Among mammals, brain size varies significantly even when body size is controlled. Although a number of hypotheses have been proposed to explain this variation, much of this research focuses on primates while relatively few studies have examined carnivore species. Carnivores provide an ideal group in which to examine factors that influence brain variation, as they experience a variety of social, environmental, and life history conditions. The present project examined some of these factors in combination with an analysis of brain volumes in selected species in the Felidae and Procyonidae families. Using computed tomography, the first study investigated the influence of social life history on intraspecific variation in brain size in African lions (Panthera leo), a gregarious species in which females are more social than males, in comparison to cougars (Puma concolor), a species where both males and females are solitary. The results indicated that, in African lions, females had a larger frontal cortical volume, a region involved in executive control of cognition and regulation of social behaviors, compared to males. In contrast, no sex difference in frontal cortical volume was found in cougars. These data demonstrate the importance of including sex as a variable in comparative analyses, and highlight the relationship between regional brain size (e.g. frontal cortex) and behavioral specializations (e.g. sociality). The second study examined the relative influence of social life history, forepaw dexterity, and arboreality on brain size and organization in selected species in the family Procyonidae. Procyonid species exhibit continuums of behaviors related to social and physical environmental complexities: the mostly solitary, semi-arboreal and highly dexterous raccoons (Procyon lotor); the exclusively arboreal kinkajous (Potos flavus), which live either alone or in small polyandrous family groups; and the social, terrestrial coatimundi (Nasua nasua, N. narica). Interspecific analysis of virtual endocasts revealed that the social coatimundi had the largest relative frontal cortical volume compared to the other two species. The arboreal kinkajou had the largest relative cerebellum and brainstem volumes compared to the other two species. The dexterous raccoon had the largest relative posterior cerebrum volume which includes the somatosensory cortex compared to the other two species. Additionally, intraspecific analysis revealed female coatimundis possessed a larger relative frontal cortical volume than males, while no sex differences were present within the other two species. Social life histories differ in male and female coatimundis but not in either kinkajous or raccoons. This analysis in the three procyonid species supports the comparative neurology principle that behavioral specializations correspond to an expansion of neural tissue devoted to that function. The last study sought to determine if increased brain size and regional brain variations reflect an increase in neuron number. Nissl stained brain sections from a dog, Canis familiaris, raccoon, P. lotor, and spotted hyena, Crocuta crocuta, were analyzed using cytoarchitectonic criteria and stereology. The regions of interests included: proreal frontal, orbital frontal, primary somatosensory, and primary visual cortices. Stereological analysis revealed that as brain size increases, neuron density decreases. Intraspecific comparisons revealed that regional neuron number and volume increased in brain regions that subserve behavioral specializations. Interspecific comparisons revealed that brain regions mediating specialized behaviors are not only larger in size but also contain more neurons. Overall, these findings in selected carnivore species support the idea that differences in sociality, environmental, and life history factors hypothesized to influence brain variations in primates, also correspond to regional and total brain variations in carnivores.
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