Abstract
Large relative brain size is a defining characteristic of the order Primates. Arguably, this can be attributed to selection for behavioral aptitudes linked to a larger brain size. In order for selection of a trait to occur, the trait must vary, that variation must be heritable, and enhance fitness. In this study, we use a quantitative genetic approach to investigate the production and maintenance of variation in endocranial volume in a population of free-ranging rhesus macaques. We measured the endocranial volume and body mass proxies of 542 rhesus macaques from Cayo Santiago. We investigated variation in endocranial volume within and between sexes. Using a genetic pedigree, we estimated heritability of absolute and relative endocranial volume, and selection gradients of both traits as well as estimated body mass in the sample. Within this population, both absolute and relative endocranial volume display variation and sexual dimorphism. Both absolute and relative endocranial volume are highly heritable, but we found no evidence of selection on absolute or relative endocranial volume. These findings suggest that endocranial volume is not undergoing selection, or that we did not detect it because selection is neither linear nor quadratic, or that we lacked sufficient sample sizes to detect it.
Highlights
Large relative brain size is a defining characteristic of the order Primates
We found a substantial amount of variation in absolute endocranial volume within the combined sample of males and females (mean = 96.13 ± 9.02 ml, coefficient of variance (CV) = 9.39), among females, and among males
The mean values of female and male absolute endocranial volume were significantly different (p = < 0.001), which can be partially explained by moderate body size dimorphism in rhesus macaques, and the correlation between brain size and body size across mammals (Table 1, Fig. 1a)
Summary
Large relative brain size is a defining characteristic of the order Primates. Arguably, this can be attributed to selection for behavioral aptitudes linked to a larger brain size. We estimated heritability of absolute and relative endocranial volume, and selection gradients of both traits as well as estimated body mass in the sample Within this population, both absolute and relative endocranial volume display variation and sexual dimorphism. Variation in relative brain size across mammals[1,2,3] is thought to be attributable to selection in response to different social and/or ecological conditions favoring adaptations for specific behavioral attributes that are linked to a larger brain[1,2,4,5,6,7]. A large relative brain size is a defining characteristic of primates[11], who display a variety of complex social and ecological behaviors. Population of rhesus macaques (Macaca mulatta) to improve our understanding of the evolution of brain size in a nonhuman primate species
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