Dietary sequestration of defensive steroids in nuchal glands of the Asian snake Rhabdophis tigrinus

Abstract

The Asian snake Rhabdophis tigrinus possesses specialized defensive glands on its neck that contain steroidal toxins known as bufadienolides. We hypothesized that R. tigrinus does not synthesize these defensive steroids but instead sequesters the toxins from toads it consumes as prey. To test this hypothesis, we conducted chemical analyses on the glandular fluid from snakes collected in toad-free and toad-present localities. We also performed feeding experiments in which hatchling R. tigrinus were reared on controlled diets that either included or lacked toads. We demonstrate that the cardiotonic steroids in the nuchal glands of R. tigrinus are obtained from dietary toads. We further show that mothers containing high levels of bufadienolides can provision their offspring with toxins. Hatchlings had bufadienolides in their nuchal glands only if they were fed toads or were born to a dam with high concentrations of these compounds. Because geographic patterns in the availability of toxic prey are reflected in the chemical composition of the glandular fluid, snakes in toad-free regions are left undefended by steroidal toxins. Our findings confirm that the sequestration of dietary toxins underlies geographic variation in antipredatory behavior in this species and provide a unique example of sequestered defensive compounds in a specialized vertebrate structure.antipredator defense ͉ bufadienolides ͉ chemical defense ͉ maternal provisioning ͉ toads

Widespread convergence in toxin resistance by predictable molecular evolution

“…1). In addition to Afro-Asian varanids (17), we find convergent resistance-conferring amino acid changes in the phylogenetically distinct viperid, elapid, and natricine snake lineages (including members of the toad toxin sequestering genus Rhabdophis) (19), representing convergent molecular evolution over ∼165 million years of separation (20). This constitutes compelling evidence of adaptive molecular convergence underlying phenotypic convergence.…”

Maternal provisioning of sequestered defensive steroids by the Asian snake Rhabdophis tigrinus

“…Each individual has a series of 10 -20 pairs of these nuchal glands that extends caudally behind the head (Nakamura 1935;Smith 1938;Toriba and Sawai 1990). The contents of the nuchal glands irritate mucous membranes, cause corneal injuries (Nakamura 1935;Kitazume 1953;Kawashima 1957Kawashima , 1959Suzuki 1960;Asahi et al 1985), and contain cardiotonic steroids known as bufadienolides (Akizawa et al 1985a,b;Hutchinson et al 2007). Bufadienolides act by inhibiting the sodiumpotassium pump in a digitalis-like fashion (Melero et al 2000).…”

Natural products from the integument of nonavian reptiles

“…107 Mori and Burghardt, 108 however, postulated that this snake acquires these compounds from toads, sequestering them from their prey for defense against their predators. Hutchinson et al 107 investigated this hypothesis by comparing the nuchal gland fluids of wildcaught snakes from areas in Japan inhabited by toads with fluids from snakes inhabiting Kinkazan Island, where toads are absent. All snakes except those from Kinkazan possessed bufadienolides 76-95.…”

“…Laboratory studies demonstrated that hatchling yamakagashis, even those from Kinkazan, rapidly and consistently accumulated bufadienolides in their nuchal glands when reared on North American toads (Bufo fowleri and B. terrestris), but not on prey (fish or frogs) that lack these compounds. 107 The toads contain mostly the conjugated bufadienolides 96-103 and 82. Yamakagashis that were fed toads accumulated 76-78, 83, 86-88, and 91 in their nuchal glands, none of which they possessed upon hatching.…”