n”- - — mums METABOLIC um AND m mamvs smmsecmcz 8N THE conow ML 955409024. Thus ‘09 Ha. Dunn of M. 5. MlCHiGAN STATE UNIVERSITY Jame: R. Bowers E969 ABSTRACT RESTING METABOLIC RATE AND ITS ADAPTIVE SIGNIFICANCE IN THE COTTON RAT; SIGMODON. by James R. Bowers The resting metabolic rates were measured and analyzed from five species of cotton rats, s. alleni, g, fulviventer, §, leucotis, g, ochro- gnathus and s, hispidus. In all, 103 specimens were studied by means of open-circuit indirect calorimetry modified from Depocas and Hart (I957) and Brown (1968). The measured resting metabolic rates were compared to determine the amount of individual variation (between individuals of the same species from a single locality), geographic variation (among samples of the same species from different localities), and interspecific varia- tion (between samples of different species). The resting metabolic rate of two laboratory populations, §, h, toltecus and s, ochrognathus, were statistically distinct from each other and all other laboratory populations. Those populations not statistically distinguishable, demonstrated a resting metabolic rate that was weight dependent in accordance with the work of Brody (I9h5) and Klelber (l96l). Variation from the expected resting metabolic rate, that is observed in the xerophylic species, g. ochrognathus, follows expectations when environmental influence is considered. Low metabolism seems to James R. Bowers give this species a selective advantage by allowing it to maintain a lower body temperature, a reduction in energy necessary to maintain basal level, greater water preservation and a greater reproductive potential. The hydrophylic subspecies, §, h, toltecus, due to excessive available energy in the form of food, reduced necessity for activity and proximity of water for intake and cooling is thought to demonstrate a high metabolism due to the lack of necessity for physiological control. It is indicated that genetic differences between p0pulations cause divergent standard energy production regardless of the immediate environmental conditions. Metabolism as an index, in some cases, seems to measure a local type adaptation, indicating that metabolism in the case of Sigmodon is an ecotypic phenomenon. RESTING METABOLIC RATE AND ITS ADAPTIVE SIGNIFICANCE IN THE COTTON RAT, SIGMODON. By James RI Bowers A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Zoology I969 ,. / 5/ .9- 25'. 6 i ACKNOWLEDGMENTS I would like to exptess my appreciation to members of the Michigan State University - Museum field parties of l96h, '65, '66, and '67, who, under the direction of Professor Rollin H. Baker and with the financial assistance of the National Science Foundation (GB-2227) and the M.S.U. Development Fund, collected all Mexican samples of Sigmodon. Other specimens used, were generously contributed through the kind cooperation of Ronald Gaertner (Arkansas), Robert Packard and Daniel Womochel (Texas) and Merle Kuns (Panama). I also acknowledge the valuable assistance given by Professor William Dawson who provided encouragement and guidance in various aspects of metabolic analysis. Thanks also go to Theodore Fleming for his assistance in obtaining habitat and climatological information concerning Panama and to my graduate student associates for their continuous help and advice. Thanks go also to Professor Wayne VanHuss and the Human Energy Research Laboratories for their assistance in obtaining the equipment necessary for this study. Special thanks go the the members of the guidance committee con- sisting of Professors R. H. Baker, H. H. Hagerman, and W. D. VanHuss, for their continual advice and encouragement throughout this project. I would also like to express special appreciation to my wife Delores, for assisting in any way she could throughout this study. V. VI. CONTENTS INTRODUCTION.. .............. .... .............. .... A. PurposeOOOOOOOOOOOOO ........... O ..... 00...... 8. Literature ReViCWOOO... ..... 0.0.0.0.... ..... EXPERIMENTAL...... ........... ................ ..... A. Animals.O...I.0.00......OOO...0...OOOOOOOOOOOOOOOOOOOOOO 8. Methods..... 000000000000 .00... ...... O ....... RBSULTS............. ..... ..................................... DISCUSS'ONOCOOOOCOOOOOOOCOOOOOOOOOOOOOOOOOOOOOOOOOOOOCOOOOOOOO A. Individual Variation................... ..... B. Intraspecific Metabolic Variation........... C. lnterspecific Geographic Variation...................... D. Factors Influencing Metabolic Rate.......... SUMMRYOOOOOOOO ...... 00.000.000.000...OOOOOOOOOOOOOO ....... 0.. LITERATURE CITED... ..... ..... ..... ..... ........... l2 l2 I8 26 29 29 29 3l 32 Al '+3 LIST OF TABLES Table Page I. Individual Heights of Population Samples ........... ........ l9 2. Resting Metabolic Rates of Population Samples...... ..... ... 27 LIST OF FIGURES Figure Page I. Geographic distribution of the two major groups of the cotton rat, Singdonooooooooooooooooooooo0000000000000oooooooo L} 2. Geographic distribution of species within the Sigmodon fUIViventer-groupoocoocoooooooo00.0000000000.00.00.00... ...... 6 3. Approximate geographical location of the studied popul- ations of the cotton rat, Sigmodon............................ l4 h-A Diagram of metabolic chamber illustrating air inlet T tube, vertical translucent plate, and air outlet T tube....... 22 4-8 Diagram of top of metabolic chamber illustrating position of periscope, transparent plexiglass, chamber insert gasket, and thermometer position.............................. 22 4-6 Side view of top of metabolic chamber illustrating chamber insert gasket, periSCOpe, light repelling flap, and centigrade thermometer........................................ 22 5. Diagram illustrating the flow of air through the oxygen monitoring systemt.......OOOOOOOOOOOOOOOOO......OOOOOOOOO ..... 23 6. Resting metabolic rates for the studied populations of Sigmodon...................................................... 28 7. Regression of metabolism (Oxygen consumption cc/g/hr) on body weight (grams) for all studied populations of Sigmodon... 28b 8. Regression of metabolism (Cal/day) on weight (grams).......... 3h 9. Comparison of mean oxygen consumption with altitude of trapping locality of populations of Sigmodon.................. 