Adaptive radiation of the ateline primates

Abstract

We propose a synthesis of anatomical, behavioral and ecological data in reconstructing the phylogeny and evolution of the ateline primates. The atelines are a monophyletic group divisible into an alouattin lineage, including Alouatta and the Miocene fossil Stirtonia, and an atelin lineage, including Lagothrix and its sister-group, the Ateles-Brachyteles clade. Body size increase was important in the origins of atelines and in their subsequent diversification. Larger body size perimitted an increased reliance on leaves (probably immature) by comparison with other platyrrhines, even in predominantly frugivorous forms. A novel locomotor pattern involving climbing, some forelimb climbing/hauling and hanging by the prehensile tail, which involved the evolution of many features derived for platyrrhines, was present in the first atelines, perhaps also connected with relatively large body size. The last common ancestor (LCA) of atelines, which morphologically resembled Lagothrix more than any other living form, probably lived in fluid multi-male polygynous groups characterized by female dispersal and had a moderate degree of sexual dimorphism. Alouattins became strongly committed to an energy minimizing adaptive strategy based upon a bulky, nutritionally poor diet composed mainly of leaves. Alouatta thus travels relatively little daily, uses a non-acrobatic style of deliberate quadrupedalism, defends resources inexpensively by long distance calling, lives in cohesive polygynous groups that influenced the development of high sexual dimorphism, and has a proportionately small brain size. These are derived characteristics which facilitate their exploitation of a wide variety of habitats, frequently in sympatry with atelins. Ancestral atelins were probably larger than the LCA. They engaged in more forelimb-dominated climbing and probably ranged widely in fluid, polygynous social groups. The least known modern form, Lagothrix, may occupy a hard-fruit feeding niche, in contrast to the soft-fruit specialist Ateles. A lithe, brachiating body plan that resembles hylobatids typifies Ateles and Brachyteles, suggesting a common ancestral foraging strategy involving rapid bursts of energy expenditure. The subsequent evolution of shearing postcanines in Brachyteles, which converges on Alouatta superficially, is interpretated as a derived adaptive compromise possibly related to the greater seasonality of the Atlantic Coastal Forest of southeastern Brazil, where the genus probably arose, in comparison to inland tropical forests. Energy saving features of Brachyteles include contracted home ranges and low intra-group aggression.