Candidate: Catherine E. Merovich

Degree of: Doctor of Philosophy

Department: Biological Sciences

Title:Bufadienolides in the Chemical Defenses of the Toads, Bufo Americanus and Bufo Fowleri

Committee: Dr. Stephen Malcolm, Chair
Dr. David Cowan
Dr. Charles Ide
Dr. David Karowe
Dr. Stephen Kohler

Date: Friday, March 18, 2005 10:00 a.m.-12:00 p.m.
2734 Wood Hall

Abstract: I investigated the steroidal chemical defenses (bufadienolides) in two bufonid species Bufo americanus and Bufo fowleri . By the nature of their complex and biphasic life cycles, toads, like other amphibians are important components of aquatic and terrestrial habitats and are prey to numerous invertebrates and vertebrates. Although most amphibians appear to rely on cryptic coloration and hiding as defenses, some species have evolved chemical defenses that may promote greater distributions and abundances. Bufadienolides are potent inhibitors of the membrane enzyme, Na + /K + adenosine triphosphatase, and as such are presumed to be important anti-predatory compounds in toads although much of their chemical ecology is poorly understood.
My objectives were to address the chemical ecology of toad bufadienolides by investigating: 1) ontogenetic variation in bufadienolides, 2) bufadienolide composition of B. americanus and B. fowleri , 3) effectiveness of bufadienolides against predators, 4) bufadienolide induction in newly metamorphosed and adult toads, and 5) individual and geographic variation in bufadienolides. I hypothesized that B. americanus would have a more extensive bufadienolide profile and a more effective suite of chemical defenses than those of B. fowleri and that this could account for distributional differences in these toads.
Results indicated that there is variability in numbers and concentrations of bufadienolides among toad developmental stages in both species, but cumulatively no difference in total mean concentrations. Bufadienolide presence appears to be consistent with predictions of a palatability model. Toads raised with predatory cues did not possess larger amounts of bufadienolides following metamorphosis than controls. Therefore, bufadienolides appear not to be inducible following metamorphosis.
In adults, more bufadienolides were detected in B. americanus but their mean total concentrations were not larger than in B. fowleri . These results suggest that distributional differences might be explained by variation in bufadienolide types and may not be affected by bufadienolide concentrations. However, seven bufadienolides were statistically different between the toad species. Polar bufadienolides with relatively short retention times were more concentrated in B. americanus while less polar bufadienolides with longer retention times were more concentrated in B. fowleri . In B. americanus there was much overlap in bufadienolide types and concentrations from all collection sites which suggests possible geographic variation in bufadienolides is not apparent at this scale of investigation. Simulated encounters with predators revealed highly variable individual responses among toads in no generalized pattern. Histological examinations of parotoid glands showed that the internal structure of these glands is similar to that of other toad species and that the effects of manual compression of the glands is similar between B. americanus and B. fowleri .
Responses of an aquatic (Dytiscid) and a terrestrial ( Thamnophis ) toad predator to either chemical cues from toads or to toad tadpoles were assessed. Results of the snake tongue-flick bioassay revealed that snakes (adults and neonates) respond 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. Further, toad parotoid secretions elicited investigative and predatory behaviors in these snakes. Because snakes showed elevated tongue-flick rates with parotoid cues than with toad skin cues, parotoid chemicals may present a more concentrated toad cue and are not necessarily deterrents to predation. Also, the Dytiscid predators equally consumed both B. americanus and B. fowleri showing no discrimination between species or between toad developmental stages.

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