Frogs have different types of call to suit different occasions, a new study finds. It was known that birds sing different songs depending on context, yet this kind of behaviour has not been documented among the so-called simpler animals, the anurans (frogs and toads). This study, conducted by researchers from Indian Institute of Science (IISc), Bengaluru, and Indian Institute of Science Education and Research (IISER), Pune, has been published in the journal Animal Behaviour.
Semantics and frogs
When we humans speak, we use words to build up sentences. Only some arrangements of words have meaning and this is what we use. The same appears to be true of frogs, too. Some typical needs among frogs are to communicate a mating call to attract a member of the opposite sex and to guard one’s territory against other males. It is important that the calls given out in such different contexts be unambiguous, so that both, the calling frog and the listening frog, understand the difference. However, frogs have a limited repertoire of “notes,” and hence, using different notes for different contexts is impossible. “We wished to address whether these frogs arrange their notes into sequences in different ways in different contexts,” says Ananda Shikhara Bhat, the first author of the paper, who is with the Department of Biology, IISER Pune.
The group studied two species of frog – Humayun’s Night Frog (Nyctibatrachus humayuni) and Amboli Bush Frog (Pseudophilautus amboli).
Two species
“While N. humayuni produces calls with two notes (ascending and descending), P. amboli produces calls with six-note types,” says K.S. Seshadri from Centre for Ecological Sciences, IISc, Bengaluru, who along with Anand Krishnan of Department of Biology, IISER, Pune, led the study. Dr. Seshadri explains that they found that alone, individuals of N. humayuni produce a single ascending note but in the presence of another male of the same species, the individual adds up to seven descending notes to the ascending note. “In contrast, individuals of P. amboli use a different set of note types [note type 1–3, as classified by the authors] when alone or when in the presence of another male but use note types 4–5 when engaging in a physical fight with another male,” he adds.
Shikhara Bhat and Varun Sane who is now a researcher at the University of Cambridge, U.K., recorded the calls of N. humayuni in June–July 2019 from roadside streams in Adarwadi and Matheran in Maharashtra. The calls of P. amboli were recorded at Sirsi in Karnataka, in July 2020. “Calls from 19 N. humayuni and 50 P. amboli individuals were recorded and used for analysis,” say the researchers.
This is the first study to examine “sequences” of vocalisations in frogs. “To the best of our knowledge, our novel data analysis method, named ‘co-occurrence analysis’, is the first of its kind that is free of assumptions about the underlying data, and is widely applicable to analysing vocal sequences of all sorts of animals,” says Shikhara Bhat.
Endemic to region
“Both species of anurans are endemic to the Western Ghats. The two species were chosen intentionally because they were common where they are found and belong to two distantly related lineages of anurans,” says Dr. Seshadri. This allowed the researchers to examine if and to what extent the vocalisations varied.
“Ask anyone if they know how frogs call, and they will likely say ‘Croak’ or ‘Ribbit’. Our work shows that anurans go beyond this simplistic [picture]. Some of the anurans, half the size of our thumb, are capable of stringing together sounds with varying levels of complexity depending on the context.” says Dr. Seshadri.
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