doi:10.1038/nindia.2014.28 Published online 25 February 2014
The first ever acoustic analysis of croaking of the elusive purple frogs found exclusively in India's Western Ghats suggests that the unique earless species might be using a hitherto unknown mechanism to hear the calls of its peers1 . The research into vocal behaviour of the endangered pig-nosed Nasikabatrachus sahyadrensis also suggests that unlike other frogs it actually calls from below the ground, where it mostly lives.
Once a year when pre-monsoon showers lash the Western Ghats, the rotund purple frogs with pig-like snouts emerge from their underground homes. The males start calling from under a thin layer of soil, waiting for the females to arrive. Teams of researchers have been studying the habits of this elusive frog, which became a celebrity of sorts after a study in 2003 proved using genetic methods that its closest relatives are not in India but in Seychelles2 .
In the latest study, the researchers recorded the calls of the male purple frogs. They found that after the pre-monsoon showers, the males called from under 2-3 cm of soil, at the opening of a loosely packed tunnel that they use for quick getaways. The frogs spent a lot of energy while calling (video here). As they caught their breath, their large vocal sacs were filled with air and then the abdomen constricted pushing the air out. The entire body of the frog convulsed powerfully, and the earth around the purple frog moved in tandem with his song.
Ashish Thomas and co-researchers from the Systematics Lab at Delhi University started recording the calls. Then they lay in wait to catch the calling male and measured its length and weight so that they could scientifically analyse the calls. Very often, the frogs retreated up the tunnel when they heard the researchers approaching, and they had to dig rapidly to get them out. Though they recorded calls from 30 males over three field seasons, they could finally use calls from only 10 males, and analysis about 200 call recordings in total.
"I feel the frogs may be sensing vibrations from the ground, a possible side effect of their underground lifestyle," Sathyabhama Das Biju, one of the authors of the paper and team leader at the Systematics Lab at Delhi University, told Nature India.
After the calls, the researchers say, a female within earshot generally responds to the male, and comes out of hiding to mate. The females are about three times the size of the male and take them on their backs, then hop off to find a spot most suitable to lay eggs, mainly in or near streams.
Females lay hundreds of eggs in each clutch. An earlier study3 reported one female laying 3600 eggs in total in several bouts. The male remains on the female's back throughout. There is no parental care and the female returns underground after her "explosive" egg-laying. Tadpoles emerge, armed with clinging mouth parts which they use to attach to rocks in flowing water. Adult frogs head back underground, and emerge only after the pre-monsoon showers the following year.
A typical ear has an external eardrum, a middle ear and an inner ear. Most frogs have an external eardrum and a functional hearing apparatus. Purple frogs don't have an eardrum or inner ear. How they perceive each others' calls has been a mystery.
The study found that the females were responding to the "advertisement" calls. The researchers also observed an interaction between males that were close to each other — they engaged in repeated bouts of call overlap and call alternation.
The closest relatives of the purple frog are found 3000 km away, on the Seychelles islands. They belong to the family Sooglossidae, and are quite tiny (about 4 cm from tip of the nose to rear end) compared to the large purple frog (about 5-9 cm long). They also lack eardrums but have inner ears inside their heads. They actually hear through their mouths. The frog's mouth resonates at a frequency close to the pitch of fellow male calls, and the cochlea sense the sound.
Millions of years ago, when the world's landmasses were together, the Indian subcontinent was part of a supercontinent called Gondwanaland. When 160 million years ago Gondwana started breaking up, the Indian subcontinent, Madagascar, Seychelles, Australia and Antarctica remained together for another 30 million years as other continents drifted away. At this point, the purple frog and the Seychelles frogs were around. Antarctica and Australia then broke away, followed by Madagascar 90 million years ago. Seychelles (with it the Sooglossidae frogs) drifted apart 65 million years ago. Finally, India inched toward Eurasia, to dock 55 million years ago like a ship pulling into port, and forming the Himalayas.
The relationship between the purple frog and the Seychelles frogs is an example of how the Indian subcontinent acted as a "biotic ferry", carrying varied life forms from Gondwana to Eurasia.
"The purple frog is found from the Malappuram district, just north of the Palghat Gap, all the way down to the foothills of the Agastyamalai hills in Kollam district," says Robin Abraham, who has also worked on the purple frog. "There is not enough data about the frog, and more studies need to be done to track its interesting life history," he says.
The difficulty for researchers is, the species surfaces just once a year, and only for a few days at a stretch.