36 IO. Regression of metabolism on mean annual temperature........... 37 ll. Regression of metabolism on mean annual precipitation......... 40 I. INTRODUCTION A. Purpose Mammals respond in two ways in withstanding the stresses exerted upon them in their environments. The first response is behav- ioral, in which the animal may try to avoid the stress or perhaps seek a physical means to allay it. The second response to the stress is by concomitant changes in some or many functions of the animal's physiology. By choosing and studying one physiological function it should be poss- ible to evaluate its adaptive significance and how it affects the survival of a particular animal population in its environment. Also it should be possible to compare and contrast particular populations of mammals (within and between species) subjected to varying envir- onmental stresses, with respect to one physiological parameter. Although a great deal of recent information is available con- cerning the adaptation of homeotherms to their environment, the bulk of this information deals with animals that have developed highly special- ized mechanisms to allow them to survive in extreme environmental conditions. The less noticeable differences that occur between related populations occupying different habitats have been mostly unstudied. Such investigations have great potential not only in demonstrating how selective pressures in the environment act upon a species to produce an adapted phenotype but also in showing how the adaptations acquired by a species may allow them to live in diverse climatic zones. Metabolic differences observed between congeneric groups of mammals have been given two kinds of explanations. According to the first explanation, genetic differences between species in the same genus -2- will cause divergent standard energy production regardless of the immed- iate environmental conditions surrounding the experimental animals. The second explanation is that all quantitative differences in standard energy production are functions of the influence exprted on an indiv- idual and the population by the immediate environment. By measuring and comparing the metabolic rate (the integral portion of the standard energy production) of selected samples of cotton rats of the genus Sigmodon, this study will assist in demonstrating the validity of the initial hypothesis. By resting metabolic rate for the homeothermic cotton rat is meant its rate of metabolism within the region of thermoneutrality when the animal is quiet (following Brown, I968, and modified from Scholander _£'gl., I950). Cotton rats of the genus Sigmodon are excellent mammals to use in congeneric comparative experiments. The Sigmodon fulviventer-group has four species which are located in diverse habitat types in south- western North America, while the Sigmodon hispidus-group has a single wide-ranging species, which occurs in many of the habitat types also occupied by representatives of the S, fulviventer-group (see Figure I). This study compares representatives of five geographically-separated populations of S, hispidus and representatives of each of the four species belonging to the S, fulviventer-group with respect to resting metabolic rate (see Figure l and Figure 2). B. Literature Review Metabolism varies among species of small mammals as indicated by the work of Morrison (l9h8), Hatfield (I939), Cook and Hannon (l95h), Benedict and Lee (I936), Lee (I939, l9h0) and Benedict and Petrik (I930). Figure l. Geographic distribution of the two major groups of the cotton rat, Sigmodon. S, fulviventer-group ----- 'S. hispidus-group -------- 6/;7 / 4/ % ////, / .‘.. I” '4’ 0 'o 0'0 ' '0'0 '0' 0 “0:" ' ' ’0’9'9'” ‘30: 0 ° '0 o I . IVVSGOQO’E " . [5" "’€% \' 00 O . S 9.2.. .5’I9.0:O.’Q’.' ‘ .(Q‘I..' C .0 9.6.“ ‘e (I no :, / / . 0°;.0:'o'o 0‘ , o o o o 00/} o, ' ‘ '0' "0 ’o'o ‘,%:e.o:o:o° j 0.9.0 0 ,0 O. :'°o'o°o . / *9 // ;, f: “.1 W . /, O ’ I . o I o 0.. /.. 0"...” o ’ ’ o o o .. - 7' - \\\ ’f mtg? . ‘ . o l 7 I/ I .. I / //’/ ’1 / ’0 a 0 0 00/ / scum 260 405 «To 060 xoToo 1Ioo P400 of kilo-otora Figure 1. Figure 2. Geographic distribution of species within the Sigmodon fulviventer-group. - ______ a ‘S. fulvnventer ng ‘S. leucotis --------- ZZ? S. 'S. ochrognathus ----- .m 9335 -6- uuouo-onua 00¢," Sad GOOu 80 660 GO. 0°“ 0 DI L k D D D b b 50. .. q III... 0\ I 003" i‘ ' ..oo/\\l u t. -7- Pearson (I947, I9A8) noted that among animals of different species but of the same weight range there was wide variation in metabolic rate, and thus felt metabolism was not closely associated with taxonomic, ecological or anatomical factors. Cook and Hannon (I95h) found that metabolic variations within a Species could be identified when the species was segregated according to geographical subspecies. The rate of oxygen consumption is influenced by activity, temperature, nutrition, body size, season and time of day as well as previous oxygen experience and genetic background (Prosser and Brown, I962). Keeping other variables constant, Brody (ISAS) suggests the use of a power function of body weight, BMR (Cal/day)= 70.5 w0'7 (w = body weight in Kg), as a general reference for making inter- and intraSpecific comparisons of basal metabolic rate. It has been shown by Morrison (I9h8) that this relation pertains to rodents. Kleiber (I96I) also suggests the use of a power function of body weight, BMR (Cal/day) - 70 w0'75 (W = body weight in Kg), to make the same general reference. Variables inherant in the differential laboratory treatment of experimental animals may affect their metabolism. For instance, the ingestion of protein may elevate the expenditure of energy from the basal level of a mammal by lS-hO percent, while the specific dynamic action for fat is about l2 percent basal elevation and for carbohydrate it is about 5 percent (Hoar, I966). In some instances acclimatization to different altitudes has been thought to alter metabolism. In mammals gradual acclimatization at high altitude results in increased oxygen capacity of the blood due to increase in the number of circulating erythrocytes, increased myoglobin and an increased cardiac output (Murie, I96I; Mitchell and Edman, I95I). -8- However, either no metabolic changes or only a slightly reduced oxygen consumption has been found