Males of most frogs and toads typically produce a small number of stereotyped, repetitive calls that have a restricted number of functions. Identified calls include the advertisement, aggressive, distress, warning and release calls (in Narins et al., 2000). The advertisement call often consists of a single note or a series of notes that is repeated frequently for many hours and can be highly stereotyped in its structure. These characteristics are highly advantageous when frogs call in high assemblages and with a high level of background noise. However, an increasing number of research studies have demonstrated that the calls of anurans are not necessarily as simple as once thought and in some species can be highly complex.
The male Emei music frog (Babina daunchina) from southwest China calls from an underground burrow that serves as a nest for egg laying and tadpole development. Its call typically consists of 4-6 repeated notes with a 150 millisecond gap in between. Research by Yue et al. (2017) showed that the first note in a series of calls conveys more information and is most important than subsequent notes. The researchers examined the brain wave response of females and found that the first call note of the male elicited a greater electromagnetic response compared to the following call notes. The importance of the first call note has been noted in other species from different taxa including zebra finches and some gecko species, where the initial call contains subtle information about the caller which is used in mate selection. The first call note in Emei music frogs may therefore contain information which is crucial for species recognition, individual discrimination and call type identification (Yue et al., 2017). Producing calls is potentially dangerous as it increases predation risk. Therefore encoding greater information in the initial call is biologically advantageous as it conveys most information without increasing predation risk. However, this situation may become more complex as a male may perform call masking which aims to interfere with the call of a neighbouring male whilst enhancing their own call. For example in Kassina fusca, a frog species from the African savannah, an individual male may try to produce its second call very close to the first call of another rival male. Known as backward masking, this results in the second call of the initial male sounding more like a first call and masks that of its neighbour. Call masking is little known in frogs and further research is required to understand its importance in mate selection.
Call matching is another form of competitive vocal interaction between males, which has been described in a few species of frogs. In these species, the number and complexity of calls that are produced by a male are approximately matched by its neighbour (Gerhardt et al., 2000). Since calling is energetically costly and increases risks of predation, males may match the number and type of call of a neighbouring male. The Australian quaking frog, Crinia georgiana is a species with a complex mating system. Males produce calls that typically consist of between one and 11 pulsed notes. Research by Garhardt et al. (2000) found when a male emitted 2-4 calls a neighbour would match the number of notes, but at a higher number of calls e.g. 8, call matching was less often employed. This suggests that the risk of predation at a higher number of calls is too risky and it is not worth males matching the number of calls of a rival.
The Madagascar bright-eyed tree frog Boophis madagascariensis is endemic to Madagascar. It is a large brown tree frog with conspicuous ‘spines’ on its elbows and heels. During the day these frogs hide in the leaf-axils of large plants but at night males emerge to call from shallow water. Initial studies on this species have shown that it has a surprising range of calls and the sounds emitted by one male tended to trigger the same call notes in neighbouring males (call matching). In addition, small groups of male frogs form choruses of similar calls which are initiated by one chorus leader. Research by Narins et al. (2000) recorded 28 different call types by males which is the largest known call repertoire of any amphibian (Narins et al., 2000). Morphological analysis of the frogs did not reveal any unusual structures which could produce such varied sounds. Instead, the researchers suggest that different neural wiring in this species may allow greater complexity of calls. For example, removing the stereotyped order of calls which are produced by some tropical species, increases call complexity. The function of so many calls remains untested but Narins et al. (2000) suggest that it may be linked to call matching as males often call in choruses consisting of similar calls. Counter call matching is when, for example, a male emits a longer, presumably more attractive call, and his neighbour is forced to increase the duration of his call to match or exceed that of the competitor. This may result in evolutionary pressure for males to develop more varied and complex calls.
Gerhardt, H.C., Roberts, J.D., Bee, M.A. & Schwatz, J.J. (2000) Call matching in the quacking frog (Crinia georgiana). Behavioral Ecology & Sociobiology, 48:243-251.
Narins, P.M., Lewis, E.R., McClelland, B.E. (2000) Hyperextended call note repertoire of the endemic Madagascar treefrog Boophis madagascariensis (Rhacophoridae). Journal of Zoology, London., 250: 283-298.
Yue, X., Fan, Y., Xue, F., Brauth, S.E., Tang, Y. & Fang, G. (2017) The first call note plays a crucial role in frog vocal communication. Scientific Reports, 7: 10128, Doi: 10.1038/s41598-017-09870-2.