Date Published: October 18, 2011
Publisher: Hindawi Publishing Corporation
Author(s): Karen J. Weinstein.
This study compares limb lengths and joint diameters in the skeletons of six macaque species (Macaca assamensis, M. fascicularis, M. fuscata, M. mulatta, M. nemestrina, and M. thibetana) from a broad range of habitats and climates in order to test whether ambient temperatures, latitude, and altitude influence interspecific variation in limb morphology in this widely dispersed genus. Analysis of variance, principal component analysis, and partial correlation analysis reveal that species from temperate latitudes and high elevations tend to have short limbs and large joint diameters for their sizes while species from tropical latitudes and low elevations tend to have long limbs and small joint diameters. Interspecific variations in intra- and interlimb length proportions also reflect phylogeny and subtle differences in locomotion. The results of this study suggest that climatic conditions are important factors among many ecological variables that influence limb morphology in this geographically widespread genus.
Ecogeographic patterns, such as Bergmann’s and Allen’s rules, have long been of interest to evolutionary biologists testing hypotheses about the evolution of geographic variation within and among closely related species. These ecogeographic rules, which state that body mass and appendage lengths vary by climate and latitude among geographically dispersed endothermic species in response to thermoregulation, have been invoked to explain variation in body size and proportions in many mammals, birds, and other vertebrate species [4–10]. Among primates, ecogeographic patterns have been used to explain variation in body size in lemurs  and baboons [12–14], cranial size, body size, and relative tail length in macaques [15–20], and body size and proportions in modern humans [21–27] and Pleistocene hominins [28–34].
Interspecific variation in overall size and limb lengths relative to body size and joint diameters suggest that Macaca limb morphology conforms, in part, with Bergmann’s and Allen’s rules. As latitude and altitude increase and average winter temperature decreases, body size and joint diameters tend to increase, and relative limb lengths tend to decrease in a pattern similar to that illustrated by Fooden [17, 19] for relative tail length variation in members of the fascicularis species group. Species from temperate latitudes and higher elevations, such as Tibetan and to some degree Japanese and rhesus macaques, tend to have short limbs and large joints for their size, while species from lowland tropical regions, such as crab-eating and pig-tailed macaques, tend to have long limbs. It is important to acknowledge that these climatic variables do not provide information about the adverse and additive effects of humidity and precipitation on the ability to conserve or release body heat. Thus, future work should also incorporate these climatic factors into studies of ecogeographic variation.
As a genus, Macaca offers important insights into the significance of ecogeographic variation in primate evolution. As a geographically widespread genus with many closely related species with recent genetic divergence from one another, species within this genus are unique among nonhuman primates for inhabiting regions outside the tropics that extend into temperate- and high-altitude regions. Results of this study indicate that ecogeographic factors are important sources of variation in postcranial morphology within this genus. Climatic conditions, including ambient temperature, altitude, and latitude, influence variation in limb lengths and proportions and overall body size that function to control thermoregulation. These same climatic conditions also affect seasonality in diet, foraging strategies, and reproductive ecology within and among Macaca species. Although not directly tested here, this study also suggests that subtle interspecific differences in locomotion and substrate use, which also are influenced by ecological conditions, are important sources of variation in inter- and intralimb length proportions within macaques. Thus, as this study of macaques demonstrates, ecogeographic conditions should be important factors considered in explanations of the recent evolution of postcranial morphological variation in closely related primate taxa.