in tissues from animals acclimated to high altitudes (Ullrich, I956). The environmental temperature of an animal onld affect the animal directly, only by changing the body temperature in accordance with the environment, if it were outside the thermoneutral zone of the animal. A change in body temperature arising in this way would alter the metabolism of the animal. It would be expected that an animal from an area with high environmental mean temperature has a lower metabolic rate than an animal from an area with low environmental mean temperature (Sullivan and Mullin, I95h). Metabolic studies on small birds and mammals have produced conflicting results concerning temperature's effect on metabolism. When measured at temperatures below thermoneutrality, rats (Gelineo, l93h), hibernators during summer (Gelineo, I939 and Kayser, I939), and rabbits (Gelineo, I9h9) demonstrate that a decrease in acclimation temperature results in higher heat production at the same exposure temperature. However, differences in thermal history neither affected t al., I9SI) or oxygen consumption in rats (Adolph, I950 and Sellers or hamsters (Adolph, I95l) at the same low temperature nor affected that of the white-footed mouse at any temperature (Hart, I953). According to Hart (I953), the demonstration that differences in thermal history do not affect oxygen consumption at the same exposure temperature indicates that white-footed mice acclimate to changes in temperature only by changes in heat production. There is no evidence in the case of white-footed mice that there is an increase in overall body insul- ation with acclimation to decreased temperatures, comparable to that -9- found in larger mammals (Hart, I953). Scholander (I950), observing tropical mammals found many of them are well furred and are thus adapted to maintain body temperature by lowering the body-to-air gradient through insulation. He also found they either protect themselves against heat by sweating and panting, avoid the sun by seeking shade, remain inactive during the heat of the day or resort to water for cooling. Water availability in the environment appears to be an important determinant in the adaptation of an animal to its environment. If an animal functions optimally at a low resting metabolic rate; this low rate of metabolism may be considered to be adaptive, if the animal noimally inhabits an area of low water availability. In decreasing its metabolism an animal may decrease its insensible water loss by lowering the volume of air passing into its lungs and through its respiratory passages, thus decreasing the amount of water lost as vapor through evaporation (Murie, I96I). An even greater evaporation modif- ication may be found when considering the skin. While studying Peromyscus maniculatus sonoriensis, Chew (I955) found that excitement, which is accompanied by a rise in metabolic rate, increases evaporation from the skin. The physiological control for this phenomenon is not known. Physiological control of either or both of these areas of insensible water loss would give a selective advantage to animals occupying envir- onments of low vapor pressure and minimal water availability (Murie, I96I). Chew (I931) and Lindeborg (I955) have shown that some rodent species do differ in insensible water loss. Correlated with lower metabolic rate is the reduction in caloric requirements. The reduction in neces- sary energy requirements might be reflected in the size of an animal's -10- foraging range, the population density and the availability of food in different seasons (Murie, I96I). Adaptations such as those mentioned above may seem unimportant when considering the ability of some rodents to survive on below normal water intake (Lindeborg, I952; French, I956; Chew and Hinegardner, I957) or to achieve a constant water intake on changed diets (Williams, I959). However, survival tests give no direct evidence of reproductive ability and it is a reasonable assumption that those animals having Optimal food and water intake will be the animals with the greatest reproductive success (Murie, I96I). Traits such as those mentioned above, although subtle, may have tremendous selective consequence and must be considered for their effects in any study of metabolism. II. EXPERIMENTAL Ame: A total of l03 animals containing representatives of all five species included in the genus Sigmodon (see Baker, I969, in press), were used in this study. Wild parental stocks of these rodents in the M. S. U. Museum Live Animal Laboratory were obtained mostly by field trips to México under the direction of Rollin H. Baker with financial support from the National Science Foundation (GB-2227). Other animals were contributed through the kind cooperation of Ronald Gaertner (Arkansas), Robert Packard and Daniel Womochel (Texas) and Merle Kuns (Panama). When animals subjected to the metabolic tests were descendants of these wild- caught stocks, they are marked as such with asterisks in the following listing. Also, listed with the names of the species (and subspecies) studied is a number which corresponds to the approximate collecting locality on Figure 3, the locality of capture and the number of individ- uals used. As shown below in most cases ten animals from each p0pul- ation sample were used. Sigmodon hi5pidus The wide-ranging hispid cotton rat occurs from Nebraska south- ward to Perfi (see Figure l), and local populations are adapted to a variety of hot and cold, mesic and xeric, grassy and brushy, and latitudinal and altitudinal variable situations. Representatives of this species used are from five localities: Sigmodon hispidus texianus (Audubon and Bachman) I. Arkansas: Benton Co., 6 km. E Rogers 10* -12- -13- Figure 3. Approximate geographical location of the studied populations of the cotton rat, Sigmodo . For population and locality key refer to experimental animal section. -14- .m 98mg ’ " “’ OhOOOIOHdH oonm can" one" can one so. oea guano -15- Environmental characteristics: cool winters and hot summers, mesic, 360 30' N lat, 422 m elevation, luxuriant grass and brush. Sigmodon hispidus berlandieri Baird 2. Texas: Lubbock Co., Lubbock. 