Tooth shape and feeding ecology
There is a large variation in the shape of the teeth. The first principal component (PC1) that represent almost half of the variation in our sample (46.98%) differentiates very thin, slender teeth with a pulp cavity that runs along the full length of the tooth from short, stout teeth with a thick layer of tissue (Fig. 4). PC2 differentiates teeth with a large base and a large maximal angle of curvature but not necessarily curved all along, from short, stout teeth that are mildly curved along their length. The distribution of species in the morphospace is not structured by phylogeny (Supplementary Material 6) but species that eat long or hard prey are mostly positioned on the left side of the plot while species that eat bulky, slippery prey, and species that must hold their prey are mostly positioned on the right side.
The linear models, with and without accounting for size, show similar results: the main mechanical challenge (size: F=1.78, P=0.001, no size: F=3.13, P=0.0001), prey hardness (size: F=2.94, P=0.0002, no size: F=4.48, P=0.0001) and foraging substrate (size: F=2.24, P=0.002, no size: F=3.8, P=0.0001) are significantly related to tooth shape, but prey shape is not (all P>0.6). For each factor, the best models are the ones not accounting for size (Supplementary Material 7). The post-hoc pairwise comparisons show significant differences in tooth shape between snakes eating hard versus slippery and bulky prey and species that must hold their prey (Table 1). Tooth shape also differs between snakes feeding on hard prey and those feeding on softer prey (Table 1). Durophagous specialists have short, stout, barely medially curved teeth. Their posterior curvature is regular but not high. Their pulp cavity is short, so they have a relatively thicker layer of hard material (dentine and enamel). Teeth of species feeding on bulky, slippery, medium, soft prey and that hold their prey are long and slender, with a long pulp cavity (i.e., relatively less hard material). They are medially curved and show an elbow-like curvature near the base of the tooth (Fig. 5).
Terrestrially feeding species have short and stout teeth but relatively more slender than durophagous species. Aquatically feeding snakes, however, have highly curved teeth, in both the medial and posterior direction. They also show an elbow-like shape but compared to species feeding on slippery prey they have very thin, needle-like tooth tips (Fig. 5).
Prey shape is not significantly associated with tooth shape in snakes.