equation was used for temperatures below the

thermoneutral zone (Joyce and Blaxter, 1964):

Hm = Hm25 + k (A1 - A2)

where: Hm = heat production at ambient tempera-
tures below TNZ (kcal.)

Hm25 = heat production at 25° 0. (kcal.)

A1 = oxygen consumption at temperature
below TNZ (liter/hr.)

A2 = oxygen consumption at 25° C.
(liters/hr.)

1k = 4.68, no. of kilocalories per liter of
oxygen utilized.

The different values of the constant, k, compensates for the
difference in R.Q.'s. This difference is due to increased fat

catabolism at subcritical temperatures (Graham gt gt., 1959).

3.7 Temperature Measurements

Temperatures were measured using copper constantan
thermocouples referenced to an ice water bath with EMF's
monitored on an adjustable range, adjustable zero, strip
chart recorder, (Mosley Model 7100B) with a full scale
sensitivity of 0 to 50° C. The temperature recording system

was calibrated daily to the nearest 0.1° 0. against a

 

26

National Bureau of Standards thermometer. An automatic

stepping relay allowed sequential monitoring of nine tempera—

tures on one recording channel.

Rectal temperature was measured with a thermocouple

mounted in a flexible catheter inserted 10 cm. into the lower

colon.

Skin temperatures were measured as follows:

a)

0)

Ear skin temperatures were obtained by locating
the thermocouple on the dorsal side of the ear,
anterior to the midline and approximately 5 cm.
from the distal tip. Thermocouples were held in
place by spraying the ear with an adhesive (Ace
Spray Adhesive) and taping the thermocouple in
place with porous adhesive tape (Figure 2).
Thermocouple placement was adjusted so that there
was less than a 2° 0. difference between ears to
eliminate placement directly over a large blood
vessel. The mean skin temperature of the two cars
was used for statistical analysis and other
computations.

Face skin temperature was measured with a thermo-
couple, attached as described above, to the mid—
line of the face approximately 5 cm. anterior to
the eyes.

Forelimb skin temperature was measured midway be-

27

«tween the knee and pastern joints on the left
foreleg. The thermocouple was held in place by
a 1 cm.2 area of nylon screening attached to a 1
inch elastic band (Figure 2) placed around the
limb.

d) Stifle skin temperature was measured midway between
the hook and stifle joints on the outside of the
left rear leg. Procedures for thermocouple attach-
ment were as described for the forelimb.

e) Trunk skin temperatures were measured approximately
10 cm. lateral to the dorsal midline and 10 cm.
posterior to the scapula on the animal's left side.
Procedures for thermocouple attachment were the
same as described for the forelimb with the elastic

band belting the trunk.

3.8 Evaporative Wgter Loss

Respiratory and skin evaporative water losses were
measured by a relative humidity transducer (Hygrodynamics
Model 15—7012) in a measured air flow system. Electric pom
tentials of 0 to 5.4 v. output (corresponding to O to 100%
relative humidity) were monitored on an adjustable range, ad-
justable zero strip chart recorder (Mosley Model 7100B). The
adjustable air flow system serving the respiration hood

(Figure 3) was operated at a rate of 60 liters/min. and that

 

 

 

28

of the skin capsule as shown in Figure 4 at 2.5 liters/min.

A vacuum pump withdrew air samples at the rate of 2.5
liters/min. from the respiratory hood and the ventilated skin
capsule (Figure 4). A reference air sample was drawn from
within the chamber at a rate of 2.5 liters/min. Water
evaporation was calculated by:

H20 loss = A(RHO - RH1)FA
where: H20 loss = water loss (gm.)

A

II

gram of water in one liter
of saturated air (gm./liter)

RHO = relative humidity in chamber (%)

RH1 = relative humidity of sampled
air (%)

FA = air flow (liters/min.)
Heat of vaporization was calculated by the formula described
by Kleiber (1961):
Hvap = K1 e K2 T
where: Hvap = heat of vaporization (kcalygm.H20)

K1 = 0.5959 latent heat of vaporization
at 00 Co

K2 = correction constant for temperature

T = temperature in degrees centigrade

3.9 Whole Body Insulation
Whole body thermal insulation was calculated as de~

scribed for sheep by Webster and Blaxter (1966) as follows:

29

Hm

tissue insulation (°C./kcal.- m2ohr.)

where: It
Tr = rectal temperature (00.)
Tg = mean skin temperature (°0.)

HIn = heat production per unit body sur-
face area per unit time (kcal./m2okun)

Surface area was calculated as 0.09 x wt. (kg.) 0.667 (correc-

tion factor for sheep) (Mitchell, 1927).

3.10 Thermal Circulatory Index

Thermal circulatory index was calculated for all skin
surface temperatures according to the following formula
(Burton and Edholm, 1955):

T.C.I. = TS - TA

Tr - Ts

where: TCI thermal circulatory index

skin surface temperature (°0.)

#3
ll

TA = ambient temperature (°0.)

_ rectal temperature (°0.)

l-3
I

3.11 Panting Index
Panting index was calculated according to the following

equation (Adams, 1968):

Panting index = (Hl)p - (Hl)r
(HmIP - (HmIr

30

where: (Hl)p

heat loss during panting

(Hl)r = heat loss during rest
(Hm)p = heat production due to panting
(Hm)r = heat production during rest

3.12 Mean Body Temperature
The formula of Hardy and DuBois (1938) was used to cal-

culate mean body temperature as follows:

where: T- = mean body temperature

b
Tr = rectal temperature
T§ = mean skin temperature

K1 = 0.80, portion of body mass repree
sented by core

0.20, portion of body mass repre»
sented by shell

(as
A)
II

3.13 Body Heat Content

Body heat content was calculated as described for sheep
by Blaxter gt git. 1959 using the mean body temperature
formulated by Hardy and DuBois (1938) and the specific heat
(0.83) of body tissue reported by Schafer (1898). This value

was calculated as follows:

BHC =(T5)(K1)(M)

 

where: BHC
Tb
M

K1

31

= body heat content (cal.)
= mean body heat (KO)
= body mass (gm.)

= 0.83, specific heat of body tissue
of sheep

3.14 Mean Skin Temperature

Mean skin surface temperature was obtained by proportion~

ing trunk temperature

and the mean of all extremity tempera-

tures (ear, face and forelimb) in the ratio 9:1 as described

for sheep by Webster and Blaxter (1966):

where: T- =

Te+

where: T6 =
Tf _

Tfl

3

mean skin temperature (°0.)

0.9. portion of skin surface repre-
sented by the trunk

0.1, portion of skin surface repre-
sented by extremities

Tf + Tfl

= mean extremity skin
temperature ( C )

mean ear skin temperature (°0.)

face skin temperature (°0.)

forelimb skin temperature (°0.)

3.125 Emperimental Procedures

A total of 48 trials were conducted (replication of a

seriues of 24 trials) on three sheep at eight ambient tempera-

32

tures (5° intervals ranging from 10 to 45° 0.),

Following training, the animals were loosely haltered
and led from the annex room (Sec. 3.3) to the environmental
chamber (Sec. 3.4). A 20 min. preconditioning period at
25° 0.. considered to be thermoneutral on the basis of a
series of pilot trials, preceeded each trial. , Following prea
conditioning the temperature and humidity were adjusted to
the appropriate treatment values. Due to the limits of the
humidity control system humidity was controlled in three
ranges as:

Treatment Temp. (°0.) Relative Humidity (%)

10, 15, 20 35 i 5
25. 30. 35 55 i 5
no, 45 65 i 5

With the attainment of the selected temperature, skin
and rectal temperatures as well as oxygen consumption and
reapiratory rate were recorded at 5 min. intervals on pre-
Viously prepared data sheets. Respiratory and cutaneous
eVaporative water loss were recorded at 10 min. intervals.
Each trial was conducted for a minimum of one hour or until
all parameters had reached a "steady state". This was defined
as a stabilizing (or reversing) of all parameters for two
Consecutive 5 min. periods. Animals were not subjected to

the experimental procedures more than once daily.