10* Environmental characteristics: cold winters and hot summers, moderately xeric, 330 “0' N lat, 988 m elevation, sparse grass and brush. Sigmodon hispidus toltecus (Saussure) 3. Veracruz: Isla del Toro l0* Environmental characteristics: hot, mesic, 220 30' N lat, 6 m elevation, luxuriant coastal grass. Sigmodon hispidus mascotensis J. A. Allen A. Jalisco: l0 km. sw Autlén 10* Environmental characteristics: warm, mesic, I90 30' N lat, l342 m elevation, luxuriant grass, brush and montane tropical forest. Sigmodon hispidus chirjguensis J. A. Allen 5. Canal Zone: Chiva Chiva Road 3 6. Canal Zone: France Field 7 Environmental characteristics: hot, mesic, 90 N lat, I00 m elevation, luxuriant grass and brush. Sigmodon alleni The brown cotton rat lives along the Pacific slope and coastal plain in southwestern Mexico from Sinaloa to Oaxaca. It ranges from sea level in tropical areas to as high in elevation as 3050 m on -I6- sub-boreal mountain sides (see Figure 2). Representatives of this species studied are from two localities (also see Baker, I969, in press): Sigmodon alleni vulcani V. Bailey 7. Michoacan: l0 km. W Capécuaro 540'2* Environmental characteristics: cool, mesic, 2lO N lat, 2059 m elevation, brush, some grass, forest. Sigmodon 211331 planifrons (Nelson and Goldman) 8. Oaxaca: l3 km. SSW Juchatengo 6¢ Environmental characteristics: cool mesic, l90 N lat, I92] m elevation, brush, some grass, mixed boreal-tropical forest. Sigmodonlfulviventer The buff-bellied cotton rat is an inhabitant of elevated grass- lands on the eastern slopes of the Sierra Madre Occidental from southern Arizona and New Mexico southward to Michoacén (see Figure 2). Represent- atives of this species are from three localities (also see Baker, I969, in press): Sigmodon fulviventer minimus J. A. Allen 9. Durango: ll km. NNE Boquilla l* Environmental characteristics: cool winters and warm summers, moderately xeric, 260 20' N lat, I952 m elevation, dry bunch grass, mixed cactus, acacia, mesquite, some juniper and oak. Sigmodon fulviventer fulviventer. J. A. Allen IO. Durango: Hda. Coyotes 9* Environmental characteristics: cold winters and cool summers, mesic, Zho 40' N lat, 2h75 m elevation, moist bunch grass in meadow surrounded by pine-oak forest. -17- Sigmodon fulviventer melanotis (V. Bailey) ll. Jalisco: 2 km. NW La Barca 2'? 8* Environmental characteristics: cool winters and warm summers, moderately mesic, 200 30' N lat, l525 m elevation, mixed grass and weeds in fallow fields. Sigmodon leucotis The white-eared cotton rat is an inhabitant of openings covered with mixed bunch grass and brush surrounded by pine-oak forest in the higher mountains of central and southern Mexico (see Figure 2). Rep- resentatives of this species were from one locality (also see Baker, I969, in press): Sigmodon leucotis V. Bailey l2. Durango: Hda. Coyotes h‘* 6"" Environmental characteristics: cold winters and warm summers, mesic, 2A0 AO' N lat, 2h75 m altitude, moist shrub with rocky substrate at edge of pine-oak forest. Sigmodon ochrognathus The yellow-nosed cotton rat is an inhabitant of the arid mountains of the extreme southwestern parts of Texas, New Mexico and Arizona southward into Coahuila, Chihuahua and Durango (see Figure 2). Repres- entatives of this species are from two localities (also see Baker, I969, in press): Sigmodon ochrognathus baileyi J. A. Allen l3. Durango: 3 km. NE Boquilla 8 l4. Durango: Il km. NE Boquilla 2 -]8- Environmental characteristics: cool winters and warm summers, moderately xeric, 260 20' N lat, I952 m elevation, sparse bunch grass, mixed cactus, acacia, mesquite, some juniper and oak on dry rocky hillsides. General»Comments The animals used for experimentation were kept in groups of 2-5 individuals, in large galvanized steel and hardware cloth cages (20.5” X l3.75” X l2”) and given a diet of Purina Mouse Breeder Chow and water (2g libidum). The ambient temperature and the relative humidity of the quarters in which the animals were confined were maintained at approximately 20-230 C and 30 percent, respectively. The wild-caught animals were allowed to acclimate to laboratory conditions for at least 90 days, while the laboratory-raised individuals were allowed to attain adult weight prior to study. Only healthy-appearing adult animals (males and non-pregnant females) were used for experiment- ation. Animals were judged to be adults by the following criteria: (a) If they weighed 6h grams or more (see Table I) and (b) when laboratory- reared, if they ranged in age from 90 to 200 days. The two sexes were used together in all calculations since no general sex-specific differ- ences in oxygen uptake were discerned. Similar observations for Peromyscus have been reported by Hayward (I965) and Musser and Shoemaker (I965). The experiments were performed during a period from January to December, I968. B-fls‘efleié. The resting metabolic rate of the cotton rats was estimated by Table I: Subspecies It!) 0 :I' O texianus berlandieri lm O l:- 0 la) 0 l:- O toltecus . mascotensis lm 0 I:- chiriquensis lm O l:- 0 [tn 0 lm . vulcani lm o o planifrons It!) 0 Pt 0 minimus fulviventer |m 0 Pa O melanotis |m O —h 0 [m . leucotis lm . ochrognathus -19- Location Arkansas: Texas: vi Veracruz: Jalisco: vic. Rogers c. Lubbock Isla del Toro vic. Autlén Canal Zone: vic. Chiva Chiva Road and France Field Michoacén: vic. Capécuaro Oaxaca: vic. Juchatengo Durango: Durango: Jalisco: Durango: Durango: vic. Boquilla vic. Coyotes vic. La Barca vic. Coyotes vic. Boquilla mean l9l.6 l90.9 87.8 ll6.2 lO9.8 l20.l l55.6 l6l.8 l66.4 l09.3 l28.6 ll5.l Individual Weights of Population Samples of Cotton Rats Weight (grams) range (l70.6-227.5) (152.9-221.9) ( 7o.1-103.5) ( 70.8-135.0) ( 6h.h-Ih3.8) ( 83.6-2lh.0) (l28.9-l76.6) (l6l.8) (133.2-l97-7) ( 76.1-187.5) ( 9h.8-ISS.5) ( 99.7-1u2.u) -20- measuring oxygen consumption, in an individual monitoring system in which the oxygen uptake was recorded at intervals of IS minutes. During each experiment a pre-weighed adult animal was kept in a black plexiglass chamber (see Figure A). Cotton was placed in the chamber to allow the animal to build a nest in an effort to quiet it. The chamber contained a centigrade thermometer for internal heat measurements and was fitted with a “periscope” to allow for observation of the animals at any time during measurement (see Figure h-C). In the back of the chamber was a 2” by l/h” vertical