Occurrence and evolution of cannibal behaviour in extant snakes

Extant snakes (Serpentes) are a highly diverse group of squamate reptiles, which have independently evolved key morphological adaptations to consume a large variety of vertebrate and invertebrate prey. While these predator-prey interactions have been widely addressed by several studies, little is known regarding the occurrence of cannibal behaviour in snakes, with scattered reports restricted mostly to natural history notes or incidental records. Here we provide an extensive review of cannibalism in extant snakes, with 503 case events available from the literature, that encompass at least 207 species in 15 families, in both captivity and the wild. For all case events, including those with and without location data, cannibal incidents occurred mostly within the Colubridae (29.0%), Viperidae (21.2%), and Elapidae (18.9%). As cannibalism in snakes has been hypothesized to be a random event, we test whether theories of foraging and feeding strategies apply to cannibal behaviour, finding that prey and predator body size are positively correlated (R = 0.81). Furthermore, we explore the evolution of cannibal behaviour in extant snakes, inferring a maximum-likelihood ancestral character estimation of cannibalism over a family-level phylogenetic tree, which revealed independent evolution of this behaviour at least 11 times during the evolutionary history of snakes. Cannibalism also appears to be correlated with macrostomate mandibular morphotypes, occurring only in Alethinophidia, and being absent in most snakes that possess mandibular morphotypes with reduced mobility. We conclude that cannibal behaviour appears to be widespread in extant snakes, and possibly represents an opportunistic behaviour that can be related to their evolutionary history, dietary and morphological specialization, environment, and other ecological correlates.