A structural approach to cranial variation in the genus 'Alouatta' (Primates, Atelidae)

Howler monkeys (genus 'Alouatta') display a peculiar cranial architecture, characterized by a high degree of airorhynchy. This condition may be evolutionarily influenced by a largely folivory diet (with selective pressures on mandibular and splachnocranial dimensions), social structure (related to the development of vocal sacs), or by the interaction between these or other more subtle factors. In terms of functional craniology, airorhynchy may be related to structural changes associated with the morphology of the supraorbital anatomy, with the spatial relationships between face and cranial base, or with morphogenetic variations of the pteric area. However the evolutionary changes associated with the cranial organization in 'Alouatta' are still currently unknown. This study is aimed at investigating the variation and covariation patterns in howler monkeys, considering both interand intrageneric morphological differences. We also investigated the structural role of the pteric area within the cranial functional matrix, in order to test possible relationships between its variation and the degree of airorhynchy. To address these objectives we applied landmark-based analysis and multivariate statistics to a comparative dataset of atelid adult skulls. Our results suggest that the cranial architecture in howlers is influenced by an allometric vector, which associates higher degrees of airorhynchy with splachnocranial enlargement, basicranial lengthening and neurocranial flattening. On the other hand, the relationship between pterion and airorhynchy could not be confirmed. Either way some minor morphological differences were identified, suggesting that variations of the pteric area may be instead related with the relative development of the masticatory apparatus.