Do Tiger Keelback Snakes (Rhabdophis tigrinus) Recognize How Toxic They Are?

Animals that depend on defensive chemicals acquired from food may face a decision when attempting to deter predatory attacks: Should they exhibit antipredator behavior that relies on the toxicity of the sequestered chemicals or should they adopt other behaviors that can avoid predation without using the chemical defense, such as flight? Thus, it is reasonable to assume that animals that sequester prey toxins have evolved the ability to flexibly change their antipredator responses according to the amount of toxin-resource they have consumed. We tested this hypothesis using an Asian snake, Rhabdophis tigrinus, that sequesters toxins from toads consumed as food and stores them in specialized organs back of the head, called nuchal glands. We reared hatchling snakes from toad-free and toad-rich islands on controlled diets and examined their subsequent antipredator responses after 3 and 6 month feeding. Juveniles from a toad-free island that had been fed a diet including toads for 3 months showed antipredator displays associated with the nuchal glands more frequently than those that had been fed a diet without toads. They showed a similar, but less clear, tendency after 6 months feeding. Juveniles from the toad-rich island did not show a clear tendency of dietary effect. We discuss possible reasons for the different dietary effects between the 2 populations and between the ages. Our results, along with previous related phenomena in other animals, suggest that the ability of self-toxicity-recognition may be widespread in animals that sequester defensive toxins from facultative food.