Does brain size matter? | Mosaic
relationship between body weight and brain weight re- mains unclear. . weight and body composition measurements taken by the same investigators on the. age, weight, height and body mass index of Arak Medical Sciences students. Int. J. Morphol. brain size, and their relation with other measurements are used in. But brain size relative to body size doesn't seem to be a particularly realised that the relationship between brain and body size is not linear – smaller calculating the total number of neurons in different parts of their brains.
We collected brain sizes and MMRs from the literature and calculated residuals from the least-squares regression line describing the relationship between body mass and each variable of interest.
We then analyzed the correlation between residual brain size and residual MMR both before and after controlling for phylogeny using phylogenetic independent contrasts. We found a significant positive correlation between maximum metabolic rate and brain size across a wide range of taxa. Conclusions These results suggest a novel hypothesis that links brain size to the evolution of locomotor behaviors in a wide variety of mammalian species.
In the end, we suggest that some portion of brain size in nonhuman mammals may have evolved in conjunction with increases in exercise capacity rather than solely in response to selection related to cognitive abilities. Introduction A large amount of recent research has detailed associations between exercise, neurogenesis, cognition, and the size of certain brain structures in both humans and nonhuman animal models  — .
Aerobic exercise increases levels of compounds such as brain derived neurotrophic factor BDNFinsulin-like growth factor 1 IGF-1and vascular endothelial growth factor VEGF which appear to lead to exercise-induced neurogenesis in the rodent and human hippocampus .
Additionally, Chapell et al. While these studies point to clear links between neurobiology and exercise within individuals and within a species, to date, no study has examined the relationship between exercise and neurobiology interspecifically. Here, we test the hypothesis that exercise and neurobiology are related across a broad range of mammalian species.
Testing this hypothesis requires comparative measures of both neurobiology and exercise capacity. Among mammals, the most extensive neurobiological data available are overall brain sizes.
The size of the brain, relative to body mass, is often considered a determining factor in cognitive abilities  — behavioral flexibility and intelligence  — . Examining the association between exercise and total brain size is therefore ecologically and evolutionarily relevant. However, there are limitations to examining overall brain size, rather than the evolution of specific brain components . Increases in brain size are the result of changes in the size of different brain components, and examinations of total brain size do not take into account the fact that selection on brain components may differ substantially .
Despite this downside to the use of total brain size, it remains the only neurobiological variable for which we have a substantial mammalian database.
Additionally, overall brain size may be more appropriate in this study, since we are not concerned with correlations between specific cognitive functions and behavior, but with a more general correlation between physical activity and neurobiology. In addition to a measure of neurobiology, we require an index of athleticism and exercise frequency across a wide range of mammals.
MMR is the maximum level of oxygen consumed by an individual during exercise before anaerobic metabolism begins supplying energy . These findings suggest that increased social complexity in the ground squirrels goes hand in hand with larger body mass and brain size, which are tightly coupled to each other. Introduction The social brain hypothesis SBH [ 12 ] postulates that the cognitive demands imposed on individuals by living in complex social groups constitute a driving force for the evolution of large brains.
Individuals living in stable groups face information processing demands associated with dyadic and polyadic social interactions needed to negotiate conflicts over the communal use of resources. Because the SBH was developed to explain the extraordinary enlargement of brain size in primates, including humans, it is not surprising that the strongest empirical support for this hypothesis came from studies correlating relative and absolute primate brain size with various indexes of social complexity, including social group size, grooming clique size, number of females in a group, male mating strategies, deception rates and the frequency of coalitions for a review, see [ 3 ].
A correlation between relative brain size and the size of a social group probably appears also in cetaceans [ 4 ]. In carnivores, ungulates, bats and birds, by contrast, relative brain size is associated with bonded social organization i. Thus, primate sociality seems to be different from that of other vertebrates. It has been argued that bonded relationships have been generalized to all social partners in the anthropoid primates [ 39 ].
In line with this notion, it has recently been suggested that bonded sociality imposes a selective pressure favouring evolutionary encephalization i. Interestingly, to our knowledge, the SBH has never been tested in rodents, the most speciose mammalian order. Among rodents, the ground squirrels tribe Marmotini provide an ideal model for testing the SBH in an explicit phylogenetic framework. They feature very similar ecologies and share many life-history traits, e.
