Big Ones Fight Back, Small Play Dead: First Records of Juvenile Thanatosis in Epicrates cenchria (Linnaeus, 1758) (Serpentes, Boinae)

Thanatosis, or death-feigning, is an antipredator behaviour characterized by voluntary inhibition of movement and reduced responsiveness to external stimuli, but remains poorly documented in several snake lineages. Here, we report two cases of thanatosis in juvenile Rainbow Boas, Epicrates cenchria, from northern Mato Grosso, Brazil. In the first episode, a juvenile exhibited tonic immobility, contortions, supination, and cocked-head movements after approximately 44 hours in captivity. In the second episode, another juvenile displayed tonic immobility, stiffness, and mouth-gaping after approximately 48 hours. A literature survey of defensive behaviours in Boidae recovered 13 references, but none provided records that met the strict technical definition of thanatosis. Thus, the observations presented here represent the first formal record of thanatosis in the subfamily Boinae. Interestingly, an adult E. cenchria examined under similar handling conditions did not exhibit thanatosis. Although based on a small number of observations, this contrast suggests a possible ontogenetic component in the expression of thanatosis in E. cenchria, potentially reflecting the lower effectiveness of other antipredator strategies in smaller individuals. We encourage standardized observations of defensive behaviour across individuals of different sizes and age classes to assess the generality and ecological significance of this behaviour in E. cenchria and other boid snakes.