Habits and characteristics of arboreal snakes worldwide: Arboreality constrains body size but does not affect lineage diversification

Arboreal lifestyles represent common and major habitat shifts among snakes. Major habitat shifts are often facilitated by particular traits that confer advantages in the new environment. Although studies have examined the habits and characteristics of arboreal snakes at the level of individual species or small clades, a broad survey has never been performed across all snakes. We surveyed the literature to identify all known arboreal snakes and summarize their general characteristics. We then tested for associations between diversification rates and arboreal habits and reconstructed ancestral states using the hidden state speciation and extinction (HiSSE) approach. Finally, we tested for an association between body size and arboreality by fitting multi-peak Ornstein-Uhlenbeck models. We expected that transitions to an arboreal lifestyle might open new ecological opportunities, leading to increased diversification rates, and that different selective pressures for arboreal snakes might lead to changes in body size evolution. We found that generally, arboreal snakes are most frequently nocturnal, oviparous, reptile-eating, brown/banded/patterned snakes inhabiting the Neotropics (assessing each trait individually, not jointly) and that arboreality has no effect on rates of diversification. Ornstein-Uhlenbeck models showed that arboreal snakes tend to be longer than non-arboreal snakes, that rates of body size evolution are generally lower in arboreal snakes, and that arboreal body sizes are pulled more strongly towards their optimum than in non-arboreal snakes. These results suggest that morphological evolution is more constrained in arboreal snakes than in non-arboreal snakes.