Bufadienolides in the chemical defenses of the toads, Bufo americanus and Bufo fowleri

I investigated the steroidal chemical defenses (bufadienolides) of Bufo americanus and Bufo fowleri. By the nature of their complex, biphasic life cycles, toads, like other amphibians are important components of aquatic and terrestrial habitats and are prey to numerous invertebrates and vertebrates. Bufadienolides are presumed to be important anti-predatory compounds although much of their chemical ecology is poorly understood. I investigated (1) ontogenetic variation in bufadienolides, (2) bufadienolides from adult parotoid secretion, (3) effectiveness of bufadienolides against a terrestrial predator, and (4) effectiveness of bufadienolides against an aquatic predator. I hypothesized that B. americanus would have a more extensive bufadienolide profile and a more effective suite of chemical defenses than B. fowleri and that this could account for distributional differences in these toads. Results showed variability in numbers and concentrations of bufadienolides among toad developmental stages, but cumulatively no difference in total mean concentrations. Bufadienolide concentrations across developmental stages appeared to fit Brodie and Formanowicz's (1987) model, but were more pronounced in eggs. Bufadienolides did not appear to be inducible following metamorphosis. More bufadienolides were detected in adult B. americanus but their mean total concentrations were not larger than in B. fowleri suggesting that distributional differences might be explained by variation in bufadienolide types rather than by bufadienolide concentrations. However, seven bufadienolides were statistically different between toad species. In B. americanus there was much overlap in bufadienolide types and concentrations from all collection sites. Repeated expressions of parotoid glands revealed highly variable individual responses among toads. In tongue-flick bioassays, terrestrial snake predators responded to chemical stimuli from both B. americanus and B. fowleri with more tongue-flicks and greater tongue-flick attack scores than snakes exposed to distilled water. Because snakes showed elevated tongue-flick rates with parotoid secretions than with toad skin stimuli, parotoid chemicals may present a more concentrated toad stimulant and are not necessarily deterrents to predation. Also, aquatic Dytiscid beetle predators equally consumed both B. americanus and B. fowleri suggesting no discrimination between species or between toad developmental stages.