33

The sheep were fed daily at 4:00 a.m. Trials were con-
ducted between 8:00 a.m. and 4:00 p.m. to insure that the
animals were in a comparable post absorpative state.

Means calculated from values recorded during the last
20 min. of each trial were used for statistical analysis and

other computations.

3.16 Statistical Analysis

Means, standard deviation, simple correlation and re-
gression equations were calculated by the computer laboratory
(3600 Computer). Duncan's new multiple range test was used

to compare treatment means.

IV. RESULTS

4.1 Rectal Temperature

Mean rectal temperature responses are reported in Figure
5. Rectal temperature-was correlated with-ambient temperature
(r = 0.65). Data reported in Table-1 indicate that a signi-
ficant increase in rectal temperature (P<:.05) was recorded
at 45° C. as compared to all other ambient temperatures. A.
significant difference was not found among rectal temperatures
recorded from 10 to 40° C. ambient temperature. The mean.
rectal temperature for all observations was 39.9 i 0.48° C.
Rectal temperature increased 0.03° 0. per degree increase in
ambient temperature. Linear regression analysis of rectal
temperature (y) on ambient temperature (x) is shown in.
Appendix III. A significant difference (P<1.05) in rectal
temperature among sheep or between trials at a.given tempera-
ture was not observed. The mean change in rectal temperature,
defined as the absolute change-from preconditioning to treat-
ment, was -0.42 1 .42° C. Change-in rectal temperature was

negatively correlated with ambient temperature (r = -0.57).

4.2 Skin Temperature
Mean skin surface temperatures are related to ambient

temperatures (Figures 6 through 10). Correlation coefficients

34

 

35

relating skin surface temperatures to ambient temperatures
ranged from r = .93 to r = .97 (Appendix II).

Although all skin surface temperatures increased as
ambient temperature increased, Table 1 shows that the responses
varied with the site of measurement. Ear and forelimb skin
temperatures increased significantly (P4<.05) with each 5° 0.
increase in ambient temperature. Face and stifle skin tempera-
tures rose significantly from 10 to 30° 0. and from 10 to
35° 0.. respectively. Trunk skin temperature was constant
from 10 to 15° C. and from 35 to 40° C. with significant
increases (P (.05) recorded at other temperatures.

At ambient temperatures of 35° 0. or less, skin surface
temperatures were always higher than the ambient temperature
(Figure 11). Conversely, when ambient temperature was 40° 0.
or above, skin surface temperatures were always lower than
ambient temperatures. Face, ear and trunk skin temperatures
were higher than rectal temperature when the ambient
temperature was 45° C. (Figure 11). Figure 11 shows that
the means of various skin temperatures converge at 30° C.

The rates of skin temperature change per degree centi-
grade increase in ambient temperature are presented in
Table 3. These data demonstrate that ear skin temperature
is the most responsive to changes in ambient temperature

while the stifle is the least responsive to changes in

 

36

ambient temperature.

The ambient temperature interval where skin surface
temperatures changed the most varied with place of measure-
ment (Table 3). The face, trunk and forelimb temperatures
changed the most from 15 to 20° 0.. stifle-temperature from
20 to 25° C. and ear temperature from 25 to 30° C. The
weighted mean skin temperatures were positively correlated
with ambient temperature (r = .96) and increased signifi-
cantly (P<(.05) with each increase in ambient temperature
above 15° C. (Figure 12).

Statistical comparisons indicated that significant
differences (P<<.05) in mean-skin temperatures did not

occur among sheep at a given temperature.

4.3 Body Heat Content
A range of 21,179 to 21,429-kcal. was observed in body
heat content (Figure 18). Whole body heat was positively

correlated with ambient temperature (r = .94).

4.4 Whole Body InSulation

AS noted in Figure 12, whole body insulation increases
as ambient temperature decreases from 45 to 15° 0. This
relationship is represented by the regression equation
I = .0046 + .000103X.

There was not a significant decrease (P<(.05) in

 

 

 

37

tissue insulation at 35, 40, and-45° C. ambient temperature
when compared to ambient temperatures below 35° C. Figure
12 shows that low insulation values were recorded at 10° 0.
These values are significantly lower than the values at
15° 0.

Whole body insulation is negatively correlated with
ambient temperature (r = -.83) and skin temperature (r:<.60)

but inversely related to skin thermal circulatory indexes.

4.5 Thermal Circulatory Index

Thermal circulatory index (TCI) is positively related
to ambient temperature in the skin areas measured. Table
4 lists the means and percentage change of each TCI. These
data indicate relative low but highly variable TCI values in
areas of the body characterized by a high surface area to
mass ratio. Stifle and trunk areas which overlay muscle
and subcutaneous fat have consistantly high TCI values with.

lower percentage change.

4.6 Respiratory Rate

Mean respiratory rates demonstrate that respiratory
frequencyvms constant at ambient temperatures from 10-to 30°Ch
(Figure 19). Mean respiratory rate increased to a maximum
of 179 breaths per min. at 45° 0., with a minimum value of

19 breaths per min. at 15° 0. Respiratory rates increased

 

38

significantly (P<(.05) at each interval of ambient tempera-
ture above 30° 0. (Table 2). Although significant differences
in respiratory rate were not observed at temperatures below
30° 0.. there was an increase in mean respiratory rate at

100 0. when compared to 15, 20, and 25° 0.

Respiratory frequency was more highly correlated with
ambient temperature (r = .83) than with rectal temperature
(r = .68). A linear response was observed from 10 to 30° 0.
Then an exponential increase as ambient temperatures in-
creased above 30° C. Table 2 shows that water loss per
breath decreases significantly (P<:.05) at 40° c. Linear
regression analysis of RR/Rewl (y) on ambient temperature
(x) is given in Appendix III. A significant difference did

not occur in respiratory rate between or among sheep.

4.7 Respiratory Water Loss

Respiratory evaporative water losses (Rewl) are plotted
in Figure 21. Rewl was the lowest at 25° c. which coincides
with low respiratory rates. Rewl values increase signifi-
cantly (P<:.05) at high ambient temperatures (35, 40, and
45° 0.). increasing 63% between 30 and 35° C. Rewl was
correlated (r = .79) with respiratory rate. When Rewl is
plotted as a function of RR. a linear relationship exists
at respiratory rates below 30° C. At temperatures above 30° 0.

the characteristic relationship between Rewl and RRiwusnot

 

39

linear. -A decrease in Rewl/RR is observed as ambient
temperature increases (Figure 22). This relationship, as
opposed to Rewl, was linearly related to ambient tempera-
ture and is represented by the regression equation y = .045
+ .00049X (Appendix III). Variations in Rewl/RR were
observed at ambient temperatures below 40° 0. However,
consistent values were obtained among animals at 40 and
45° 0. There were no significant differences (P<(.05)
among sheep in RR. Rewl or Rewl/RR responses at a given
ambient temperature. Panting indexes are tabulated in
Table 5 and plotted in Figure 23.

Rewl was correlated with skin water loss (r = .70),
rectal temperature (r = .52)—and with changes in rectal
temperature (r = .67). The latter two were linearly related

with Rewl.

4.8 Cutaneous Water Loss

Cutaneous evaporative water loss is plotted as a
function of ambient temperature in Figure 24. This figure
shows a nine-fold increase in cutaneous water loss was ob-
served between 10 and 45° 0.. with values at 40 and 45° 0.
significantly higher (P<:.05) than at ambient temperature
less than 40° C. A 133% increase in cutaneous water loss
occurred between 35 and 40° C. This increase was

appreciably more than the significant increase in Rewl

 

40

observed between 30 and 35° C. The data in Table 2 also
demonstrate that sweating at 35° C.was significantly
higher (P<:.05) than at ambient temperatures from 10 to
25° 0. Cutaneous and respiratory evaporative water loss
were correlated (r = .74) with trunk skin temperature. No
significant difference (P<(.05) in sweating at a given

ambient temperature was found among sheep.

