ARE INDUCED DEFENSES COSTLY? CONSEQUENCES OF PREDATOR-INDUCED DEFENSES IN WESTERN TOADS, BUFO BOREAS

Abstract

Induced defenses are widespread in nature, and in amphibian larvae they are often expressed as altered behavior and changes in tail shape, color, and size. Theory predicts that induced defenses should be costly in the absence of a predator threat. No costs have been found for these defenses after metamorphosis. In this study, we tested for induced defenses in western toads, Bufo boreas, and measured larval and postmetamorphic con- sequences of these responses. Larvae were raised in either the presence or absence of nonlethal predator cues. Defense responses to these larval treatments were measured during the larval stage and shortly after metamorphosis using both predator bioassays and quan- tification of the putative chemical defense common in toads, bufadienolides. We found no differences in larval morphology, growth rate, or development rate between the predator and control treatments. In the larval bioassays, some types of invertebrate predators con- sumed significantly fewer of the B. boreas larvae that were reared with predator cues compared to the control treatments. Bufadienolides were not present in B. boreas larvae. In the postmetamorphic bioassays, tiger salamanders (Ambystoma tigrinum) had longer handling times when consuming B. boreas that had developed in larval environments without predator cues compared to predator-treatment B. boreas. However, postmetamorphic B. boreas from predator cue larval environments had significantly higher concentrations of bufadienolides than did those from larval environments without predators, suggesting that these defenses are ineffective against tiger salamanders. Our results demonstrate that there is plasticity in the chemical defenses of toads and suggest that induced larval defenses may incur costs that are only apparent after metamorphosis.