translucent plate which allowed light into the chamber, for observations, using the periscope. The chamber was fitted with an air inlet T-tube and an air outlet T-tube placed directly opposite and at 900 angles to each other, to assure maximum mixing of chamber air (see Figure h-A). Tygon tubing was used for all connections in the air line. Air, fed to the chamber, flowed from a low pressure pump at a rate of approximately 500 cc/minute through a tube containing a water absorbant (Drierite) then into the animal chamber (see Figure 5). The air leaving the chamber flowed first through a rotameter for measuring flow rate, then into a moisture- absorbing train containing more Drierite, before entering a Beckman Oxygen Analyzer (Model E2). Metabolic measurements were taken between the hours of 9:00 A. M. and 9:00 P. M., when the captive rats appeared to be the least active. Each animal was allowed to remain for one hour in the chamber, to become adjusted to this environment, prior to having its oxygen uptake recorded at IS minute intervals. Measurement of oxygen uptake was continued until two successive I5-minute-interval measurements demonstrated that minimal oxygen uptake was reached. Usually, two similar readings, with a l5- -2]- Figure A-A. Diagram of metabolic chamber illustrating air inlet T tube (A), vertical translucent plate (B), and air outlet T tube (C). Figure h-B. Diagram of top of metabolic chamber illustrating position of periscope (A), transparent plexiglass (B), ghamber insert gasket (C), and thermometer position (0). Figure h-C. Side view of top of metabolic chamber illustrating chamber insert gasket (A), periscope (B), flight repelling flap (C), and centigrade thermometer (D). -22- c3 —-------------b--- 8H A B ..lx Figure h-A 8 l/2” ~4' l-—'— r-----—---------b- | ‘ O l ------EIEE‘E Av- -'|-'---|' L---------- ------ T m _\N: .L Figure h-B Figure h-C -23- .Amv LoN>_mc< com>xo :mExoom «£0 pcm .on LouoEmHoL .on Lonsmzo o__onm»oE .Amv money mc_ IQLOmnm ou_cm_cc .Am mc_LoH_coE com>xo ecu smsoesu L_m mo 30_m ecu mc_umLom:___ Emcmm_c .m we:m_m -24- minute interval separating readings, were obtained for an individual animal within a two hour period, although for some highly “active” animals as long as five hours were required. From the minimal oxygen uptake and the rate of gas flow, oxygen consumption (volume of dry gas at standard temperature and pressure) was calculated using the applicable formula modified from Depocas and.Hart (I957). The weight of the cotton rats prior to the experimental run was used for calculation of all weight-specific, resting metabolic rates. Since cotton rats starved in advance for l2 hours became highly active in the metabolic chamber, it was decided to not deprive them of normal amounts of food prior to experimentation. However, the animals received no food during the period of measurement. Brown (I968) found that Neotoma maintained similar metabolic rates while being deprived of food for as long as seven hours, during which time the animals became postabsorptive. He suggested that specific dynamic action is not an important variable in metabolic studies such as his own. Assuming that the metabolism of other small rodents may be somewhat similar to that of Neotoma, starving the experimental cotton rats was not thought to be a necessity. The weights of the studied populations (see Table I) did not demonstrate variance homogeneity, thus were analyzed using the non- parametric Kruskal-Wallace one-way Analysis of Variance (Siegel, I956). Using this test it was determined that the weights of three populations (S, h, berlandieri, S, h, texianus, and S, h. toltecus) differed sign- ificantly from the other eight populations. Due to the above variation the three differing populations could not be included in an analysis of variance concerning metabolism. Since only one sample was measured -25- of S. fulviventer minimus from Boquilla, Durango, it is excluded from various statistical analyses. The Newman-Keuls method (Winer, I962), a modified q statistic, was used to evaluate the differences found between the eight populations that could be analyzed. The above mentioned eight experimental populations were used in determining linear relationships demonstrated through linear regression analysis (Li, l96h). The three populations that differed from the eight used in analysis were added to the linear plots after the relationships were determined. III. RESULTS The resting metabolic rates for the five population samples of .S. hispidus, the two of S, gfljfialL, the two of S, fulviventer, and one each of S, leucotis and S, ochrognathus, respectively, are listed in Table 2. and charted in Figure 6. Variation of resting metabolic rate within population samples is shown to be extensive in certain populations (see Figure 6). This is due to variation in weights of animals within each experimental population (see Table I), which was caused by limited choice of exper- imental animals. The relationship between body weight and metabolism ( O2 consumed cc/g/hr) Is demonstrated in Figure 7. The slope and intercept of the regression line were calculated using the mean metabolism (02 consumed cc/g/hr) of.S. ochrognathus, S, £11521 planifrons, S, 21135; vulcani, S, leucotis, S, h, chiriquensis, S. h. mascotensis, S. f. fulviventer and S, f, melanotis. -26- -27- Table 2. Resting Metabolic Rates of sample populations of Sigmodon Subspecies Location Average and Extreme oxygen consumption (cc/g/hr) S, h, texianus Arkansas: vic. ROgers l.3l ( .96-l.56) S, h, berlandieri Texas: vic. Lubbock l.33 (l.02-l.75) S. h. toltecus Veracruz: Isla del Toro 2.42 (2.Il-2.89) .§~.h- mascotensis Jalisco: Vic. Autlén I.S3 (l.23-l.57) .S. h. chiriquensis Canal Zone: vic. France l.68 (l.lO-2,35) Field and Chiva Chiva Road .§-.£- vulcani Michoacén: vic. Capécuaro l.62 (I.33-l.95) S, g, planifrons Oaxaca: vic. Juchatengo l.33 (l.l0-l.58) .§-.E- fulviventer Durango: vic. Coyotes l.47 (l.22-l.85) .§-.E° melanotis Jalisco: vic. La Barca l.54 (l.l4-l.8l) S, leucotis Durango: vic. Coyotes l.45 (l.38-l.54) .