On the other hand, the ground squirrels display levels of sociality which range from solitary and single-family female kin groups to large groups including both males and females [ 1415 ]. The most complex social systems probably evolved through several steps, including the retention of daughters within the mother's range, relaxation of distinction between adjacent litters and superposition of male territories, maintained beyond the breeding season, over female ranges [ 15 ].
- Does brain size matter?
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- Absolute, not relative brain size correlates with sociality in ground squirrels
Solitary lifestyle is considered ancestral for ground squirrels [ 1516 ] and sociality evolved repeatedly among them, e. The repeated evolution of different levels of sociality allows statistical testing of the effect of sociality on brain size. In asocial ground squirrels hereafter referred to as social grade 1juveniles disperse shortly after weaning and social interactions, even between kin, are mainly agonistic [ 15 ].
Brain-to-body mass ratio - Wikipedia
Asocial ground squirrels do not display complex social relationships or social bonding e. Species living in single-family female kin groups grade 2 develop long-term social relationships among females, which cooperate in territorial defence and anti-predator vigilance [ 19 ].
Animals occupy individual burrows, but related females from neighbouring burrows closely interact and defend a common territory, which can be held over several generations [ 20 ]. In favourable habitat patches, female kin clusters form large colonies counting several dozen individuals [ 12 ]. Members of a colony interact and recognize each other by individually recognizable vocalization [ 21 ]. In more social species grades 3—5males are incorporated in family groups, i. Typically, a dominant male and several related females and their offspring form a cohesive group, the members of which are engaged in amicable contacts, the communal use and maintenance of burrows, and defence of a common territory from conspecifics from other groups [ 15 ].
The social complexity of the egalitarian polygynous harems grade 5 may be further augmented by the presence of an additional adult male s and non-reproductive helpers, and by a complex dominance hierarchy within family groups [ 1522 ]. In species where both sexes disperse and groups consist of related and unrelated individuals, social bonds develop early in the ontogeny and greatly influence dominance rank, dispersal decision and reproductive success in adults of both sexes e.
Some species are monogamous because males are not able to monopolize more than one female [ 2627 ]. Taken together, along a continuum of sociality social grades 1—5; for further details, see Material and Methods and electronic supplementary materialthere are trends towards increasing social complexity and bonded social organization [ 28 ], and therefore increasing relative brain size would be expected according to the SBH.
In this study, we utilized ground squirrels as a model to test the predictions of the social brain hypothesis by examining the relationship between brain size and social system complexity. Most species of ground squirrels exhibit male-biased sexual dimorphism in body mass [ 29 ] and their sociality is primarily based on social bonds among females [ 1415 ].
Because earlier reports have shown that social challenges may exert different selection pressures on males and females [ 30 — 33 ], we assessed sexual dimorphism in brain size and tested its potential association with sociality and performed all analyses for males and females separately.
We assumed that female brain size would be more tightly associated with social system complexity than male brain size. Material and methods The ground squirrels tribe Marmotini Pocock, ; sensu [ 34 ] form a monophyletic group within the family Sciuridae. They are widely distributed in the Northern Hemisphere, inhabiting grass-dominated biomes such as montane grasslands, temperate grasslands steppes, prairiessemi-deserts and tundras.
Endocranial volume was taken as a proxy for assessing brain volume. In total, the endocranial volume was measured in museum specimens of 63 species the mode 15 specimens per sex-species category from the tribe Marmotini.
The museums visited to collect this material are specified in the electronic supplementary material table S1 ; museum identification numbers of the examined skulls are available on request.
Brain-to-body mass ratio
Only adult dental wear pattern was used for skull ageingundamaged and located specimens were examined. Once the skull was filled, the lead shot was decanted and weighed, and the weight subsequently transformed to volume using a calibration line. All measurements were taken by a single person J. Mean body mass and the hind foot length were taken as proxies for body size. Body mass is a composite measure of body size integrating many aspects, and as such it is the most often used measure in comparative studies of brain size [ 36 ].
However, at the same time, body mass varies in many ground squirrels seasonally and it reflects the current individual body condition. Therefore, we primarily used species mean body mass taken from the literature for references, see electronic supplementary material, table S1.