Chemical communication in frogs and toads
The use of chemical communication to facilitate courtship in salamanders and newts is well recognised. Since anurans (frogs and toads) predominantly use acoustic cues during courtship, the use of scent has previously been thought of as a less important form of communication. However, studies over the past two decades have shown that chemical cues are more important than was first realised in the lives of frogs and toads.
During the breeding season of frogs and toads, the male will often form amplexus with the female and to help facilitate a grip on the female’s back, male frogs develop keratinized, often spiny nuptial pads on their thumbs and forearms. Glands are present below the surface of the nuptial pads and these release a secretion onto the keratinized surface of the nuptial pads. It has been proposed that these produce glue-like substances to enhance the male’s grip on the female. In addition, they may also act as attractant pheromones for reproduction. A recent study on the common frog (Rana temporaria) has shown that these chemicals serve a purpose in reproduction. Willaert et al. (2013) have found that chemicals known as amplexins are secreted at the surface of the male’s nuptial pad, probably during amplexus. The small spines on the male’s nuptial pad causes wounds on the female and these may allow the secreted molecules to seep directly into the female’s circulatory system. Such a direct delivery of protein pheromones into the circulatory system is only known from some species of plethodontid salamanders from North America. In these species, males develop a special gland during the breeding season which they use to rub pheromones into the female’s skin. It is therefore possible that delivery of pheromones through the skin is more common than initially thought within anurans.
Chemical communication may be used to serve different purposes during courtship in anurans. The anuran family Hyperoliidae (reed frogs) is common in sub-Saharan Africa, Madagascar, and the Seychelles, and consists of over 200 species in 18 genera. Reed frogs develop a colourful gular patch on their vocal sac during the breeding season. The vocal sac is used to produce advertisement calls and results from experimental work on reed frogs by Starnberger et al. (2013) has shown that the colourful gular patch produces pheromones (Figure 1). This study confirmed the presence of volatile pheromones in the gular patch on the vocal sac of reed frogs from several of the genera known at present including the most species-rich ones (Hyperolius, Afrixalus, and Heterixalus). Starnberger et al. (2013) propose that the male produces volatile substances that are emitted while he is calling and these circulate in the air to attract females, and/or to keep rival males at a distance. Male reed frogs often call from females in mixed groups with other closely related species. Starnberger et al. (2013) propose that advertisement calls alone may not be sufficient for females to sufficiently identify males from their own species. Therefore the additional chemical cues emitted in the form of pheromones may aid in species recognition.
Male dwarf African clawed frogs (genus Hymenochirus) have also been observed to have an ability to attract females using pheromones. Members of this genus are highly aquatic and endemic to central Africa, occupying slow-moving water bodies within areas of rainforest. As with many other male frogs and toads, individuals within the genus Hymenochirus possess skin glands during the breeding season. The importance of these has remained unknown for many decades. However, Pearl et al. (2007) have found that the breeding glands of male dwarf African clawed frogs (Hymenochirus sp.) release a mate-attractant chemical. In an experimental y-maze, females showed a positive response to water housing males and to water containing homogenized breeding glands. Results from this study were among the first to demonstrate a mate-attractant function for anuran breeding glands. Since many anuran species possess breeding glands, these results suggest that pheromone communication could be more widespread among frogs and toads than previously believed.
Since this discovery in 2007, further research has focused on searching for other anurans which use chemical signals. In the Mantelline subfamily of Mantellid frogs, males have enlarged femoral glands that cluster on their ventral thighs (Figure 2). An analysis of macroglands from several Mantelline species by Veneces et al. (2007) found species specific differences in the volatile compounds. The authors suggested that the chemicals help the frogs identify individuals of the same species in multi-species choruses. These and similar studies are adding to our knowledge and understanding of how anurans use chemical signals. The chemical systems used by amphibians are varied and complex and future research should look at further investigating the use of chemicals in anuran reproduction.
References
IUCN SSC Amphibian Specialist Group. 2014. Hymenochirus boettgeri. The IUCN Red List of Threatened Species 2014: e.T58154A18396612. http://dx.doi.org/10.2305/IUCN.UK.2014-3.RLTS.T58154A18396612.en. Downloaded on 08 September 2017.
Pearl, C.A. Cervantes, M., Chan, M., Ho, U., Shoji, R. and Thomas, E.O. (2007) Evidence for a mate-attracting chemosignal in the dwarf African clawed frog Hymenochirus. Hormonal Behaviour, 38 (1): 76-74.
Starnberger, I., Poth, D., Peram, P.S., Schulz, S., Vences, M., Knudsen, J., Barej, M.F., Rödel, M-O., Walzl, M. and Hödl, W. (2013) Take time to smell the frogs: vocal sac glands of reed frogs (Anura: Hyperoliidae) contain species-specific chemical cocktails. Biological Journal of the Linnean Society, 110 (4): 828–838, https://doi.org/10.1111/bij.12167.
Willaert, B., Bossuyt, F., Janssenswillen, S., Adriaens, D., Baggerman, G., Matthijs, C., Paulsels, E., Proost. E., Raepsaet, A., Schoof, L., Stegan, G., Treer. D., Van Hoorebeke, L., Vanderbergh, W. and Van Bocxlaer, I. (2013) Frog nuptial pads secrete mating season-specific proteins related to salamander pheromones. The Journal of Experimental Biology, 216: 4139-4143.
Vences, M., Walh-Boos, G., Hoegg, S., Glaw, F., Spinelli Oliveira, E., Meyer, A. & Perry, S., 2007. Molecular systematics of mantelline frogs from Madagascar and the evolution of their femoral glands. Biological Journal of the Linnean Society, London, 92: 529–539.
Woodley, S. (2015) Chemosignals, hormones, and amphibian reproduction. Hormones and Behaviour, 68: 3-13.