Craniofacial modularity and the evolution of cranial kinesis in the adaptive radiation of Furnariidae (Aves: Passeriformes)

Abstract

The role of phenotypic integration and modularity in the evolution of skull morphology in birds has been a subject of debate in recent years. Furnariids, a spectacular avian adaptive radiation, are distinguished in their cranial morphology as the only passerines with two types of cranial kinesis, constituting a great model to test whether the evolution of novelties in cranial kinesis was associated with shifts in patterns of evolutionary modularity and allometry in the avian skull. Our analyses by means of geometric morphometric tools and phylogenetic comparative methods show that the beak and neurocranium of furnariids evolved in a modular fashion and shaped by the cranial kinesis evolution. Besides, species with prokinesis show a higher degree of modularity and morphological disparity, lower phenotypic rates, as well as higher contribution of allometry in the evolution of the beak morphology than species with proximal rhynchokinesis, suggesting, as observed in several vertebrates, that the functional demands associated with higher degrees of cranial kinesis promote rapid integration throughout the skull. Prokinetic - robust morphotypes and proximal rhynchokinetic - gracile morphotypes, have repeatedly evolved by evolutionary convergence in both cranial modules, which suggests the existence of functional trade-offs and long standing adaptive optima related to cranial kinesis.