Ecological correlates of cranial evolution in the megaradiation of dipsadine snakes

Background Dipsadine snakes represent one of the most spectacular
vertebrate radiations that have occurred in any continental setting, with
over 800 species in South and Central America. Their species richness is
paralleled by stunning ecological diversity, ranging from arboreal
snail-eating and aquatic eel-eating specialists to terrestrial
generalists. Despite the ecological importance of this clade, little is
known about the extent to which ecological specialization shapes broader
patterns of phenotypic diversity within the group. Here, we test how
habitat use and diet have influenced morphological diversification in
skull shape across 160 dipsadine species using micro-CT and 3-D geometric
morphometrics, and we use a phylogenetic comparative approach to test the
contributions of habitat use and diet composition to variation in skull
shape among species. Results We demonstrate that while both habitat use
and diet are significant predictors of shape in many regions of the skull,
habitat use significantly predicts shape in a greater number of skull
regions when compared to diet. We also find that across ecological
groupings, fossorial and aquatic behaviors result in the strongest
deviations in morphospace for several skull regions. We use simulations to
address the robustness of our results and describe statistical anomalies
that can arise from the application of phylogenetic generalized least
squares to complex shape data. Conclusions Both habitat and dietary
ecology are significantly correlated with skull shape in dipsadines; the
strongest relationships involved skull shape in snakes with aquatic and
fossorial lifestyles. This association between skull morphology and
multiple ecological axes is consistent with a classic model of adaptive
radiation and suggests that ecological factors were an important component
in driving morphological diversification in the dipsadine megaradiation.