The relationship between body size and feeding ecology is well established for primates. It is argued that the evolutionary history of modern New World monkeys and, in particular, the path to attainment of current body size is significant in understanding the similarities and differences between dietary strategies and other ecological parameters of similar-sized monkeys. Current interpretations of New World monkey evolutionary relationships are reviewed. Based on a synthesis of available body weights and the assumption that the earliest New World monkeys weighed close to 1 kg, similar to modern Aotus and Callicebus, predicted patterns of body size change in each lineage are given. Restrictions on directions of body size change in primates are discussed, and it is shown that "Stanley's Rule" offers a good explanation for differing body size ranges in New and Old World anthropoids. Predicted ecological correlates to body size drawn from the mammalian literature are offered and tested using data on New World monkeys, which show some concurrence and several interesting departures from predicted patterns. Sexual dimorphism in body weight of New World monkey species is reviewed, based on the new summary of body weight data given.
Neotropical primates show a remarkable range in body size, spanning two orders of magnitude from the tiny pygmy marmosets (100 g) to the woolly spider monkeys (11,000 + g). Even among the "smaller" platyrrhines, the range is large. In addition, these primates demonstrate a wide diversity in degrees and directions of sexual dimorphism, in both body size and canine size, from marked positive dimorphism (males larger than females), through monomorphic species, to negative dimorphism. Potential correlates or causes of the patterns of dimorphism in body size are investigated, including overall body size, natural selection for life history strategies, sexual selection, diet, habitat, and phylogenetic inertia. Focus is especially on those genera that show species-specific variation in dimorphism (e.g., Saguinus, Pithecia). Results are contrary to those for cross-primate or catarrhine studies, but complementary to recent studies on strepsirhines. They suggest that sexual selection is the primary determinant of degree and pattern of sexual dimorphism in platyrrhines, but that there is also a dietary effect. Natural selection may have some effect, although not the parameters analyzed here. Body size, habitat (primary vs. secondary forest preference), and phylogenetic inertia or constraints do not have any effect on the presence of sexual dimorphism in body weights in New World monkeys. o
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