§- ochrognathus Durango: vic. Boquilla l.34 (I.l5-2.0I) S. ochrognathus -23- gr) .Axm mo.uhfis.. m_m>LoO:_ oocoo_wcoo ucmocoa mm moumo_vc_ Lon .mo_uco> ago new .cmoE co_um_:aoa any webmo_oc_ oc__ .muco~_co; any .omcmc maumo_nc_ m:__ .mo_ueo> ugh .covOEm_m mo mco_um_:aoa uo_v:um ecu to» mount o__oamuoE mc_pmom .o mczm_u Ierl ensus Irlqu texuanus fulviventer mascotenSIs toltecus f. melanoti h. berland leucotis h. ch h- h- h- a. i. S. a. planifrons vulcani § §_ g. [s s c: g. §. ,§ L""'"""1 l—————J .L i 4- A.;\m\uuv L rm._ .cho No 3 Metabolism (Log cc/g/hr) .24 L9 2.0 2.2 2.3 Body Weight (Log grams) Figure 7. Regression of metabolism (Oxygen consumption cc/g/hr) on body weight (grams) for all studied populations of Sigmodon. Key: 0 S, h. texianus 0S, 3. berlandieri AS. I_'I_. toltecus IIS. I1. mascotensis nS. h. chiriquensis -28 b- . g, vulcani g, planifrons ..fl. fulviventer .fi. melanotis leucotis . ochrognathus OIDOOF flnflnknflnkflkn IV. DISCUSSION A. Individual Variation Using as many as lO animals from each population for testing (see Methods), it was found that individual variation was often consid- erable (see Table II). Maximum range of variation in resting metabolic rates were found for the sample of S, hisEidus chiriquensis (l.25). Minimum range was found in‘S. leucotis (.l6). The remaining nine populations measured have ranges between .35 and .86. Variation in these values in the ubiquitious S, hispidus (from .35 to l.25) may reflect their greater tolerance to climatic conditions while the more ecologically-restricted S. leucotis (.l6), S, alleni (.48 and .62),‘S. ochrognathus (.86), and S, fulviventer (.57 and .63) show a lesser amount of variation. B. Intraspecific Metabolic Variation Sigmodon hispidus Population samples of the hispid cotton rat were purposely chosen to Include representatives from various habitats in which this ubiquitious species occurs. As would be expected (see Prosser and Brown, I962), population samples with a higher mean body weight, which were found in the more northern localities, have resting metabolic rates averaging lower than those of samples of the smaller-sized animals taken from the more southern and warmer climates (see Table 2. and Figure 6). The sample from Isla del Toro in Veracruz (S, h, toltecus) -29- -30- has the highest average metabolic rate. Assuming that this average of 2.42 cc/g/hr of oxygen consumed is normal for this population, I would guess that this high figure might be the result of a combination of influences: its small size, its insular isolation, and its mesic, tropical savanna-grass habitat. As shown in Figure 7, the resting metabolic rates of all of these populations, except for the Veracruz animals, are not significantly different at the 95 percent confidence level. Sigmodon alleni The two population samples came from somewhat similar, cool, montane areas, at the junction between the higher pine-oak forest and the lower humid tropical forest, although, as shown in Table I, the Oaxacan population sample (S,'g. planifrons) averaged 35 g per individual heavier than those animals in the Michoacén population sample (§3.2- vulcani). The heavier weight may account for the lower average resting metabolic rate determined for the former sample (see Table 2). Sigmodon fulviventer The resting metabolic rates of the population samples from the northern part (Durango) and the southern part (Jalisco) of the Mexican Plateau are essentially the same (see Table 2). Any variation observed for these population samples seemed to be in direct correlation with the mean weight of the animals studied. The major environmental dif- ferences between habitats of these populations are in altitude and temperature, the Durangan locality being substantially higher and colder. -3]- C. lnterspecific Geographic Variation Sigmodon fulviventer and Sigmodon ochrognathus ‘S..fi. minimus and S, ochrognathus occured in the same locality near Boquilla, Durango. S, j, minimus was found occupying the flat, grassy areas, while S, ochrognathus occupied the dry, rocky hillsides. The resting metabolic rate of S, ochrognathus, due to body size, would be expected to be higher than that of S, fulviventer of the same area (see Table l and Figure 7). However, the former's metabolic rate was statistically lower than that of the one specimen of S. fulviventer measured. A low resting metabolic rate may give S, ochrognathus of this xeric area a selective advantage by allowing it to maintain a lower body temperature, a reduction in the quantity of energy necessary to maintain basal level, greater water preservation and a greater repro- ductive potential. Sigmodon fulviventer and Sigmodon leucotis 'S. j, fulviventer and S, leucotis occurred in the same locality near Coyotes, Durango. However, S. i. fulviventer occupied the moist bunch grasses and meadows of the area while S, leucotis was found in the moist shruby and rocky areas. When statistically analyzed with 95 percent confidence, these two species were found not to vary in oxygen consumption (see Figures 6 and 7) indicating an ecotypic adaptation occurs in these cotton rats. Sigmodon hispidus and Sigmodon alleni The S. hispidus population taken from near Autlén, Jalisco and -32- the two‘S. alleni populations from Michoacén and Oaxaca, respectively, occupied somewhat similar habitats. As would be expected, their rest- ing metabolic rates do not differ significantly (see Figure 7). 0. Factors Influencing Metabolic Rate Disease or Adverse Endocrine Condition According to Newburgh (I945) and Gaertner (I963) any abnormal condition such as disease or adverse endocrine control will have an effect on the oxidative rate. An adverse endocrine condition, although not superficially noticeable, might be directly demonstrated in the fail- ure of a population to reproduce. Due to the above considerations ten individuals measured from a S, S, planifrons population, which was slowly dying out in the laboratory, were eliminated from inter- and intra- specific comparisons. In their place the measured values for six S, g, planifrons, made using a modification of closed circuit indirect calor- imetry, taken by Ostrander (unpublished, I966) were used. The other wild-caught or laboratory-reared animals used in this study appeared normal outwardly, from the standpoint of health. Weight Linear regression analysis was used to determine the linear relationships between S. leucotis, S, ochrognathus, S,li. fulviventer, S. i. melanotis, S. g. vulcani, S. h. mascotensis