“Though cold-blooded, their passions are strong […]. To speak of music, when applied to the discordant and overwhelming sounds emitted by male bullfrogs and some other species, seems, according to our taste, a singularly inappropriate expression. Nevertheless, certain frogs sing in a decidedly pleasing manner”

– Charles Darwin, The Descent of Man and Selection in Relation to Sex (1874)

 

While I had heard these words before, I only truly began to appreciate them while standing knee-deep in an ephemeral monsoon stream deep within the forests near Adarwadi, in the northern Western Ghats, with a microphone in my hand. From the soft, musical trills of night frogs (Nyctibatrachus sp.) to the harsh croaks of common toads (Duttaphrynus sp.), visitors to these ancient rainforests find themselves surrounded — indeed, almost overwhelmed – by the sheer diversity of frog calls ringing through the jungle every night. To be a male frog in the Western Ghats is to be engaged in an extremely fierce contest, singing your heart out amidst a thousand other frogs to gain the attention of potential mates. A colleague and I were spending a week in Adarwadi, recording the calls of Humayun’s Night Frog (Nyctibatrachus humayuni) in the monsoon of 2019 as part of a summer project with Dr. Anand Krishnan, a biologist at IISER Pune.

Male frogs emit vocal calls for a variety of reasons, including attracting mates and defending territories. Often, these calls are in the form of a sequential series of vocal ‘notes’ (a note is a unit of vocalization, and plays a role analogous to the role of words in the English language). We wanted to know whether these frogs arranged their notes into particular sequences the way we arrange words in our own songs to form meaningful lyrics.  N. humayuni is found in fast-flowing streams, and like most frogs, is active at night. By day, we scouted out potential areas to find these frogs, encountering various denizens of the rainforest such as the elusive Oriental Dwarf Kingfisher along the way. By night, we listened for the musical notes that males of this species possess and carefully pointed a microphone at them, standing in pitch darkness so as not to disturb them. All the while, it was imperative that we kept the equipment safe from water, both by making sure we had a sure footing in the streams and by keeping an eye out for any signs of the torrential downpours the Western Ghats are known for.

A male Humayun’s Night Frog (Nyctibatrachus humayuni), calling.

Oriental Dwarf Kingfisher

When we got back from the field, we analysed these recordings, poring through the data for tell-tale signs of structured sequences of vocalizations. We quickly recognised that these frogs possessed two types of notes, which we called an ‘ascending note’ (AN) and a ‘descending note’ (DN) (see the video below, for repertoires of both species). The frogs used these two types of notes in different ways – when they were vocalizing alone, undisturbed by the presence of other males of the same species, the frogs mostly used only ANs, whereas when they heard other male N. humayuni also calling around them, they added DNs to their songs, perhaps in an effort to outdo the competition, or perhaps to mark off their own presence to other males as an indicator of territorial behaviour.

Later that year, I discussed these findings with Dr. Seshadri KS, a biologist at IISc Bangalore. We began to wonder whether other frogs which possess larger ‘vocabularies’ show similar patterns. To investigate this, I spent the monsoon of 2020 at a homestay in Sirsi, far away from pandemic-stricken cities. This time, the target was the Amboli Bush Frog (Pseudophilautus amboli), a diminutive bush frog with a large 6-note vocabulary. These bush frogs are common throughout the central Western Ghats, and males call to attract mates, engaging in fierce territorial fights during the breeding season, for calling sites. We showed that these frogs used different note types in different behavioural contexts. Further, using measures derived from a field of mathematics called information theory, we showed that these frogs used simpler arrangements during territorial disputes, though they used more complex arrangements when vocalizing in the presence of neighbours.

Amboli Bush Frog (Pseudophilautus amboli)

Two Amboli Bush Frogs, fighting.

Traditionally, frogs have been viewed as “simple” creatures, and consequently, nobody had looked at whether frogs arrange their notes into specific sequences, despite the fact that birds and mammals were known to be able to do this. We were able to show using various measures that individuals of both N. humayuni and P. amboli modified their vocal sequences according to behavioural context, using different arrangements of notes in different contexts.

A cartoon about behavioural contexts.

As we wrapped up the study and began writing it up, I was struck by the realisation of how little we know about the vast majority of the denizens of these rainforests that we visit so often. The jungle is teeming with life, and for those who look, each of these creatures has a story to tell. As the late E.O. Wilson wrote in ‘The Diversity of Life’, “The unsolved mysteries of the rain forest are formless and seductive”. Wilson was speaking primarily of taxonomic biodiversity, but at least in my view, these organisms exhibit a bewildering array of fascinating behaviours to cope with the complexities of life in a hyper-diverse rainforest. Just as biodiversity has been recognised as worth recognising, ‘behavioural diversity’ is also worth recognising, for both utilitarian and cultural reasons.

The next time you find yourself in a tropical forest in India, keep an eye out for the smaller, less-charismatic inhabitants that call the place home. With patience and diligence, these often-overlooked organisms will surely provide some tantalising glimpses into the world of mysteries that Wilson spoke of.

References:

Bhat, A. S., Sane, V. A., Seshadri, K. S., & Krishnan, A. (2022). Behavioural context shapes vocal sequences in two anuran species with different repertoire sizes. Animal Behaviour 149, pp 111-129, https://doi.org/10.1016/j.anbehav.2021.12.004