Absolute, not relative brain size correlates with sociality in ground squirrels

Matějů, Jan; Kratochvíl, Lukáš; Pavelková, Zuzana; Řičánková, Věra Pavelková; Vohralík, Vladimír; Němec, Pavel

doi:10.1098/rspb.2015.2725

Cited by 22 publications

(19 citation statements)

References 70 publications

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“…These findings add to the series of recent papers that have reported no link between relative brain size and sociality in mammals 5 , 6 , 8 , 9 , 80 (but see 10 ) and fish 4 , 90 . However, they are in stark contrast to previous studies in primates, cetaceans, carnivores and insectivores 62 – 65 , 91 that have found a positive relationship between C R and social group size.…”

Section: Discussionsupporting

confidence: 77%

“…In other words, C R is in fact indicative of absolute brain size, and that is what drives the correlation in primates. Interestingly, a recent test of the SBH in another rodent group (ground squirrels of the tribe Marmotini) 6 , revealed that there is no link between relative brain size and sociality, but that social species tend to be larger and hence have absolutely larger brains. This relationship between body mass and sociality (and, correspondingly, the neocortex ratio) is opposite in mole-rats, and thus contrary to the SBH.…”

Section: Discussionmentioning

confidence: 99%

“…An exception is a recent study reporting larger brain size in cetaceans living in mid-sized groups 10 . The hypothesis has only recently been tested in rodents for the first time and the results revealed that, in ground squirrels, sociality is not associated with larger relative brain size, but that social species tend to have larger bodies and correspondingly absolutely larger brains 6 , suggesting that a possible link between body size and sociality may be mediating the effect on brain size.…”

Section: Introductionmentioning

confidence: 99%

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Sociality does not drive the evolution of large brains in eusocial African mole-rats

Kverková

Bělíková

Olkowicz

et al. 2018

Sci Rep

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The social brain hypothesis (SBH) posits that the demands imposed on individuals by living in cohesive social groups exert a selection pressure favouring the evolution of large brains and complex cognitive abilities. Using volumetry and the isotropic fractionator to determine the size of and numbers of neurons in specific brain regions, here we test this hypothesis in African mole-rats (Bathyergidae). These subterranean rodents exhibit a broad spectrum of social complexity, ranging from strictly solitary through to eusocial cooperative breeders, but feature similar ecologies and life history traits. We found no positive association between sociality and neuroanatomical correlates of information-processing capacity. Solitary species are larger, tend to have greater absolute brain size and have more neurons in the forebrain than social species. The neocortex ratio and neuronal counts correlate negatively with social group size. These results are clearly inconsistent with the SBH and show that the challenges coupled with sociality in this group of rodents do not require brain enlargement or fundamental reorganization. These findings suggest that group living or pair bonding per se does not select strongly for brain enlargement unless coupled with Machiavellian interactions affecting individual fitness.

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Section: Discussionsupporting

confidence: 77%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Sociality does not drive the evolution of large brains in eusocial African mole-rats

Kverková

Bělíková

Olkowicz

et al. 2018

Sci Rep

Self Cite

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“…Finally, Matějů et al. () published a study on the correlation between brain/body masses and sociality using a total of 63 species of the tribe Marmotini (Xerinae). Although these quantitative data are extremely valuable, they were obtained on a limited number of species from different subfamilies.…”

Section: Introductionmentioning

confidence: 99%

Virtual endocast of the early Oligocene Cedromus wilsoni (Cedromurinae) and brain evolution in squirrels

