Embryonic Olfactory Learning in Western Toad (Bufo boreas) Larvae and Metamorphs

Speaker: Gena Bentall

The olfactory sense is well developed in amphibians and is important to many aspects of their natural history. It is used to locate food sources, recognize predator attack cues, recognize kin and navigate between resources. Western toads, native to the western United States, have been known to migrate long distances between their home range and breeding sites, often returning to the same breeding site year after year. Previous experiments have shown that toads rely on a variety of senses to navigate successfully, and that these senses can overlap. Toads that have had their olfactory sense experimentally impaired are disoriented, indicating that chemical cues may be of primary importance to navigation. At what point during toad development are preferences for these cues learned?
This experiment tests the hypothesis that chemical cues present during the embryonic stage of Western toads will influence behavior in later stages, and that larvae and metamorphic (newly terrestrial) toads will show a preference for these cues. Late stage Western toad embryos were treated in the lab with various natural plant extracts foreign to the natural habitat of this toad species. In preference tests done with larvae, behavior differences between treatment and control larvae were seen, with some treatment larvae showing a significant preference for their treatment odorant. This preference was maintained when animals were given a choice between their odorant and water from their natal pond. Further preference testing will be done when treatment larvae have completed metamorphosis. These results indicate that larvae can respond to odors present during their embryonic stage and further studies will confirm whether or not this odor memory in maintained. Embryonic olfactory learning would seem adaptive if the imprinted memory of chemical cues from a natal site decreased energy expended by toads seeking an optimal breeding site.