Dawn of snake sociality: resource defence and social behaviours of Lycodon semicarinatus foraging on sea turtles

Snakes are traditionally considered to be the least social among non-avian reptiles. Therefore, social behaviours that are not related to mating have rarely been studied in wild snakes. Nonetheless, in two species of snakes, Lycodon semicarinatus and Oligodon formosanus, intraspecific interactions unrelated to mating have been reported among individuals that forage on sea turtles and cluster around sea turtle nests. In this study, we conducted a field survey on L. semicarinatus on a sandy beach on Okinawa Island for four years by route census and fixed videography to quantify interactions between individuals that occurred at sea turtle nests. Snakes repelled conspecifics from the nest by using a ritual combat dance. Whether the snakes attacked the conspecific depended on the opponent at the nest. Some snakes waited for the conspecific to leave the nest. This decision-making may be based on individual recognition by chemical cues, because the snakes exhibited frequent tongue-flicking toward the occupant before contacting. Their tongue-flicking duration was significantly longer when the subsequent response was attacking than when it was fleeing. These intraspecific interactions imply the occurrence of territoriality and suggest the possible occurrence of a dominance hierarchy in L. semicarinatus. A comparison of L. semicarinatus with a previous study on O. formosanus revealed that they express different social interactions, although the two species use a similar feeding environment. Oligodon formosanus removes conspecifics from the nest by simply biting, but L. semicarinatus competed for the priority to access the nest by combat dance, and its decision-making whether to challenge the combat dance changed flexibly depending on the opponent. In addition, several L. semicarinatus that did not challenge the combat dance waited for hours near the opponent that were foraging on the nest. Lycodon semicarinatus exhibited behaviours to avoid encounter and fights with conspecific individuals. Although there are differences in the social level between the two snake species, the exploitation of a food resource that contains large numbers of prey in one place, such as sea turtle nests, may be an ecological factor that favours the evolutionary development of social behaviours in solitary snakes.