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The Evolution of Body Size in the Diverse Lesser Apes Open Access

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The highly specialized locomotor behaviors exhibited by extant hylobatids are generally considered to have evolved alongside a reduction in body size from the last common ancestor of all hominoids. However, among the four currently recognized hylobatid genera there is a greater variation in body size than usually recognized. Symphalangus is nearly twice the size of the three smaller genera, creating two discrete morphs of hylobatids. Furthermore, the large array of body mass estimates for putative stem hominoids, as well as a lack of early fossil hylobatids, and the still contentious phylogenetic relationships within this clade make the reconstruction of body size evolution in hylobatids problematic. Within the context of anthropoids, this study models the evolution of body mass in hylobatids and the hominoid last common ancestor. Using a large sample of extant primates, as well as six fossil catarrhines, ancestral body size was estimated under three evolutionary models: maximum-likelihood under constant-variance (cvREML) and multiple-variance Brownian motion (mvREML), and multiple-variance Brownian motion using reversible jump Markov chain Monte Carlo (mvMCMC). As phylogenetic relatedness of fossil taxa is unresolved, and body mass estimates are based on a limited number of specimens, this study tests the impact of their inclusion. The impact of the phylogenetic position of the small-bodied Miocene catarrhine Pliobates cataloniae is specifically tested here, by including it as a stem catarrhine, or alternatively as a stem hominoid. Model choice has a larger effect on ancestral body mass predictions here than the inclusion of fossil taxa, or the phylogenetic position of Pliobates. Predictions of ancestral body mass are generally consistent across the three methods, but both multiple-variance models significantly outperformed the constant-variance model, and mvMCMC model outperformed those based on maximum-likelihood. Both the use of multiple-variance models and the inclusion of fossil taxa constrain the impact of the extremely large-bodied great apes on the predictions for hominoid last common ancestor (LCA), Estimates from the mvMCMC model predict a ~19-27 kg hominoid LCA, and a ~8.3-8.8 kg LCA for hylobatids. This is just larger than in Nomascus, suggesting that the ~11kg Symphalangus is secondarily enlarged, while ~6.9 kg Hoolock and especially ~5.5 kg Hylobates have continued a trend of reduction since the hominoid last common ancestor. Interestingly, the geographic range of Symphalangus falls entirely within the range of Hylobates, suggesting that divergence in body size among hylobatids may be coincident with an ecological niche differentiation.

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