Functional adaptations in the craniofacial morphology of Malagasy primates: shape variations associated with gummivory in the family Cheirogaleidae.
The infraorder Lemuriformes is characterized by a high level of homoplasy that clouds the evolutionary signal. The analysis of the morphological disparity of the Malagasy primates' crania and mandibles demonstrates the high determinism of functional specializations and developmental constraints, regardless of the phylogeny. In the present work, the weight of functional constraints linked to diet--a putative source of homoplasy--is analyzed first at the level of the infraorder Lemuriformes as a whole, and secondly at the level of a single family, the Cheirogaleidae, chosen because it contains taxa with two different diets (omnivory and gummivory). Malagasy primates are characterized by a great variety of dietary habits: some of them are omnivores, folivores, frugivores, gummivores, insectivores or even specialized hard-object feeders. All cheirogaleids feed on tree exudates, but while gum consumption is occasional or limited for Microcebus, Cheirogaleus and Mirza, gums are the dominant food source for Phaner and Allocebus. Craniofacial shape variations are analyzed using geometric morphometrics: methods based on landmark identification (Procrustes superimpositions) are chosen for cranium shape analysis, and methods based on outline decomposition (elliptical Fourier functions) for study of mandible shape. The morphospaces obtained at the level of the infraorder appear to be highly constrained by dietary habits, especially in the case of the mandibles. At the finer level of the family Cheirogaleidae, the analyses permit (1) separation of craniomandibular shape variation associated with the two dietary categories and (2) among the omnivorous category, to distinguish variation associated with the percentage of gum consumption.