4.9 Heat Production

Changes in heat production are plotted in Figure 26.
A minimum mean value of 65.1 kcal./kg.3/4/hr. was observed
at 30° C. The relationshipretween heat production and
ambient temperature is non-linear. A significant (P<:.05)
increase in heat production at 10° 0. when compared to
higher temperatures is noted in Table 2. There was no sig-
nificant difference (P<(.05) among temperatures from 15 to
35° C. or between 40 and 450 0.. although there is a signi»
ficant increase in heat production at 45° 0. when compared
to 35° C. The increase in heat production at 10° C. was the
largest observed being 57% and 26% greater at 15 and 45° 0.,
respectively (Figure 26). Heat production was not highly

correlated with any other single variable (Appendix II).

 

V. DISCUSSION

The rectal temperature of homeotherms varies within
"normal limits". The dimension of these limits is not uni-
versal; some homeotherms are more precise regulators of
rectal temperature than others. Data reported here indicate
that the "normal limits" of rectal temperature for the sheep
are relatively wide with significant differences (P<:.05) in
rectal temperature observed over the range of ambient
temperatures studied. Rectal temperature ranged from 38.8
to 41.10 C. and increased approximately 0.0270 C./°C. change
in ambient temperature over the range of 10 to 45° C. (Figure
5).

The relatively wide "normal limit" for internal
temperature in sheep is also reported by others (Blaxter gt
gt., 1959a; Brook and Short, 1960; Slee and Sykes, 1967; and
Webster,1968). This wide "normal" range of rectal tempera—
ture has important implications. Rectal temperature is a
physiological measurement used as an indicator of health,
disease and other physiological states. The influence of
ambient temperature on the rectal temperature of sheep
should therefore be considered when rectal temperature is

used as an index of these states in sheep exposed to

41

 

42

different thermal environments. Chemical reactions are
accelerated as temperature rises according to the Q10 law.
An increase in"deep'body temperature would increase (or a
decrease if body temperature would decrease) the rate of
physiological reactions. When evaluating data recorded cone
currently with changing"deep'body temperature, it is
important to consider the Q10 effect on physiological
reactions.

The rate of heat exchange between the skin surface and
the external environment is dependent upon the thermal
gradient. Control of cutaneous blood flow by vasomotion
regulates the rate of heat flow from the core to the skin.
Therefore, vasomotion is capable of modifying sensible heat
loss. vasomotor variability, termed "tissue insulation" or
its reciprocal "tissue conductance" is a thermoregulatory
response (Hammel, 1968), and is the only adjustable means
of distributing internal heat to the skin. The present study
demonstrates that the sheep has the ability to modify
sensible heat loss by varying tissue insulation (Figure 12).
Whole body tissue insulation was significantly higher
(P‘<305) at ambient temperatures above 30° 0., when compared
to values calculated from 15 to 30° C. (Table 3). These
values are similar to those of Blaxter (1962) who reported

3.5 and 1.2° C. X 10'3/kcal.m2/24 hrs. for shorn sheep

   

43

exposed to cold and heat stress respectively.

Whole body insulation is inversely related to ambient
temperature, from 15 to 45° C. (Figure 12). These findings
are teleologically compatable with the maintenance of
homeothermy. Conversely, a decrease in whole body insulation
was observed at 10° 0. Similar findings have been reported
by Blaxter gt gt. (1959a). This phenomenon can be explained
by the hypothesis that during shivering, which was observed
at 10° 0., the fascial muscles are supplied with blood. The
opening of capillary beds within the muscles would result in
decreased tissue insulation.

Skin surface temperatures are a function of cutaneous
blood flow and environmental temperature. Changes in either
of these determinates is reflected by changes in skin sur-
face temperature. The difficulty in distinguishing the in-
fluence of vasomotion from that of changes due to environs
mental temperature can introduce erroneous conclusions
regarding cutaneous blood flow, and consequently, the regua
lation of sensible heat loss.

The thermal circulatory index (TCI) (Burton and Edholm,
1955), used to distinguish vasomotor changes, indicated that
vasomotor responses vary with the region of measurement
(Figures 13 through 17). Wool free regions with high sure
face area to mass ratio (ears, face and forelimb) had rela-

tively large changes in TCI values compared to regions of

44

small surface area to mass ratio (Table 4). These areas,
therefore, may play an active role in modifying sensible
heat loss. Conversely, trunk and stifle regions, which are
fleeced areas and are characterized by underlying sub-
cutaneous fat and muscle respectively, varied less in
cutaneous blood flow as reflected by relatively constant TCI
values. The exception was noted in a substantially elevated
TCI value of the trunk at 10° C. (Table 4). A concurrent
decrease in tissue insulation at 10° C. (Figure 12) supports
the contention thattflood flow must increase in these areas.
This occurrence is attributed to shivering which was ob-
served at 10° 0. With the increased TCI of the trunk value
accounted for, it is concluded that on the basis of TCI
values the skin of the trunk and stifle (Figures 16 and 17)
do not respond to thermal changes (between 10 and 45° C.
ambient temperature) by vasomotion.

The magnitude of skin surface temperature change can be
used to identify vasomotion. Slee (1968) defined vasomotion
in sheep as the point where the change in skin surface
temperature exceeds 0.5° C./°C. change in ambient temperature.
This relationship was based on the observation that when the
change was less than this value, skin temperature variations
were attributed to environmental temperature.

Using the criteria of Slee, data reported here indicate

 

45

that skin temperature changes of about 0.4° 0./°C. change in
ambient temperature can be attributed to environmental
temperature. A similar ratio between skin surface tempera-
ture and environmental temperature was observed at all measure-
ment sites above 30° C. which suggests that vasodilation was
complete at 30° C. ambient temperature. It is concluded that
when temperature changes of more than 0.4° C./°C. change in
ambient temperature may be indicative of vasomotion.

Table 3 indicates large differences in the magnitude of
change among the various skin regions. The change in skin
temperatures of the trunk and stifle approximates 0.4° C./°C.
change in ambient temperature which infers that these areas
are without vasomotion. Conversely, changes in skin tempera-
tures of the ear, face and forelimb areas were greater than
0.4° 0./°C. change in ambient temperature and assumed to re-
sult from vasomotion in these regions. These findings
parallel those of the thermal circulatory index. On the
basis of thermal circulatory indices and the magnitudes of
skin surface temperature change per degree change in ambient
temperature, it is concluded that the ear, face and fore»
limb are capable of modifying sensible heat loss by vaso-
motion. Conversely, the trunk and stifle regions appear to
lack this ability.

Table 3 demonstrates that the ambient temperature

interval where the greatest change in skin temperature was

 

46

observed varies with the region of measurement. Ear skin
temperature had the greatest change (0.8° C./°CTA) between
25 and 30° C. while the face and forelimb changed the most
between 15 and 20° 0., with changes of 0.60 c./°c.qj~q and
0.6°C./°CJ&, respectively. These data question the value of
surface temperature changes in the skin for defining
thermoregulatory parameters in sheep, since these changes
are largely dependent on the region of measurement. The use
of ear skin temperature to determine the thermoneutral zone
(Blaxter gt gt., 1959a) in sheep appears to be a result of
circumstance; large changes of ear skin temperature coinciding
with feasible values for the thermoneutral zone.- The use of
skin surface temperatures to define the thermoneutral zone
and critical temperatures in sheep is therefore not come
pletely justified.