2016

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Extant squirrels exhibit extensive variation in brain size and shape, but published endocranial data for living squirrels are limited, and no study has ever examined brain evolution in Sciuridae from the perspective of the fossil record to understand how this diversity emerged. We describe the first virtual endocast for a fossil sciurid, Cedromus wilsoni, which is known from a complete cranium from Wyoming (Orellan, Oligocene), and make comparisons to a diverse sample of virtual endocasts for living sciurids (N = 20). The virtual endocasts were obtained from high-resolution X-ray micro-computed tomography data. Comparisons were also made with endocasts of extinct ischyromyid rodents, the most primitive rodents known from an endocranial record, which provide the opportunity to study the neuroanatomical changes occurring near the base of Sciuridae. The encephalization quotient of C. wilsoni is higher than that of Ischyromys typus from the same epoch, and falls within the range of modern terrestrial squirrel variation, but below the range of extant scansorial, arboreal and gliding sciurids when using cheek-tooth area for the estimation of body mass. In a principal components analysis, the shape of the endocast of C. wilsoni is found to be intermediate between that of primitive fossil taxa and the modern sample. Cedromus wilsoni has a more expanded neocortical surface area, especially the caudal region of the cerebrum, compared with ischyromyid rodents. Furthermore, C. wilsoni had proportionally larger paraflocculi and a more complex cerebellar morphology compared with ischyromyid rodents. These neurological differences may be associated with improvements in vision, although it is worth noting that the size of the parts of the brain most directly involved with vision [the rostral (superior) colliculi and the primary visual cortex] cannot be directly assessed on endocasts. The changes observed could also relate to balance and limb coordination. Ultimately, the available evidence suggests that early squirrels were more agile and visually oriented animals compared with more primitive rodents, which may relate to the process of becoming arboreal. Extant sciurids have an even more expanded neocortical surface area, while exhibiting proportionally smaller paraflocculi, compared with C. wilsoni. This suggests that the neocortex may continue increasing in size in more recent sciurid rodents in relation to other factors than arboreality. Despite the fact that both Primates and Rodentia exhibit neocortical expansion through time, since the adoption of arboreality preceded major increases in the neocortex in Primates, those neurological changes may be related to different ecological factors, underlining the complexity of the inter-relationship between time and ecology in shaping the brain in even closely related clades.

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“…However, recent analysis with larger datasets, advanced statistical methods, and alternative hypothesis testing suggests that ecology, including diets of dispersed high quality food items, is a better predictor of brain size than sociality among primates (DeCasien et al 2017;González-Forero and Gardner 2018). There is lack of support for the social brain hypothesis in other diverse taxonomic groups such as marmots (Matějů et al 2016), carnivores (Benson-Amram et al 2016, and bats (Pitnick et al 2006). In contrast, it is well documented that toothed-whales (Odontoceti) have relatively large brain size and complex socio-ecological systems (Marino 1998;Boddy et al 2012;Montgomery et al 2013;Fox et al 2017).…”

mentioning

confidence: 99%

The allometry of brain size in mammals

Burger¹,

George

Leadbetter

et al. 2018

Preprint

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Why some animals have big brains and others do not has intrigued scholars for millennia. Yet, the taxonomic scope of brain size research is limited to a few mammal lineages. Here we present a brain size dataset compiled from the literature for 1552 species with representation from 28 extant taxonomic orders. The brain-body size allometry across all mammals is = −1.26 ( ) 2.34 . This relationship shows strong phylogenetic signal as expected due to shared evolutionary histories. Slopes using median species values for each order, family, and genus, to ensure evolutionary independence, approximate ~0.75 scaling. Why brain size scales to the ¾ power to body size across mammals is, to our knowledge, unknown. Slopes within taxonomic orders exhibiting smaller size ranges are often shallower than 0.75 and range from 0.24 to 0.81 with a median slope of 0.64. Published brain size data is lacking for the majority of extant mammals (>70% of species) with strong bias in representation from Primates, Carnivores, Perrisodactyla, and Australidelphian marsupials (orders Dasyuromorphia, Diprotodontia, Peramelemorphia). Several orders are particularly underrepresented. For example, brain size data are available for less than 20% of species in each of the following speciose lineages: Soricomorpha, Rodentia, Lagomorpha, Didelphimorphia, and Scandentia. Use of museum collections can decrease the current taxonomic bias in mammal brain size data and tests of hypothesis.

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Absolute, not relative brain size correlates with sociality in ground squirrels

Cited by 22 publications

References 70 publications

Sociality does not drive the evolution of large brains in eusocial African mole-rats

Sociality does not drive the evolution of large brains in eusocial African mole-rats

Virtual endocast of the early Oligocene Cedromus wilsoni (Cedromurinae) and brain evolution in squirrels

The allometry of brain size in mammals

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