Ecological associations with arboreality during the evolution of body shape in Neotropical colubrid and dipsadid snakes

Morphological evolution is frequently associated with ecological diversification, and ecomorphological patterns often express functional relationships involved in the range of tasks a group of individuals performs in a given environment. In some snakes, morphological traits related to locomotion seem to converge in association with an arboreal lifestyle, although such ecomorphological associations so far have not been described for species-rich lineages such as Colubridae and Dipsadidae. In addition to microhabitat usage, body shape may differ according to activity period if the surface area-to-volume (S/V) ratio differs between diurnal and nocturnal species. In this study, we address evolution of body shape in Colubridae and Dipsadidae from these two perspectives. We measured 61 species of Colubridae and Dipsadidae and classified snakes according to the arboreality degree based on the level of gravity exposure: non-scansorial (terrestrial), eurytopical (broadly arboreal/terrestrial) and stenotopical (narrowly adapted arboreal); and also, according to the activity period as diurnal or nocturnal. We predicted that higher degrees of arboreality and nocturnality represent derived states that evolved in association with slender and narrower bodies with longer tails, and consequently increased S/V ratios. Our ancestral reconstructions suggest that specializations towards arboreality and nocturnality evolved more recently during the diversification processes of these colubrids and dipsadids snakes. Most morphological traits of stenotopical species were similar to those of eurytopical ones, and only differed in relation to non-scansorial snakes, and body tapering in the anterior body region differed between non-scansorial and eurytopical species. We did not identify any morphological associations with activity period, suggesting that body shape in colubrid and dipsadid snakes evolved mostly associated to different degrees of specialization to arboreal habitats.