As ambient temperature rises above critical temperature
II, (also termed hyperthermal rise and defined as the ambient
temperature where heat stress begins) heat loss from the skin
surface decreases until skin temperature equals rectal
temperature, at which point there is no net sensible heat
exchange. When ambient temperature surpasses rectal tempera»
ture, the thermal gradient is reversed and the skin surface
gains heat: an added liability to the homeotherm. Vasodilae
tion in these circumstances jeopardizes homeothermy because

blood flow through cutaneous vessels increases the rate of

 

47

heat gain.‘ The jackrabbit (Schmidt-Nielsen gt g;., 1965) and
the ostrich (Crawford and Schmidt-Nielsen, 1965) vasodilate
cutaneous vessels up to an ambient temperature near body
temperature, and then to vasoconstrict these same vessels at
still higher air temperatures. Vasoconstriction, and conse-
quent increased tissue insulation, have been reported to
reduce heat gain when ambient temperature is aboverectal
temperature. The data here do not suggest that this
phenomenon is not operative-in sheep.

Changes of body heat content have been reported as an
important thermoregulatory mechanism (Bleakley and Findlay,
1955). Blaxter gt_g;, (1959a) reported that below the
critical temperature the body heat of sheep decreased by
approximately 7 to 10 kcal./°C. decrease in ambient
temperature. ~These workers also reported that body heat
content varied approximately 300 kcal. at ambient tempera=
tures from 10 to 40° C. The present data (Figure 18) recorded
a 247 kcal. difference in total body heat between 10 and
45° 0.. this difference resulting from modifications of body
core and shell. This mechanism could serve as an important
biothermal control mechanism during acute thermal stress by
buffering a rise (or fall) in deep body temperature. How-
ever, after shell and core temperatures reach an equilibrium

(after 1 to 3 hrs.), body heat content does not change and

 

48

therefore cannot benefit homeothermy. Elevated respiratory
rates which were observed below critical temperature I were
attributed to the increased metabolic heat production which
accompanies shivering (Figure 26). Slee and Sykes (1967) re-
ported significant increases in respiratory rate of shorn
sheep between thermoneutral (30° C.) and subzero temperatures.
Subzero temperatures were not utilized in the present study.
Significant increases in respiratory rate (Table 2)
were observed during heat stress. Respiratory frequency in-
creased logarithmically above the critical temperature II
until ambient temperature equalled rectal temperature (Figure
20). Respiratory rates can therefore be predicted within a
defined range of ambient temperature. This relationship
between respiration rate and ambient temperature could be
useful in detecting respiratory insufficiencies resulting
from administration of tranquilizers, anesthetics, etc.
Respiratory rate response is frequentDy utilized in biothermal
investigations to identify the onset of heat stress (critical
temperature II). Comparison of the beginning of active heat
loss (evaporative water loss) and of respiratory rate
response (Table 2) justifies the utilization of respiratory
rate increase to estimate the initiation of heat stress
(critical temperature II). The use of respiratory rate, per

se is not valid for this estimate since numerous studies have

reported wide ranges of respiratory rate at identical ambient

 

49

temperatures.

Hales and Webster (1967) report two phases of respiratory
pattern responses to heat in sheep. The first is character~
ized by increased frequency and a decrease in tidal volume.

A second phase, characterized by decreased frequency and inn
creased tidal volume, was observed by these authors after 70
minutes of heat exposure (40 to 60° 0.). In the present
study, sheep exposed to 45° C. for one hour did not obviously
enter the second phase of respiratory responses described by
Hales and Webster (1967) .

Respiratory evaporative water loss increased exponen~
tially at ambient temperature above 30° C. (Figure 21). The
calculated theoretical maximum gross respiratory evaporative
heat loss, using the size adjusted reSpiratory minute volume
of Hales and Webster (1967), was approximately 2000 kcal./day.
During heat stress, in the present study, slightly more than
one-fourth of this potential was attained. Respiratory
evaporative water loss was similar (r = .80), but not
equivocal to respiratory frequency. This difference is ex-
plained by the changing RR/Rewl ratio (Figure 22).

Increased reSpiratory evaporative water loss by panting
(defined for this study as a significant increase in respira=
tory rate above thermoneutral levels) is accompanied by the
increased heat production due to the inherent in muscular

activity. Therefore, the net efficiency of heat loss during

 

5O

panting must be considered. Whittow and Findlay (1968) res
ported that only a portion of increased heat production
during heat stress is attributable to the muscular activity
of panting in cattle. Although many sources of increased
heat production were present (increased heart rate, elevated
catcholamines, sweat gland activity), these workers reported
that the.Q10 effect of increased rectal temperature on
general metabolic reactions accounted for a major portion of
increased heat production observed during heat stress,

Values used to calculate the panting index, considering in-
creased heat production according to the Q10 law, are shown
in Table 5. Figure 23 plots panting index as a function of
respiration rate. A non-linear response was expected because
of the changing RR/Rewl ratio during heat stress. Extension
of the panting index curve (Figure 23) suggests that when
respiratory rate reaches approximately 240 breaths per minute,
the panting index equals one; the limit of net respiratory
evaporative heat loss.

Cutaneous evaporative water loss (Figure 24) increased
significantly (P<:.05) at 35° 0. when compared to 25° C.
(Table 2). A significant difference was not observed between
30° C. and either 25 or 35° 0. The data suggest that active
sweating begins at 40° C. ambient temperature. The data show
a nonshgnificant increase in sweating occurred at 45° 0. when

compared to 400 0.. suggesting that the maximum rate had been

 

51

reached. A positive correlation (r = .74) between sweating
rate and skin temperatures suggests that increased skin
temperature may stimulate sweating. Data presented in Table
2 does not conclusively demonstrate the ability of sheep to
sweat. Increases in obligatory cutaneous water loss due to
increases in skin surface temperature could account for
significant increases in cutaneous water loss. To investi-
gate this possibility, Figure 25 compares vapor pressure in-
crease (proportional to evaporative water loss from a wet
surface) and cutaneous water loss as a function of trunk
skin temperature. The difference in the shape of these
curves indicates a limited ability for sheep to control
actively skin water losses, probably involving the secretion
of apocrine glands found in conjunction with wool follicles.
Allen and Bligh (1968) reported phasic patterns of sweat
discharge in sheep subjected to heat (400 0.). and attributed
this to variations in sweat secretory patterns. The wide
range of values obtained during heat stress, in conjunction
with the absence of variations during cold stress, adds
support to suggestions of the existence of thermally related
sweat gland activity in the sheep.

Effective heat loss due to sweating is difficult to

quantify since the heat of vaporization is in part from the

environment and the rate of sweating is not consistent among

 

52

skin areas. The measurement of skin water loss on a known
surface area of the trunk (as done in the present experiments)
therefore, cannot be multiplied by total body surface to
accurately determine total skin water loss. For these rea-
sons the rate of cutaneous water loss in the present study
should be interpreted as qualitative responses to heat
stress. However, these data do suggest the existence of an
active cutaneous evaporative water loss control mechanism.
These data shed new light on the controversy concerning
the relative importance of respiratory and cutaneous
evaporative water loss in sheep. Previous approaches to
this question have eliminated one avenue of evaporative heat
loss while measuring the other, or measured one avenue of
evaporative heat loss and related it to total heat loss.
These methods have been uniformly unsuccessful in explaining
the relative importance of Rewl and Cewl. Brockway gthl..
(1965) measured both respiratory and cutaneous evaporative
water loss simultaneously but only at temperatures below
30° 0. Their conclusions cannot be considered applicable to
heat stress. The present series of experiments suggest that
both respiratory and cutaneous evaporative water loss are

important avenues of heat loss in sheep and that each increases

significantly during heat stress. This is in contrast with
prior workers who have reported that sweating does not occur

in sheep. In the present experiment, sweating increased

 

53

eight-fold while respiratory water loss increased only
three-fold. With ambient temperatures of 40° C. and above,
the primary avenue of heat loss is evaporative water loss.

