Chemoreceptive responses of two sympatric water snakes to extracts of commonly ingested prey species

Newborn of two locally sympatric species of water snakes,Nerodia fasciata andN. erythrogaster, were exposed to skin extracts of frequently ingested prey. In southern Louisiana, fish account for 78% of the diet of adultNerodia fasciata and about 15% forNerodia erythrogaster. Anurans comprise most of the remainder of the diets of these water snakes. Responses of naive individuals were compared to the chemoreceptive response profiles of groups of snakes reared on limited diets of fish and/or frogs. All snakes were tested seven times over a nine-month period and ontogenetic changes were apparent. NewbornNerodia fasciata have a strong chemoreceptive response for fish extract that remains unchanged by early dietary restrictions for the first six months of life. After that time their responses are more variable and can be altered by recent feeding experience. In southern Louisiana, this species is the least specialized water snake, an observation supported by these laboratory results. While newbornN. erythrogaster did not respond preferentially to any of the prey extracts, a significant response rate for fish extract is established by two months of age that persists for several months regardless of dietary restrictions. A subsequent shift of the response to frog extract at eight months of age regardless of diet may reflect a maturational process. Field studies are in agreement with these results and support the notion of ontogenetic changes related to predator size and age. This is the first demonstration of an ontogenetic change in chemoreceptive response that is not related to dietary experience.