Comparison of captive and wild fascicularis-group macaques (Primates, Cercopithecidae) provides insight into cranial form changes in response to rapid environmental changes

Objectives Geographic variation is an important feature among primates, but the mechanisms underlying it are not well understood. Macaques are geographically widespread and have been translocated to captive populations, providing a prime opportunity to evaluate changes in cranial form in response to a novel environment. Clinal variation was assessed among wild fascicularis-group macaques and compared to two translocated populations to explore the relative contributions of developmental plasticity and microevolutionary forces in producing geographic variation in cranial form. Materials and Methods Forty-five 3D coordinates over the cranium were taken on 380 wild macaques and 56 captive Macaca mulatta from Cayo Santiago, PR and Beaverton, OR. Geographic shape, represented by the singular warp scores from a two-block partial least squares analysis, and centroid size were regressed against latitude, mean annual temperature, and mean annual precipitation among wild macaques. The two captive populations were compared to wild M. mulatta to assess potential changes in cranial form. Results Size and geographic shape were highly correlated with latitude in wild macaques, but neither ecological variable was important across the entire cline. Precipitation was important for shape only outside the tropics. Translocated M. mulatta are significantly larger, but not different in shape, compared to wild M. mulatta. Discussion These results are consistent with previous studies demonstrating cranial form is latitudinally variable among fascicularis-group macaques, but the potential drivers remain unclear. Size is highly plastic while geographic shape has evolved differentiation at interpopulation and interspecific levels. Variation in cranial morphology likely arises first in size, followed relatively rapidly by evolved modifications to shape.