If the data are expanded to 24 hr. these data show that at

40° C. respiratory water loss accounted for 486 kcaL/da. while
heat production was 2,280 kcal./da. Therefore, cutaneous
evaporative heat loss must have been reSponsible for a portion
of total heat loss. Accordingly, Table 2 tabulated a signi»
ficant increase in sweating at 400 C. and 45° 0. when com—
pared to lower temperatures. On the basis of these findings,
both cutaneous and respiratory evaporative water loss are
hypothesized as important avenues of heat loss in shorn sheep
during heat stress. In addition, increased respiratory water
loss precedes the onset of sweating and therefore is relam
tively more important during mild heat stress (35° 0.).
However, during severe heat stress (40 and 45° 0.) heat loss
by cutaneous evaporative water loss is relatively more
important.

Metabolic heat production increased above thermoneutral
values during both heat and cold stress (Figure 26). Signi~
ficant (P<1.05) increases in heat production above the
critical temperature II are believed due to two factors.
First, muscular activity associated with panting results in
elevated heat production. Second,a rise in"deep"body

temperature leads to increased heat production in accordance

54

with the Q10 law. Data presented in Table 5 indicate that
the Q10 effect may be greater than the effect of panting.
The importance of the Q10 effect on heat production in sheep
suggests a need for further study.

Increased metabolic heat production during cold stress
(Figure 26) is attributed to shivering. Slee and Sykes (1967)
reported the onset of shivering in shorn sheep at 16° 0.. and
moderate shivering at 5 to 10° C. Conclusive shivering was
detected in the present experiment at 10° C. which coincided
with a significant increase in heat production (Table 2). The
wide range of values of heat production (Figure 26) observed
at 10 and 15° 0. suggests individual variation in the onset
and magnitude of shivering. The need for studies specifically
designed to elucidate the factors involved in these relation»
ships are indicated.

The lowest metabolic heat production was observed at
30° C. with only a slight increase(1.8%)noted at 25° C. A
larger increase (8.5%). although non-significant (Table 2)
was measured at 20° C. Shivering was not observed con-
currently with these increases of metabolic heat production.
Since non-shivering thermogenesis has not been reported for
adult sheep, it is presumed that "thermal muscular tone" which
preceeds shivering (Burton and Edholm,1955) accounts for
increased heat production during mild cold stress (15 and

20° 0.). Critical temperature I is coincident with the

55

ambient temperature where cold stress begins.

On the basis of the initial increases in heat production
below the thermoneutral zone, the results of this study
indicate that critical temperature I of sheep (as previously
defined) is greater than 20° 0. Table 2 shows that only a
slight (1.8%) increase in heat production is noted at 25° C.
as compared to.a substantial (8.4%) increase at 20° C. These
data suggest that critical temperature I may be near 25° 0.
although the slight increase (1.8%) does not warrant this as
a conclusion. The 5° 0. intervals of ambient temperature
studied limit a more accurate assessment of critical
temperature I.. Comparable data reported by Blaxter gt gt.
(1959a) suggests 26° C. as the critical temperature for
shorn sheep on a maintenance ration. Dukes (1956) reports
critical temperatures for swine, cattle (sheared) and goats
(fasted) as 21° C, 18.3° C. and 20° C., respectively. It is
concluded that critical temperature I for sheep (as defined)
is greater than 20° c.

Hyperthermal rise, termed critical temperature II. and
defined as the upper limit of the thermoneutral zone has not
been reported in sheep. Before critical temperature II can
be accurately determined, the relationship of its determinants
(respiratory rate, respiratory and evaporative water loss) to
heat stress must be clarified. The absence of these relations

ships has predicated the impression that critical temperature

 

56

II is of minor importance. However, the role of the critical
temperature in determining the thermoneutral zone and in
describing the initiation of heat loss mechanisms identifies
it an important factor in the description of thermoregulatory
mechanisms of sheep. On the basis of increased heat loss,
this critical temperature is equal to or less than 30° C.

The investigation of physiological responses of homeo=
therms to thermal stresses requires the establishment of the
thermoneutral zone. Numerous workers (Slee and Sykes, 1968;
Blaxter gt gt.. 1959b; and Armstrong gt gt., 1960) have con»
sidered 30° C. as thermoneutral for shorn sheep, although it
can be modified by many variables (i.e., exercise, diet, etc.).
This value wraprimarily based on vasomotor activity in the car
as indicated by rapid changes in ear skin temperature (Blaxter
gt gt.. 1959a). The use of this criterion to describe the
thermoneutral zone in sheep is questionable due to the
diversity of vasomotor activity observed in different body
regions. It may be more appropriate to define the thermo«
neutral zone for sheep as the range of ambient temperature
between critical temperature I (initial cold stress) and
critical temperature II (initial heat stress). Based on these
criteria the lower limit of the thermoneutral zone for sheep
(as defined in this experiment) usequal to or greater than

20° C. while the upper limit is equal to or less than 30° C.

 

 

57

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59

TABLE 3

Ambient Temperature Interval and Magnitude of Greatest
Change of Rectal and Skin Temperatures

 

 

 

Measurement Ambient temp. interval Change per °C
site 1/ of greatest change g/ ambient temp.3/
Redal temp. 40-45 0.03 -
Ear temp. 25-30 0.8
Face temp. 15-20 0.6
Forelimb temp. 15-20 0.6
Stifle temp. 20-25 0.4
Trunk temp. 15—20 0.4

 

l/ Rectal temperature was measured in lower colon. Skin
temperatures were measured at the skin surface.

g/ Ambient temperature interval (°0.) when greatest change
in temperature parameters were recorded.

Change in temperature parameters (°0.) recorded per degree
centigrade change in ambient temperature.

60

TABLE 4

Thermal Circulatory Indices at Different
Ambient Temperatures l/

 

 

 

 

Skin Ambient Temperature °C. Percent
Area 10 15 20 25 30 Change g/
Ear 0.21 0.16 0.24 0.23 0.78 271
Face 0.58 0.50 0.90 1.00 1.03 106
Forelimb 0.35 0.39 0.59 0.68 0.91 160
Stifle 1.64 1.65 1.48 1.84 1.63 12
Trunk 1.72 1.11 1.25 1.21 1.15 55

 

1/ Thermal circulatory index = TS - TA

II.r ‘ Ts
where: TS = skin temperature (°C.)
T1. = rectal temperature (°C.)

TA = ambient temperature (°C.)

g/ Percent change = Highest.TCI - Lowest TCI

Lowest TCI X 10°

 

Partition of Q10

61

TABLE 5
and Panting Effects on Metabolic

Heat Production and Calculation of Panting Index

 

 

Inc. in Inc. in m §r
maids metabolic Q10 Panting resp.evap. Panting
temp. heat prod.t/ effectg/ effect3/ heai;lossfly7 index 5/
35° c. 252 58 194 434 2.23
40° c. 516 155 361 486 1.34
45° c. 680 278 402 510 1.24

 

1/ Increase in metabolic heat production above lowest value
recorded at 30° C. (kcal./24 hr.)

g/ Calculated 910 effect of "deep body" (rectal) temperature
on metabolic heat production (kcal./24 hr.).

3/ Effect of panting on metabolic heat production (total in»
creased heat production - Q10 effect) (kcal./24 hr.).

3/ Increase in respiratory evaporative heat loss above lowest
value recorded at 25° C. (kcal./24 hr.).

5/ Panting index = (Hl)p - (Hl)r

where:

Hm p - Hm r
(Hl)r = heat loss during rest
(Hm)p = heat production due to panting
(Hm)r = heat production during rest

(H1) — heat loss during panting

62

 

Figure 1. Picture of Temperature — Humidity Control Chamber

63

N I
Elastic Band y 0'1 Screen

   
   

 

Thermocouple Wires

Ear Skin
Surface

I
Porous Adhesive Tape

Figure 2. Schematic drawing of thermocouple attachment.

