Distinct pathways for diversification of craniofacial morphology driven by size-related constraints in Madagascar primates

Malagasy lemurs are diverse primates that underwent adaptive radiation. While this radiation promotes modifications in cranium for niche partitioning, evolutionary constraints impose common craniofacial patterns among smaller lemurs: they are predicted to be constrained to have a shorter upper jaw and laterally-facing orbits to compensate for small masticatory muscles and accommodate relatively large orbits, regardless of niche. This study aims to elucidate how these evolutionary constraints operate during the adaptive radiation of lemurs. The results demonstrated that smaller species, such as dwarf/mouse lemurs (cheirogaleids) and sportive lemurs (lepilemurids), shared an overall cranial shape despite their diverse ecological niches, as predicted. However, under the constraints related to body size, these groups are adapted to their respective niches through modifications in specific parts of the cranium, such as the rostrum. These findings suggest that, even in the presence of constraints, morphological diversity can still be produced. In contrast, larger taxa, such as Megaladapis, exhibited more diverse overall cranial morphologies, reflecting fewer constraints, and suggested novel cranial functions through modifications of the entire skull. Thus, while evolutionary constraints may limit certain aspects of morphological evolution, they can also foster diversity by channeling distinct evolutionary trajectories based on body size in adaptive radiation.