Patterns and Constraints of Craniofacial Variation in Colobine Monkeys: Disentangling the Effects of Phylogeny, Allometry and Diet

Leaf-eating monkeys (colobines) are a highly diversified subfamily with 61 species in ten genera, in which patterns and constraints of morphological evolution are still poorly resolved. In the present study, we measured the skulls of 452 specimens collected from different museums worldwide. Using one of the most extensive samples ever employed, and geometric morphometric techniques, we aimed to elucidate the evolutionary processes that have led to the craniofacial diversification of colobines. Our comprehensive analyses of the colobine cranium demonstrated that phylogeny is the first order signal to emerge, with clear interspecific patterns of differentiation. Allometric trend constrains shape variation for most colobine taxa, but to a lesser degree than phylogeny. We also confirmed that diet is significantly associated with the variation in cranial shape among colobines. In particular, the mechanical advantage of the masseter for biting at the anterior dentition is linked to seed intake. We postulate that such ecomorphological patterns explain, in part, the non-phylogenetic and non-allometric variations in the colobine skull, and indicate the importance of diet in interspecific resource partitioning, allowing for species coexistence.