 

 

64

 

 

T-

 

rII '0 ROHIEV.
_. Humidity
Transducor

III

 

 

air 65 sq. cm.
inflow —>

skin surface

 

 

 

(0)
61cm. long
air inflow
45cmdio.
/
air inflow
lo Relative
_. Humidity
[— Transducer
(b) I

to 02 analyzer.

Figure 3. Schematic drawing of evaporative water loss air sam-

pling systems (a-skin capsule, b-respiration hood).

 

65

Sample Air Reference Sample Air
from Respiratory Air from Skin
HOOd ”IQ-3b) Capsule (fig.3a)

       
 

I

 

Insulation

1. Three way valve

2. Air sampling pump

3, Relative humidity transducer
4. Recorder

Figure 4. Schematic drawing of air flow system for deter-
mination of evaporative water loss.

 

Rectal Temperature °C

42-

41-

‘57.?

w
‘9
I

38

 

66

 

l
I r I l l I I 1*
10 15 20 25 30 35 40 45
Ambient Temperature °c
Figure 5. Rectal temperature as a function of ambient

temperature (circles indicate means and
vertical lines denote ranges of six observations).

 

 

67

42

co
co
I-O—l

 

Ear Skin temperature °c

 

10 15 20 25 30 35 40 45

Ambient Temperature °C

Figure 6. Ear skin temperature as a function of ambient
temperature (circles indicate means and vertical
lines denote ranges of six observations).

46-

w 9
0° N
1 1

or
'7‘

Face Skin Temperature °c
w
G

 

68

 

26‘
224
18"
14‘
10"
' 7 l r Y T r r
10 15 20 25 30 35 40 45

Ambient Temperatu re °c

Figure 7. Face skin temperature as a function of ambient

temperature (circles indicate means and vertical
lines denote ranges of six observations).

 

 

°C

Forelimb Skin Temperature

69

4o-
42-
33. <5
34.
3o«
26-

22-

14-

10‘

 

 

T T l T I T

r r
10 15 20 2 5 30 35 40 45
Ambient Temperature °C
Figure 8. Forelimb skin temperature as a function of

ambient temperature (circles indicate means and
vertical lines denote ranges of six observations).

 

 

°c

Trunk Skin Temperature

 

70

 

46-
42« §
38‘ i §
34— §
30‘ % i
26~
22-
18‘
14-
10*
l T T Y I Y I T
10 15 20 25 30 35 40 45
Ambient Temperature °C
Figure 9. Trunk skin tenperature as a function of ambient

temperature (circles indicate means and vertical
lines denote ranges of six obserV9tions).

 

46-

N N w a; OJ 5
N 6"
. i ‘P f ‘1" '1’

Stifle Skin Temperature °c

p...
N
l

14-

10

 

71

 

Figure 10.

T T I I Y Y I

15 20 25 30 35 40 45
Ambient Temperature °C
Stifle skin temperature as a function of ambient

temperature (circles indiCate means and vertical
lines denote ranges of six observations).

 

 

4o—

42~

38‘

Skin Temperatures °c

N
N
l

y...
N
l

on

10"

 

72

 

 

r

10

Figure ll.

T r T T T T T

15 20 25 30 35 40 45

Ambient Temperatu re °C

Skin temperatures as a function of ambient temp-
erature.

 

 

9x10" °C./kcal. - m2- hr.)

Whole Body Insulationhunir

T‘

65
I

U1
l

f

w
l

N
L

'1-4

73

 

 

 

 

l I T

10 1'5 2'0 2'5 30 35 40 4'5

Ambient Temperature °C

Figure 12.. Whole body tissue insulation as a function of
ambient temperature (circles indicate means and
vertical lines denote ranges of six observations).

 

Ear Thermal Circulatory Index

2.0"

H
01
L

0.5"

71+

 

 

r T
10 15 20 25 30
Ambient Temperature °c

Figure 13. Thermal circulatory index of the ear as a
function of ambient temperature.

2.04

p—e
01
1

Face Thermal Circulatory Index

9
01
l

 

75

 

10

Figure 14.

T T
15 20 25 30
Ambient Temperature °C

Thermal circulatory index of the face as a
function of ambient temperature.

 

Forelimb Thermal Circulatory Index

2.0"

p—a
hi
1

1.0-4

P
U!
1

76

 

 

T T
10 15 20 25 30
Ambient Temperature °C

Figure 15. Thermal circulatory index of the forelimb as a
function of ambient temperature.

 

77

 

2.0-i -

g 1.5-
-=
.5
h
‘5
I!
=
.2
E3

g 1.0-1
.92
y—
.I
=
E
I—

0.5‘

I I T I I
10 15 20 25 30
Ambient Temperature 0C
Figure 16. Thermal circulatory index of the trunk as a

function of ambient temperature.

 

 

Stifle Thermal Circulatory Index

H
Ln
1

1.0-4

 

78

 

0.5"
I I T I I
10 15 20 25 30
Ambient Temperature °C
Figure 17. Thermal circulatory index of the stifle as a

finction of ambient temperature.

21,700—

2|,600-

21,500a

21,400—

2I,300-1

Body Ileat Content (kcal.)

21,200—

21,100-

79

 

I I Y r t I

T T
10 15 20 25 30 35 40 45

Ambient Temperature °C

Figure 18. Whole body heat content as a function of ambient
temperature.

8O

250a
225-
2004

.175-

In)

150-

125-

' 100-

Respiration Rate (per m

-u
01
l

50-

 

 

 

T I I Y I I T

r
10 15 20 25 30 35 40 45
Ambient Temperature °C
Figure 19. Respiratory rate as a function of ambient

temperature (circles indicate means and vertical
lines denote ranges of six observations).

81

 

2.5-i

-
0.

2

an
32. 839.38.. 32%:

40

25

 

.0

Ambient Tempe ratu re °C

Log of respiratory rate as a function of ambient

te

FiQure 20.

A

moerature.

82

0304
027-
024-
0114 V
one-

OJS‘

 

0J2-

Respiratory Evaporative lleat Loss(kch./kg./hr.)

 

OD9-

096-

093-

 

 

I I r ‘r r v

”r T
10 15 20 25 so 35 40 45
Ambient Temperature °C
Figure 21. Respiratory evaporative water loss as a function

of ambient temperature (circles indicate means and
vertical Lines denote ranges of six observations).

-3

Respiratory Evap. Heat Loss(kca|/kg/hr.)per Respiratory Ratehner min) 10

83

4-4

 

11

 

 

1'0 1'5 2'0 2'5 3'0 3'5 4'0 4'5
Ambient Temperature °C

Figure 22. Respiratory evaporative heat loss per respira-

tory rate as a function of ambient temperature.

Index

Panting

84

 

3
2..—
llllllllllllllllllllm
l _.
o—
I I I I I I T I

 

 

80 TOO

Figure 23.

T
120 T40 160 180 200 220 240 260

Respiration Rate(P°'"““-)

Panting index as a function of respiratory rate.

85

&

w

 

 

N

Cutaneous Evaporative Water loss(gm./in?/da.)

 

 

10 15 20 25 30 35 40 45

Ambient Temperature °C

Figure 2”. Cutaneous evaporative water loss as a function of
ambient temperature (circles indicate means and
vertical lines denote ranges of six observations).

Cutaneous Water loss (gm./in?/hr.)

86

 

 

 

‘ V
‘ 3 -60
I e
Q
I 5
¢
I :
§
4?
it
s .
s I
: h
S I
5‘ h
8
~‘ I
g? .
N
s h
.'
¢
3 I
s h
e I
¢t .
e‘ I
‘0 s h
9°.“ I
d’¢ .
\ s
Q 5 l
s a? ~
4%?
485? 1' '40
‘s‘é ’
5 N
Ԥ
s‘é ’
Q
“o“ "
\“‘ $’
\
.\‘° 0"" s’
‘ (
§¢’ (“0&3 s"
\“‘ fl‘
\ I
0 1"
\‘ — I —
II-I-IF‘I-I-I-'
\
‘0‘ 130
\\
9“
s“
I I I I I I I I I I I I I I
2 8 30 32 34 36 38 40
Trunk Skin Temperature 0C
Figure 25. Comparison of Vapor pressure and cutaneous
trunk

evaporative water loss as a function of

skin tenpera ure.