and S. h. chiriquensis. Confidence limits at the 95 percent level were fitted and plotted for this line (see Figure 7). Plotting the other five studied populations on this regression graph illustrates that S, h, texianus, S. g. planifrons -33- and S, h, berlandieri readily fit within the confidence intervals. Because these latter three fit within the confidence intervals they can be considered, with 95 percent confidence, to demonstrate similar correlation of metabolism with body weight. Thus, Figure 7, a linear regression of metabolism on body weight has been based on the nine pertinent populations. Using an analysis such as the above has compensated for one of the two variables present in this study, that of weight variation. Any further variation observed in metabolic values must be due to variation in genetic character of the studied subspecies (see Figure 6). Brody (I945) and Kleiber (l96l) suggested the use of a power function of body weight as a general reference for making inter- and intraspecific comparisons of basal metabolic rate (see Figure 8). Morrison (I948) demonstrated that the relationship suggested by Brody and Kleiber applies to small rodents as well as larger mammals. The values expected for Brody's or Kleiber's approximation (see Figure 8) are for basal metabolic rates and would be expected to be lower than resting metabolic values, since the non-postabsorptive condition of Sigmodon while being measured may raise the expenditure of energy from the basal level by 5-40 percent (Hoar, I966), due to specific dynamic action. As is demonstrated by Figure 8, although the energy expenditure is elevated by the non-postabsorptive state, the slope of the regression of energy production on body weight would remain constant. It has been demonstrated that variations from the supposed normal metabolic values of Sigmodon, which are found in comparing metab- olism and weight must be due to factors other than immediate environ- mental influence. Also, the resting metabolic rates recorded for the l.5‘- l.4- Metabolism (Log Cal/day) l.3‘fi l.IH -34- I.0 -..- r "‘1 I I 2.0 2.l 2.2 2.3 Weight (Log grams) Figure 8. Regression of metabolism (Cal/day) on weight (grams). (Klelbel', 196]) ci- oooooooooooooo o ooooooooooo 0-0 (Brody, I945) ------------------------------- Sigmodon experimental animals, although elevated above predicted values, would be expected due to specific dynamic action. It remains to attempt to determine those factors present in the previous environment of the animals that might influence their metabolic rate. Altitude Altitude has apparently exercised no effect on the resting metabolic rate of Sigmodon (see specimen listing and Figure 9). This may be the result of the distribution of Sigmodon in nature. Small mammals such as the cotton rat could not withstand the thermal stress to which they would have to submit, if passing into an area having an oxygen tension differing from their own enough to affect them. It has been reported that in mammals, gradual acclimatization at high altitude results in many physiological changes which are beneficial to oxygen utilization, but either no metabolic changes or only a slightly reduced oxygen consumption have been found (Murie, l96l; Mitchell and Edman, l95l; Ullrich, I956). Environmental Temperature Environmental temperature apparently has directly exercised no effect on the metabolic rate of Sigmodon.(see Figure ID). The environ- mental temperature of Sigmodon would affect the animal directly, only by changing the body temperature in accordance with the environment, if the temperature were outside the thermoneutral zone of the genus. According to Sullivan and Mullin (I954) one would expect that an animal from an area with high environmental mean temperature has a lower -35- mEOcm—oE .H .m‘ Emu—.3 .M .m‘ maoofiou ...“ ..w4 mafmcaecoo ..muO 33633: .H .WO 22633.55 .m .WD Cozucmton .m .m 0 $0026. .wl mcoctcm—a ..M .w 0 flmcouoommelm .wfl macmmxmu .m. .m. .zo_on >ox c_ vo_m_ucou_ ocm m_onE>m .covOEm_m mo mco_um_:aoa mo >u__moo_ mc_aamcu mo ov:u_u_m ;u_3 co_qu:mcoo com>xo cmoE mo com_LmaEOU .m oe:m_u Acz\m\oov co_uaE:mcoo com>xo emu: JPN L ~.N . o.N r w._ 0.... ... 3.. _ N._ . o— ! CON 0. 1.0o: .. 25 r com 0 [.000— “ Elevation (m) 1. 88 r 83 -37- #55333» .H .we flmcouoomms .m ..wB mscumcmoezoo .mo mcocuficm—q ..m. .mo 963.3 ....I. ..w 4 3083....— ..WI 2.3:; .M .m 4 76:57.3 .m ..m. O 23:20:. .H .m‘ Emcoagtco .m .nm. 0 3:2on .d .w. 9 .3o_on >ox ou Emmet m_one>m mo co_umo_m_ucov_ Lou .mesumtoaeou .maccm came :0 Em__oamuue mo co_mmucmom .o_ otam_u Acmo>\ocmcm_ucoo moccmon moqv beaumanEou _m:ccm emu: m7. .3. m. _ N. _ ... _ o. _ O o u a IN. 4 D . Acs\m\oo .mcou com>xo mOAV Em__onmyoz I m. -33- metabolic rate than an animal from an area with low environmental mean temperature. However, Hart (I953) studying the white-footed mouse found that differences in thermal history do not affect oxygen consump- tion at the same exposure temperature. The results found for Sigmodon tend to corroborate the results of Hart (I953) when statistically considered (see Figure IO). However, it must be pointed out that to ascertain the true relationship that exists between Sigmodon and its environmental temperature requires a study of the body-to-air gradient, behavior and insensible water loss. Precipitation The thermoregulatory resources of Sigmodon are expected to be limited in any environmental temperature range above thermoneutrality. Any use of body water by a rodent would be a detriment if the animal were in an environment with little water available. If ah- animal functions optimally at a low resting metabolic rate, this low rate of metabolism may be considered to be adaptive, if the animal normally inhabits an area of low water availability. According to Murie (l96l), the decrease of metabolism found in some mammals may cause decrease in insensible water loss through the respiratory passages. An even greater modification of water loss was found by Chew (I955); he noted that excitement, which is accompanied by a rise in metabolic rate, increases evaporation from the skin. Physiological control of either or both of the above areas of insensible water loss would give a selective advantage to animals occupying environments of low vapor pressure