Don’t Kiss and Tell: The Unusual Breeding Behaviour of Humayun’s Wrinkled Frog

It was a misty monsoon evening in 2001, and we were out looking for amphibians and reptiles in Amboli, Maharashtra. Partially drenched, despite our ‘branded’ gear, we soaked in our surroundings when a team member, Vithoba Hegde, exclaimed, “Listen to that call carefully, isn’t it melodious?” The whistle-like call caught our attention too. “Must be a bird,” he added. “Maybe a nocturnal one”.

He was pretty confident about his identification, but I wasn’t so sure, and neither was my field companion and partner in fun and adventure, Sameer Kehimkar. We were both confused, but I was only a Research Assistant at the Bombay Natural History Society (BNHS) at the time, and Vithoba was a seasoned naturalist who had served with the BNHS for many years, and had worked with experts, including the great ornithologist (and my first teacher), Dr Salim Ali.

Intrigued nevertheless, we cautiously approached the source of the call, leading to another discovery: It was not a single performer but an orchestra, and it appeared to be coming from the bushes. We examined the flora with our flashlights, but without success. Just as our curiosity was turning into distress, Sameer suddenly said, “Hey look at this!” In a flash, we turned our torches towards the stream, but Vithoba and I remained puzzled. Try as we may, we could not get a glimpse of this mysterious bird.

A few minutes later, we heard the call again, and it was then that we finally saw the performers. It wasn’t a bird at all, but a choir of frogs. Wasting little time, we located the rest of the “singers”, caught a few individuals, and photographed the specimens for identification.

Later, when we had returned home to Mumbai, we were able to identify the species as Nyctibatrachus sp., also known as the wrinkled frog. The frogs of the genus Nyctibatrachus are endemic to the Western Ghats, and live close to forest streams. Most species inhabit the southern part of the Western Ghats, except Humayun’s wrinkled frog (Nyctibatrachus humayuni), which is found in the northern ghats.

This remained the extent of my knowledge about wrinkled frogs, until three years later, when I heard the unique call again. This time, I was in Koyna Wildlife Sanctuary in Satara, Maharashtra. I was part of a team doing a biodiversity assessment when I heard the familiar sound early one evening. The next time I heard it, I asked my field assistant Shankar Desai (Mama) about it, Shankar a local with a deep understanding of his landscape, simply pointed towards the stream and said these are frogs.

This time, the number of frogs was much higher, and the air was filled with their melodious calls. A few were parked on the bank, some were inside the stream, and a few others were trying to climb rocks and trees. This behaviour raised my curiosity: Why were they climbing? We decided to wait and watch: a decision that yielded a wealth of natural history knowledge.

Half an hour later, we saw a few more males climb onto the leaves of nearby trees, and resume their sing-song calling. I approached the nearest male, keen to capture this behaviour on my camera, and when I pointed my torch on him I received another surprise: By the male’s side, were three clutches of eggs, with developing tadpoles clearly visible. This was extremely peculiar, because frogs almost always breed in water (since tadpoles are aquatic until they become froglets). At least that’s what I had learned. Yet, here was a frog, perched on a tree, singing his heart out, with eggs by his side.

I knew about tree frogs laying their eggs on trees, but I also knew that they are generally laid in a cloud of froth to prevent the eggs drying out. There was no froth in this case, which was another reason for my heightened curiosity. The more we looked, the more males we saw, calling from leaves, bark, and rocks — many sitting close to already fertilised eggs.

Back home, I managed to identify the species as Nyctibatrachus humayuni, and found a research paper by Krushnamegh Kunte. He provided a detailed account of the breeding behaviour of Nyctibatrachus petraeus, a similar frog from Castle Rock in Karnataka, near the Goa border.

Some of my observations, however, were not in tune with the published literature, so I decided to study the curious behaviour of Humayun’s wrinkled frogs in Koyna. From 2010-12, I collaborated with a team lead by Dr SD Biju from Delhi University and experts from the University of Brussels, Belgium, resulting in a paper we published in 2016 with Bert Willaert as a lead author. Our subject was the breeding behaviour of the Nyctibatrachus humayuni.

We found that these frogs, due to their strict habitat preference, breed close to streams. They are territorial, and males visit their respective ‘posts’ every night. Calling was initiated immediately after entering the stream, and continued from trees or other perches until midnight. If other males entered a male’s chosen territory, fights erupted, and the winner retained the territory.

The females, who are non-vocal, were usually observed foraging in the territory of these males. When they selected a suitable male, the females climbed the trees, bush, or rock to approach the male at his post, eventually touching his forehead with one of her hind limbs. This is the first physical contact. Then the female positions herself close to the male, who subsequently climbs on her back and sits there without holding her.

Normally all frog eggs are fertilised externally; males do not have reproductive organs. Males usually sit on the female’s back, while holding her down; this position is called amplexus. The female then lays the eggs while the male discharges the sperm, leading to fertilisation.

Humayun’s wrinkled frogs appear to do things a little differently. Our studies showed that the male mounts the female without holding her down. We described this as a ‘dorsal straddle’. In this position, the male lies motionless, until the female starts vigorously shaking her hind limbs and belly. With this ‘signal’ from the female, the male dismounts and moves slightly away from female, and resumes calling.

This is immediately followed by the female depositing a clutch of eggs, which she spreads evenly with her hind legs. Females appear exhausted at this stage, and remain motionless until they slowly leave the male’s post. After this, the male resumes calling, sometimes fairly close to the freshly laid eggs. In some cases, we observed males retain the post for days, oblivious to the eggs, calling every night for a new mating partner.

Initially we assumed that the male fertilises these eggs after the female has left. But in an experiment done during this study, the freshly laid eggs were covered with Ziploc plastic bags, just to prevent male contact. To our surprise, these eggs were already fertilised. Our question then was: when does male release the sperm? During dorsal straddle, or at the end when the female moves her body to dismount the male? It’s a mystery we are yet to solve.

Once the female moved away, no ‘parent’ was seen close to these eggs during the day. The males of Nyctibrachus humayuni were only seen during the night, and they paid no attention to the eggs, nor did they protect them from predators. In fact, predation was common, and we even reported a new species of predator: Thackeray’s cat snake (Boiga thackerayi). In most of the cases of predatory activity, the males usually selected other spots and resumed calling again.

Eventually, some 18-20 days later, the eggs develop into tadpoles that drop into the stream below after hatching. These tadpoles in turn, metamorphose into adults, return to the same stream, and continue their melodious performance, and the circle of life.

Humayun’s wrinkled frogs are habitat specialists and their survival is inextricably linked to the health of their forest ecosystem. Without their freshwater streams, however, they would be under serious threat. On the other hand, if their habitat is conserved, we have plenty to gain. This is because forest streams are the main source of water for the rivers originating from these landscapes. So by protecting them, we are in effect, protecting our own water sources, and civilisation at large. Not to mention, a real and rare treasure of India: Humayun’s wrinkled frog.