Vapor Pressure (mm “9)

 

87

140-

K;
5’

5

en
?

Ileat Production (mmgflm
w
6

A
‘P

20'

 

 

T I I I W7 1' I

10 15 20 25 30 35 40 45

Ambient Temperature °C

Figure 26. Metabolic heat production as a function of
ambient temperature (circles indicate means and
vertical lines denote ranges of six observations).

2.

VI. CONCLUSIONS

The "normal limit" of rectal temperature in sheep is

relatively wide.

TCI and changes in skin surface temperatures indicate
that the ears, forelimbs and face (characterized by
freedom from fleece, high surface area to mass ratio,
and overlying areas of little fat or muscle tissue) are
capable of vasomotion and are able to modify sensible

heat exchange.

TCI and skin surface temperatures indicate that the trunk
and stifle areas (characterized by the presence of fleece,
low surface area to mass ratio and overlying subcutaneous
fat and muscle tissue) are probably incapable of alters
ing skin blood flow and consequently unable to modify

sensible heat exchange.

Sheep do not appear to be capable of reducing skin blood
flow to reduce heat gain when ambient temperatures are

greater than rectal temperature.

Respiratory rate increases logarithmically from critical

temperature II until ambient temperature equals rectal

88

89

temperature. These data suggest that changes in res-
piratory rate are valid indicators of thermoregulatory

mechanisms.

Panting indices suggest that at rates exceeding 240

breaths per minute, panting is a thermal liability.

Comparison of obligatory and observed cutaneous water
loss indicated the presence of active sweating on

sheep.

It is hypothesized that cutaneous and respiratory
evaporative water loss are both important avenues of
heat loss in shorn sheep during heat stress. It appears
that respiratory evaporative water loss is relatively
more important during mild heat stress (35° C.) but

that sweating is relatively more important during

severe heat stress (40 and 45° C.).

Increases of heat production during cold stress are due
to shivering. Increases during heat stress are due to
the respiratory activity of panting but more importantly
to the Q10 effect. The latter, which has received little
attention in the past, deserves careful consideration in

sheep because of the wide "normal" range of rectal

temperatures.

10.

11.

90

The 5Q C. intervals of ambient temperature used in this
study do not allow accurate determination of critical
temperatures. The data suggest that critical temperature
I is equal to or greater than 200 C. and that critical

temperature II is equal to or less than 30° C.

0n the basis of physiological responses indicative of
heat and cold stress, the lower limit of the thermos
neutral zone for sheep (as defined in this experiment)
is equal to or greater than 20° C. while the upper limit
is equal to or less than 30° C. The use of skin surface
temperature to estimate the thermoneutral zone is

questioned.

APPENDIX

 

 

92

APPENDIX I
FREQUENTLY USED SYMBOLS

TNZ Thermoneutral zone

CTI Critical temperature I

CTII Critical temperature II

TA Ambient temperature

Tr Rectal temperature

Te Ear skin temperature

Tf Face skin temperature

Tfl Forelimb skin temperature

Tt Trunk skin temperature

Tst Stifle skin temperature

TB Mean body temperature

T; Mean skin temperature

ATr Change in rectal temperature

RR Respiration rate

Rewl Respiratory evaporative water
loss

Cewl Cutaneous evaporative water
loss

BHC Whole body heat content

Hm Metabolic heat production

TCI Thermal circulatory index

CIVD Cold induced vasodilation

It Tissue insulation

R.H. Relative humidity

93
APPENDIX II
CORRELATION COEFFICIENTS

.1.

Tr
Te
Tfl
3st
RR
Hm
Rewl
Cewl
Rewl/RR
RR
Rewl
Rewl/RR

I».

\AH O\O\O\O\O\O\
NUXOOVi—‘C‘Uifl

0000000
0 o o o o o o
\O\O\0\O\O\O\O
PFO\UIO\\'IO\

y = a + bx

94

APPENDIX III

REGRESSION EQUATION*

.1.

Ts

TB

RR
Rewl/RR
It

Hm
Cewl
Rewl
Tt
Trl
Tst
Tr
Te
Tr
Rewl/RR
Rewl

Rer

.§.

0.439
0.110
4.590
41.70 x1o'5
-1.0 x 10‘“
-O.261
0.101
0.003
0.412
0.651
0.339
0.620
0.797
0.028
-8.1 x 10"LL
0.043

4.9 x 10"LL

.2.
21.71
35.66
56.45

2.95:;10‘”3
0.6 1:103
87.27
-1.23
0.01
22.80
12.93
25.19
14.85
7.78
39.15
0.03
«1.64
0.04

BIBLIOGRAPHY

Adams. T. 1960. Environmental factors influencing thermal
exchange. Tech. Rpt. No. 59-22. Ladd AFB, Alaska.

Adams, T. (in press) "Carnivore temperature regulation",
Chap. 12 in Physiology of Temperature Regulation. Ed. by
G. C. Whittow. Academic Press.

Alexander, G. 1961. Temperature regulation in the newborn
lamb. III. Effect of environmental temperature on
metabolic rate, body temperature and respiratory quotient.
Aust. J. Agr. Res. 1211152.

Alexander, G. 1962. Temperature regulation in the newborn
lamb. V. Summit metabolism. Aust. J. Agr. Res. 13:82.

Alexander, G. and A. H. Brook. 1960. Loss of heat by
evaporation in young lambs. Nature 185:770.

Alexander, G. and D. Williams. L962. Temperature regulation
in the newborn lamb. VI. Heat exchanges in lambs in a
hot environment. Aust. J. Aqr. Res. 13:122.

Allen, T. E. and J. Bligh. 1968. A comparison of the
patterns of sweat discharge in several mammalian species.
J. Physiol. 196336.

Armstrong, D. G.. K. L. Blaxter, J. L. Clapperton, N. M.
Graham, and F. w. Wainman. 1960. Heat production and
heat emission of two breeds of sheep. J. Agr. Sci.
55:395.

Bailey, C. B. 1964. Effect of environmental temperature on
feed digestion, water metabolism, body temperature, and
certain blood characteristics of sheep. Can. J. An. Sci.
44:68.

Blaxter, K. L. 1962. The Energy Metabolism of Ruminants.
Charles Thomas Publisher, Springfield, Ill.

Blaxter, K. L. 1964. The effect of outdoor climate in
Scotland on sheep and cattle. Hannah Dairy Res. Inst.
Reprint No. 545.

Blaxter, K. L. 1965. ed. Energy Metabolism, Academic Press.
London-New York.

95

96

Blaxter, K. L.. N. MC. Graham, F. w. Wainman and D. S. Arm-
strong. 1959. Environmental temperature, energy
metabolism and heat regulation in sheep. I. Partition
of heat losses in closely clipped sheep. J. Agr. Sci.

52:25.

Blaxter, K. L., N. MC. Graham, and F. w. Wainman. 1959.
Environmental temperature, energy metabolism and heat
regulation in sheep with fleeces. J. Agr. Sci. 52:41. A

Blaxter, K. L., J. L. Clapperton and F. w. Wainman. 1966.
The extent of differences between six British breeds of
sheep in their metabolism, feed intake and utilization,
and resistance to climatic stress. Brit. J. Nutr.
20:283.

 

Bleakley. W. R. anle. D. Findlay. 1955. The effect of
environmental temperature and humidity on the skin
temperature of Aryshire calves. J. Agr. Sci. 45:353.

Brockway, J. M., J. D. McDonald and J. D. Pullar. 1965.
Evaporative heat-loss mechanisms in sheep. J. Physiol.

