Does an Asian Natricine Snake, Rhabdophis tigrinus, Have Chemical Preference for a Skin Toxin of Toads?

Abstract: Several Asian natricine snakes possess special organs called nuchal glands. Previous studies showed the nuchal glands of Rhabdophis tigrinus contain cardiac steroidal toxins known as bufadienolides (BDs) that are sequestered from toads consumed as prey. Recently, a congener has been found to ingest fireflies and sequester defensive BDs from them. As amphibians are probably the plesiomorphic diet of Rhabdophis species, it is presumed that a dietary transition of toxin source from toads to fireflies has occurred within the radiation of Rhabdophis. Because snakes heavily rely on chemical cues to recognize their prey, we predicted that species of Rhabdophis are capable of detecting BDs. To test the prediction, we conducted chemical preference tests using adults and hatchlings of R. tigrinus. We presented seven stimuli (water, cologne, earthworm, cinobufagin [a compound of BDs], toad, frog, and lampyrine firefly) to them. Both adult and hatchling R. tigrinus showed a higher response to toads and frogs than the controls (water and cologne), but, contrary to our prediction, they did not respond to cinobufagin. Adult R. tigrinus did however show a higher response to lampyrine fireflies than the controls. Our data imply that chemical resemblance between toads and fireflies elicited the reaction in adult R. tigrinus, but the chemical substances remain unknown. To identify the proximate mechanisms of the unique shift from toad-eating to firefly-eating in the snakes, further investigation is necessary.