Cranial integration and airorhynchy in Alouatta seniculus

Abstract

Howler monkeys (genus Alouatta) evolved a distinctive cranial morphology characterized by retroflexion of the facial skeleton on the neurocranial axis (airorhynchy), small and flat braincase, and a posteriorly oriented foramen magnum. This morphology can be due to distinct factors including folivory diet, specialized locomotion, and the presence of large vocal sacs. In this survey, we analyze cranial shape variation in red howler monkeys (Alouatta seniculus), to evaluate the degree and patterns of integration in face and braincase associated with airorhynchy. We investigate the relationships between facial and neurocranial morphology through Partial Least-Square (PLS) and modularity analyses, as to quantify the covariation between these two cranial districts. According to our results, airorhynchy is largely influenced by size, which is the main factor associated with adult cranial variability and sexual dimorphism. PLS approach suggests a noticeable integration between the neurocranial and splanchnocranial blocks. Modularity analysis (based on two-blocks partitions) supports this result, but its statistics is rather sensitive to operational choices associated with partitions parameters. Red howlers also display a peculiar bone pattern at the pteric area, resembling the scheme found in catarrhine primates. Our results suggest that this arrangement can be influenced by airorhynchy, although the vertical skull proportions are probably more important in this sense.