and minimal water availability (Murie, I96I). The availability of environmental water and its physiological -39.. affect on Sigmodon are demonstrated in this study (see Figure II). The most xerophytic species of the genus, S, ochrognathus, shows a rest- ing metabolic rate that is lower than the population expectation for that Species by l2.9 percent (see Figure 6). Although further study is needed to demonstrate its validity, perhaps this reduction in metab— olism is a selective advantage. The advantages of low metabolism attributable in this instance may be lower body temperature, reduction in quantity of energy necessary to maintain basal level, water preser- vation and greater species reproductive potential. On the other hand, S, h, toltecus, the most hydrophylic population studied, demonstrates a resting metabolic rate 42 percent aboVe that expected for this sub- species. Perhaps excessive available energy in the form of food, reduced necessity for activity, and the proximity of much water for both bodily intake and cooling allow the excessive metabolism demonstrated for this subspecies of S. hispidus. Perhaps no physiological determinants of metabolism are active here; rather, the metabolic rate for this sub- species may be the function of the lack of necessity for physiological control. ' It is clearly indicated that genetic differences between species will cause divergent standard energy production regardless of the immediate environmental conditions. The genetic differences between populations not necessarily being limited only to those factors that control metabolic balance, but also those concerning all behavioral and physiological processes. Metabolism as an index, seems simply to measure a local type of adaptation. It is an ecotypic parameter, not a phylo- genetic phenomenon. -90- m.m m3£umcmocsoo m_uoo:o_ m_u0cm‘oE Louco>_>_:$ mcotm_cm_n _cmu_:>. .M ...u .m am mlmlmlmlmlml «Q .cl:|.c|.c|.c m_mco:mwe_co m_mcouoomme . m300u_0u _Lu_vcm_con . mzcmwxou . .zo_mn >ox mlmltglmlml $04.0 Cu Emmet m_one>m mo co_pmo_m_ucou_ Lou .co_pmu_a_ooca .msccm came :0 Em__onmuos wo cohmmocmom .__ ocsm_u Acmo>\EE moav co_umu_a_ooLa .maccm emu: ~.m ..m o.m m.~ m.~ m.~ m.~ r b - If, (lb 4 . _. O o o Metabolism (Log Oxygen Consumption cc/g/hr) -m- V. SUMMARY Resting metabolic rates were measured for five species of lab- oratory acclimated cotton rats, Sigmodon, from the United States, México and Panama. The measurements were made by open-circuit indirect calorimetry modified from Brown, (I968). The resting metabolic rates of the studied populations were analyzed to determine variation within population samples, between geographically-separated population samples of the same species, and between species. It was found that individual variation was often considerable. The maximum range of variation in resting metabolic rate was found in the ubiquitious species, S, hispidus, possibly reflecting this species greater tolerance for a wide range of climatic conditions. The more ecologically restricted S. leucotis, S. ochrognathus,_S, 311331 and S, fulviventer demonstrated less variation. The resting metabolic rate of two laboratory p0pulations, S, h, toltecus and S, ochrognathus were statistically distinct from each other and all other experimental populations. The populations that were not statistically distinguishable from the other populations, demonstrated a metabolic rate that was for the most part weight dependent in accord- ance with the work of Brody (I945) and Kleiber (l96l). Variation from the expected resting metabolic rate observed in .§- ochrognathus follows directly its expectations when the influence exerted upon this species by its environment is considered. A low metabolic rate may give this species a selective advantage in its xeric environment by allowing it to have a lower body temperature, a reduction -42- in the quantity of energy necessary to maintain basal level, greater water preservation and a greater reproductive potential. 1S..h. toltecus may vary from the expected resting metabolic rate due to the lack of necessity for physiological controls. Excessive available energy in the form of food, reduced necessity for activity and proximity of water for intake and cooling, may allow high metabolic rates such as those observed for this population. This study indicates that genetic differences between populations will cause divergent standard energy production regardless of the immed- iate environmental conditions. Also, metabolism as an index, in some cases, seems simply to measure a local type adaptation, indicating that metabolism, in the case of Sigmodon, is an ecotypic phenomenon. -43- VI. LITERATURE CITED Adolph, E. F. I950. Oxygen consumptions of hypothermic rats and acclim- atization to cold. Am. J. Physiol. l6lz359'373. Adolph, E. F. l95l. Acclimatization to cold air; hypothermia and heat production in the golden hamster. Am. J. Physiol. l66:62-74. Baker, R. H. I969. Cotton rats of the Sigmodon fulviventer-group (Rodentia: Muridae). Univ. Kansas Publ., Mus. Nat. Hist., Misc. Publ. (in press). Benedict, F. G. and R. C. 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S. You l95l. Acclimatization to cold in rats: Metabolic rates. Am. J. Physiol. l67:65I-655. Sdholander, P. F., R. Hock, V. Walters, and L. Irving I950. Adaptation to cold in arctic and tropical mammals and birds in relation to body temperature, insulation and basal metabolic rate. Biol. Bull. 99:259-27l. Scholander, P. R., R. Hock, V. Walters, F. Johnson, And L. Irving I950. Heat regulation in some arctic and tropical mammals and birds. Biol. Bull. 99:237-258. Siegel, Sidney I956. Nonparametric Statistics. New York. McGraw-Hill Book Co. Inc. Pp. xvii‘+ 3l2, illus. Sullivan, 8. J. and J. T. Mullen I954. Effects of environmental temperature on oxygen cons- umption in arctic and temperate-zone mammals. Physiol. Zool., 27:2l-28. Ullrich, W. C., gi‘gl I956. Metabolism of rats at high altitudes. J. Appl. Physiol. 9:49-52. Williams, Olwen. . I959. Water intake in the deer mouse. Jour. Mammal. 40: 602-606. Winer, B. J. I962. Statistical Principles in Experimental Design. New York. McGraw-HIII Book Co., Inc. Pp. x +-672, illus. Ifll-iri'iib‘al.‘ S‘AIE UNIVERSITY LIBRLP'IES \l lllllgllgglljlll l 293 089 ‘Ah‘l .AL A. 1"