Brain Size and Social Complexity: A Computed Tomography Study in Hyaenidae

Abstract

The social brain hypothesis posits that the demands of living in complex social groups require increased neural processing, and that this underlies the expansion of brain areas involved in mediation of complex social behavior. However, much of the support for the social brain hypothesis is derived from comparative studies in primates. If large brains evolved as a result of selection pressures imposed by life within complex societies, as the social brain hypothesis predicts, then gregarious nonprimate species should possess large brains and exhibit comparable expansion of brain areas mediating social behavior. Our purpose here was to test a prediction of the social brain hypothesis – that increased brain size is related to social complexity – by examining species in the carnivore family Hyaenidae. Hyaenidae contains 4 extant species that span a spectrum of social complexity: the aardwolf (Proteles cristata) is solitary during the nonbreeding season, and forms monogamous pairs during the breeding season; the striped hyena (Hyaena hyaena) lives solitarily or in small groups; the brown hyena (Parahyaena brunnea) lives in groups of up to 14 individuals; and the spotted hyena (Crocuta crocuta) lives in complex hierarchically organized groups containing up to 90 animals. Computed tomography was used to create three-dimensional endocasts based on serial analysis of coronal sections of the adult endocranium. The largest brain volume, relative to body size, is found in the spotted hyena. We found no significant variation in relative brain volume among striped hyenas, brown hyenas, and aardwolves. The spotted hyena also possesses a larger anterior cerebrum volume relative to total brain volume than is found in the other hyena species; this region is composed primarily of frontal cortex. These data are consistent with the idea that expansion of the frontal cortex is driven by the demands of processing cognitive information associated with complex social lives, but other factors may drive the evolution of large brains in hyaenids.

Pride Diaries: Sex, Brain Size and Sociality in the African Lion (Panthera leo) and Cougar (Puma concolor)

“…Collectively, these findings suggest that the observed sex differences may be due to differences in frontal cortex and thus may be concomitant with differences in the social life histories of African lions. These findings lend support to previous findings that differences in social behaviors may correlate to dimorphisms in the amount of neural tissue devoted to the mediation of social behaviors [Arsznov et al, 2010;Sakai et al, 2011a].…”

“…Previously, this nondestructive imaging technique has been shown to be useful in creating virtual endocasts in species where preservation of the brain is difficult or impossible, but skull specimens are readily available [Arsznov et al, 2010;Sakai et al, 2011a;Sakai et al, 2011b]. The virtual endocasts show the external morphology of the African lion and cougar brain ( fig.…”

The Procyonid Social Club: Comparison of Brain Volumes in the Coatimundi <b><i>(Nasua nasua, N. narica),</i></b> Kinkajou <b><i>(Potos flavus),</i></b> and Raccoon <b><i>(Procyon lotor)</i></b>

“…Interestingly, Iwaniuk et al [1999] did not find a significant correlation between relative brain size and forelimb dexterity in his analysis of a broader range of fissiped carnivore species, including the Procyonidae family. Another intrafamily analysis examining the four extant species of the family Hyaenidae found a relationship between the relative amount of frontal cortex and sociality, where the most social species, the spotted hyena (Crocuta crocuta), had a greater relative amount of frontal cortex compared to less social hyaenids [Sakai et al , 2011a]. However, a recent broad taxonomic analysis of 36 carnivore species failed to find a similar relationship between frontal cortex volume and sociality [Swanson et al, 2012].…”

“…This social structure can contain upwards of 40 individuals and is comprised of linear dominance hierarchies [Gompper, 1995;Gompper and Decker, 1998;Hirsch, 2011]. The female bonded bands have been observed to contain subgroups that will forage independently and may be similar to the fission-fusion clans of spotted hyenas [Gompper, 1995], another highly social carnivore species noted for both a relatively large brain and frontal cortex [Sakai et al, 2011a]. In contrast to the gregarious coatimundi, kinkajous are typically solitary or found in small polyandrous family groups [Kays and Gittleman, 2001], while the raccoon is predominately solitary [Kaufmann, 1982].…”

The evolution of intelligence in mammalian carnivores

“…Because it is extremely difficult to find actual hyaena brains that have been properly preserved, we used CT analysis of skeletal material from adult members of the four extant hyaenid species collected in the wild (32 spotted hyaenas, eight brown hyaenas, 11 striped hyaenas and five aardwolves) [127,135] to generate virtual three-dimensional hyaena brains with which we could examine the relationship between frontal cortex volume and social complexity. We measured overall endocranial volume relative to the size of the skull from which each brain was scanned.…”