Multiple Determinants of Whole and Regional Brain Volume among Terrestrial Carnivorans

Abstract

Mammalian brain volumes vary considerably, even after controlling for body size. Although several hypotheses have been proposed to explain this variation, most research in mammals on the evolution of encephalization has focused on primates, leaving the generality of these explanations uncertain. Furthermore, much research still addresses only one hypothesis at a time, despite the demonstrated importance of considering multiple factors simultaneously. We used phylogenetic comparative methods to investigate simultaneously the importance of several factors previously hypothesized to be important in neural evolution among mammalian carnivores, including social complexity, forelimb use, home range size, diet, life history, phylogeny, and recent evolutionary changes in body size. We also tested hypotheses suggesting roles for these variables in determining the relative volume of four brain regions measured using computed tomography. Our data suggest that, in contrast to brain size in primates, carnivoran brain size may lag behind body size over evolutionary time. Moreover, carnivore species that primarily consume vertebrates have the largest brains. Although we found no support for a role of social complexity in overall encephalization, relative cerebrum volume correlated positively with sociality. Finally, our results support negative relationships among different brain regions after accounting for overall endocranial volume, suggesting that increased size of one brain regions is often accompanied by reduced size in other regions rather than overall brain expansion.

The evolution of intelligence in mammalian carnivores

“…The results from this zoo study, particularly when taken together with our earlier data on brain volumes [58], are remarkably like those obtained by MacLean et al [32] in a comparative study of problem solving by a wide array of birds and mammals on two tasks requiring self-control, another important executive function in the general domain. In both studies, the best performance was observed in the species with the largest brains (either mass-corrected or uncorrected brain volume), and social complexity failed to predict either success in problem solving or brain size in either primates or carnivores.…”

“…Diet also has important effects: carnivore species that primarily consume vertebrates have the largest brains, omnivores are intermediate and carnivores that specialize on insects have the smallest brains relative to their body size [58]. We found no support for a role of social complexity in overall encephalization, which is consistent with results from earlier carnivore studies [144].…”

“…The proportion of individuals within each species that succeeded at opening the box varied among families, with species in the bear family (Ursidae, 69.2% of trials), the raccoon family (Procyonidae, 53.8% of trials) and the weasel family (Mustelidae, 47% of trials) being most successful at opening the puzzle box, and those within the mongoose family (Herpestidae, 0%) being the least successful. Total brain volume corrected for body mass varied among the species tested, with canid and ursid species having the largest brains, and viverrid, hyaenid and herpestid species having the smallest brains [58]. Carnivore species with larger brain volumes relative to their overall body mass were significantly better than others at opening the puzzle box [33].…”

“…We videotaped all trials and extracted performance measures from videotapes using methods described elsewhere [33,149,150]. We then brought together data on success and performance measures during zoo trials with data documenting total brain size [144], the relative volumes of different brain regions and average group size for each species tested [58], and used phylogenetic generalized least-squares regressions [151,152] to identify the variables predicting success or failure in solving this non-social problem (detailed methods are available in [33]). …”

Extraordinary intelligence and the care of infants

“…However, the account complements several prior theories of human distinctiveness, notably theories based in social interaction (2,3,7,10,47), as well as reproduction and childrearing (48)(49)(50)(51). Altricial neonates require substantial parental attention.…”