THE EFFECTS OF SNAKE PREDATION ON METAMORPHOSIS OF WESTERN TOADS, BUFO-BOREAS (AMPHIBIA, BUFONIDAE)

For some anuran species, synchronous metamorphosis may function as an antipredator adaptation by swamping predators during the period of tra nsformation. We examined the levels of synchrony of emergence from the water of metamorphosing western roads (Bufo boreas) in the presence a nd absence of a lire snake predator, the common garter snake (Thamnoph is sirtalis) in a laboratory experiment. To compare between the treatm ents, we measured the time to emergence from the mater, the number of metamorphs emerging together, and the level of aggregation (before and during emergence) of the toads in each treatment. There was a differe nce between the treatments when all three factors were considered. We attributed these differences to a behavioral response in which B. bore as emerged sooner in the presence of the predator, regardless of wheth er individual toads had reached the point at which they were physicall y better suited to the terrestrial environment than the larval environ ment.