17935540

Brody, S. 1945. Bioenergetics and Growth, Reinhold Pub.,
New York.

Brook, A. H. and B. F. Short. 1960. Sweating in sheep.
Aust. J. Agr. Res. 11:557.

Burton, A. C. and 0. G. Edholm. 1955. Man in a Cold En-
vironment. Edward Arnold Pub.. London.

 

Carter, H. B. and w. H. Clarke. 1957. The hair follicle
group and skin follicle population of some non-Merino
breeds. Aust. J. Agr. Res. 8:109.

Cragg, J. B. and L. Davies. 1947. Sweating in sheep.
Nature, 159:34.

Crawford, E. C. Jr. and K. Schmidt-Nielsen. 1965. Temperature
regulation in the ostrich. Fed. Proc. 24:347.

Dawes, G. S. and J. C. Mott. 1959. The increase in oxygen
consumption of the lamb after birth. J. Physiol.
14632950

Duerden, J. E. and M.I.F. Ritchie. 1924. Development of the
Merino wool fibre. S. Afr. J. Sci. 21:480.

97

Dukes, H. H. 1956. The Physiology of Domestic Animals.
7th ed. Comstock Pub., Ithaca, New York.

Euler, C. von. 1961. Physiology and pharmacology of
temperature regulation. Pharmacol. Rev. 13:361.

Eyal. E. 1963a. Shorn and unshorn Awassi sheep. I. Body
temperature. J. Agr. Sci. 60:159.

Eyal, E. 1963b. Shorn and unshorn Awassi sheep. II. Pulse
rate. J. Agr. Sci. 60:159

Eyal, E. 19630. Shorn and unshorn Awassi sheep. III.
Respiration rate. J. Agr. Sci. 60:174.

Eyal, E. 1963d. Shorn and unshorn Awassi sheep. IV. Skin
temperature and changes in temperature and humidity in
the fleece and its surface. J. Agr. Sci. 60:183.

Freney, M. R. 1940. Chemical investigations on the fleece
of sheep. Coun. Sci. Ind. Res. Aust. Bull. No. 130.

Graham N. M°., F. W. Wainman, K. L. Blaxter, D° G. Arm-
strong. 1959. Environmental temperature, energy
metabolism and heat regulation in sheep. I. Energy
metabolism in closely clipped sheep. J. Agr. Sci.
523130

Hales. J. R. S. and M. E. D. Webster. 1967. Respiratory
function during thermal tachypnoea in sheep. J.
Physiol. 190:241.

Hammel, H. T. 1968. Regulation of internal body tempera-
ture. Ann. Rev. Physiol. 30:691.

Hardy, J. D. 1961. Physiology of temperature regulation.
Physiol. Rev. 41:521.

Hardy, J. D. and E. F. DuBois. 1938. Basal metabolism,
radiation, convection and evaporation at temperatures
of 22 to 35° c. J. Nutr. 15:477.

Hemingway, A. and C. Hemingway. 1966. Respiration of
sheep at thermoneutral temperature. Respiration
Phys 101. 1:30.

 

98

Joyce, J. P. and K. L. Blaxter. 1964. The effect of air
movement, air temperature and infrared radiation on
the energy requirements of the sheep. Brit. J. Nutr.
18:5.

Kammlade, W. G. 1947. Sheep Science. Lippincott. Chicago,
Ill.

Kleiber, M. 1961. Fire of Life. John Wiley & Sons. New
York.

Knapp, B. J. and K. W. Robinson. 1954. The role of water
for heat dissipation by a Jersey cow and a Corriedale
ewe. Aust. J. Agr. Res. 5:569.

Lee, D. H. K. 1950. Studies of heat regulation in the
sheep, with special reference to the Merino. Aust. J.
Agr. Res. 1:200.

Lee, D. H. K. and K. W. Robinson. 1941. Reactions of the
sheep to hot atmospheres. Proc. Ray. Soc. Qld. 53:189.

Lennox. F. G. 1938. Fleece investigations. Coun. Sci.
Ind. Res. Aust. Pamph. No. 83.

Lewis, T. 1930. Observations upon the reactions of the
vessels of the human skin to cold. Heart 15:177.

MacFarlane. W. V. 1964. Terrestrial animals in dry heat:
ungulates. Handbook of Physiology, Sec. 4. Adaptation
to the Environment. pp. 509. Am. Physiol. 300..
Washington, D. C.

Mendal, V. E. and G. V. Raghaven. 1964. A study of diurnal
temperature patterns in sheep. J. Physiol. 174:206.

Miller, M. C. and L. Monge. 1946. Body temperature and
respiration rate, and their relation to adaptability
in Sheep. Jo An. 5010 531470

Mitchell, H. H. 1927. Livestock investigations. Ann. Rpt.
Ill. Ag. Expt. Sta. pp. 155.

Munro, J. 1962. The use of natural shelter by hill sheep.
An. Prod. 4:343.

Powell. T. L. 1963. The use of "additional" shelter by ewes
and their lambs. An. Prod. 5:222 (Abstr.).

99

Riek, R. F., M. H. Hardy, D.H.K. Lee, and H. B. Carter. 1950.
The effect of dietary plane upon the reactions of two
breeds of sheep during short exposures to hot environ—
ments. Aust. J. Agr. Res. 1:217.

Ryder, M. L. 1955. Studies on the nutrition of wool
follicles in sheep: the anatomy of the general blood
supply to the skin. J. Agr. Sci. 45:311.

Schafer, E. A. 1898. Textbook of PhysioloSY. Hodder and
Stoughton. London. pp. 838.

 

Schmidt—Nielsen, K., T. J. Dawson, H. T. Hammel, D. Hind,
and D. C. Jackson. 1965. The jack rabbit — a study
in its desert survival. Hvalradets Skrifter No. 48:
125.

Scholander, P. F., V. Walters, R. Hook, and L. Irving. 1950.
Body insulation of some artic and tropical mammals and
birds. Biol. Bull. Woods Hole. 99:225.

Slee. J. 1966. Variation in the responses of shorn sheep
to cold exposure. An. Prod. 8:425.

Slee, J. 1968. Body temperature regulation and vasomotor
responses in Scottish Blackface and Tasmanian Merino
sheep subjected to slow cooling. An. Prod. 10:265.

Slee, J. and R. Halliday. 1968. Some effects of cold ex-
posure, nutrition and experimental handling on serum
free fattymacid levels in sheep. An. Prod. 10:67.

Slee. J. and A. R. Sykes. 1967. Acclimatization of Scottish
Blackface sheep to cold. I. Rectal temperature responses.
An. Prod-9 9:3330

Slee, J. and A. R. Sykes. 1968. Acclimatization of Scottish
Blackface sheep to cold. II. Skin temperature. heart
rate, respiration rate, shivering intensity, and skin-
fold thickness. An. Prod. 10:17.

Ster, G. 1960. Central nervous regulation of body tempera~
ture. Handbook of Physiology, Sec. 1. Neurophysiology,
Vol. II. Amer. Physiol. Soc. pp. 1173.

Webster, A. J. F. 1966. The establishment of thermal
equilibrium in sheep exposed to cold environments.
Res. Vet. Sci. 7:454.

100

Webster, A. J. F. and K. L. Blaxter. 1966. The thermal
regulation of two breeds of sheep exposed to air
temgzratures below freezing point. Res. Vet. Sci.
734 c

Webster, A. J. F. and K. G. Johnson. 1964. Distribution of
temperature in the skin of sheep exposed to moderate
environments. Nature. 201:208.

Whittow, G. C. and J. D. Findlay. 1968. Oxygen cost of
thermal panting. Am. J. Physiol. Vol. 214294.

Wodzicka, M. 1959. A study of some environmental factors
affecting wool growth and heat tolerance of sheep.
Ph.D. Thesis, Johns Hopkins Univ.

 

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