Chemical Signaling in Amphibians

The term pheromone has been used to describe chemical signaling in most invertebrate and vertebrate groups, including amphibians. Pheromones are typically defined as chemical substances (e.g., a single molecule or a blend of a few molecules) that elicit an innate stereotyped behavior or developmental change in another individual of the same species (reviewed in Wyatt 2010). Pheromones may be water-soluble, volatile, or nonvolatile. In tetrapod vertebrates, they are detected by chemosensory neurons of the olfactory system, both the main olfactory system and the accessory (vomeronasal) olfactory system (Baum and Kelliher 2009). Pheromones are usually categorized according to function as releasers, primers, modulators, and alarm pheromones. Despite years of study, it is still unclear whether pheromones operate differently from nonpheromones, especially in mammals (Doty 2010). Nonetheless, many favor the use of the term pheromone, arguing that the term has intrinsic heuristic value if restricted to chemical emissions shaped by evolution to have a signaling function within a species (Wyatt 2010). Some of the controversy regarding pheromones may arise from the difficulty of translating a concept originally developed in insects to behaviorally and cognitively sophisticated mammals. Study of amphibian chemical signaling can offer insight to the controversy over the nature of pheromones as well as provide general insights to vertebrate chemical communication. Compared to mammals, amphibians have relatively simple nervous systems and express stereotyped behaviors. A number of amphibian pheromones have been characterized and can be used to evaluate concepts related to pheromones. Further, as modern representatives of basal tetrapods, amphibians can provide insight to the evolution of aspects of chemical signaling, such as the evolution of the vomeronasal system. Finally, due to their biphasic life cycle, amphibians are a good model for understanding how chemical signaling functions in aquatic versus terrestrial environments. Chemical signaling is widespread in amphibians. Extant amphibians descended from a common ancestor about 250 million years ago (San Mauro et al. 2005) and consist of anurans (frogs and toads), urodeles (salamanders), and gymnophionans (caecilians). Chemical communication has been well studied in urodeles, for which chemical communication is a dominant sensory modality. Chemical communication has been less studied in anurans but may be more prevalent than realized (Belanger and Corkum 2009; Lee and Waldman 2002; Waldman and Bishop 2004). Compared to the other amphibian orders, little is known about caecilian chemical communication (but see Eisthen and Polese 2006; Reiss and Eisthen 2008). The goals of this chapter are to review chemical signaling in amphibians and to highlight how study of amphibians can offer insight to vertebrate chemical communication. In this review, I will use the term chemosignal to describe chemical information conveyed among members of the same species that has signaling properties. I will discuss the sources of amphibian chemosignals, the function and chemical identity of chemosignals, and the sensory detection of amphibian chemosignals. The review is not meant to be comprehensive, and readers are referred elsewhere for more information (Dawley 1998; Eisthen and Polese 2006; Houck 2009; Kikuyama et al. 2005; Reiss and Eisthen 2008; Woodley 2010).