Morphology of Howler Monkeys: A Review and Quantitative Analyses

Recognition of a particularly derived eco-behavioral strategy for the genus Alouatta has been crucial for studying and understanding its equally derived cranial and postcranial morphology. The unique architecture of the skull and mandible has very likely evolved in relation to both masticatory correlates associated with an increasingly folivorous diet as well as the use of vocal communication associated with social behavior and an energy-minimizing strategy. Comparisons of cranial morphology using three-dimensional geometric morphometrics have highlighted significant interspecific shape differences. Alouatta seniculus is the most divergent in both cranial and hyoid morphology and exhibits the most pronounced levels of sexual dimorphism in those areas. Cranial variability is expressed in facial prognathism and airorhynchy, basicranial flexure, and zygomatic height. Inter- and intraspecific differences based on these axes of variation are very likely linked to interspecific variations in diet, behavior, and life history. This is further evident in the dental anatomy of the genus, indicating adaptations to a shift to a more folivorous diet. In addition, recent studies provide further evidence for significant inter- and intraspecific variations in hyoid size and shape. Alouatta seniculus possesses the largest and most inflated hyoid bulla, and the species that occupy distributional extremes (A. palliata, A. caraya) are differentiated by highly distinct hyoid shapes. These data indicate a complex relationship between morphology and behavior, with possible biogeographic implications. In terms of postcranial morphology, the forelimb bears a mixture of features that favor quadrupedalism with restricted abduction and overhead extension, providing stable contact and support along branches. In contrast, the hind limb appears to allow for a wider range of movement in all joints, with an emphasis on thigh extension and abduction, leg rotation, and powerful grasping with a habitually inverted foot. Interspecific variation reveals traits that can be ultimately related to subtle differences in the frequency of use of different positional modes, associated with variable eco-social factors. These results, deriving from different anatomical regions, provide evidence for understanding morphological variation across howler species in terms of morphofunctional adaptations, environmental pressure, and niche partitioning.