• A calling male of the Bombay night frog. (Photo by SD Biju)
These frogs in India have been caught in a position no other frogs engage in, as far as we know.
By
Sandrine Ceurstemont

Source:
New Scientist
15 Jun 2016 - 10:18 AM  UPDATED 15 Jun 2016 - 10:20 AM

Species: Bombay night frog, Nyctibatrachus humayuni. A type of Indian night frog, an ancient group that diversified about 75 million years ago
Habitat: streams in the Western Ghats of southwestern India. During breeding season, frogs congregate on land and in surrounding vegetation after sunset

Hop on, hop off. The frog Kama Sutra just got updated. An Indian night frog has been caught in a new sexual position dubbed the dorsal straddle.

Sathyabhama Das Biju at the University of Delhi and his team spent 40 nights filming the sex lives of Bombay night frogs (Nyctibatrachus humayuni) in a forest in the Western Ghats in India. Until now, six frog mating positions have been known, but they have now spotted a seventh.

Many frogs use a behaviour called amplexus, in which a male is on top of the female, grasping her with his front legs, in what could be likened to “doggy style”. The new position is a variation of this.

But when a male straddles a female, instead of clasping her with his limbs he clings to vegetation and sperm is released on her back, rather than onto the eggs. The female then lays her eggs and the sperm trickles down to fertilise them. “It was unexpected,” says Biju.

“This behaviour appears to be unique and it is unclear how or why it evolved,” says Ryan Taylor from Salisbury University in Maryland, who studies frog courtship.

Making a splash

The amphibians would fall in the water during sex about half of the time. But unless strong currents swept them away, they would get back in the saddle. The method seems to be successful: all of the 15 egg clutches monitored by Biju and his team were fertilised.

The males also made mating calls, which are typical in frogs, but the team was surprised to find that females occasionally initiated calls when they couldn’t make contact with a vocal male. “We often heard a feeble whistle-like sound in between the high chorus of male calls,” says Biju. “At first, we did not think it could be a female.”

The night watch

Females of only 25 species of frogs worldwide are known to make vocalisations. “For centuries it was believed that female frogs were mute,” says Iris Starnberger from the University of Vienna in Austria, who studies modes of frog communication.

Biju and his colleagues also saw a tree snake eating night frog eggs, which is less usual than preying on the adult amphibians. The team noticed that males sporadically guard eggs after dark but how they might protect them is unknown.

Unique reproductive behaviour has been observed in related frogs. The kumbara night frog, for example, is the only amphibian known to coat its eggs in mud, probably to protect them. These frogs also do a brief handstand after copulation and the female deposits her eggs while upside down.

Journal reference: Peer JDOI: 10.7717/peerj.2117

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This article was originally published in New Scientist© All Rights reserved. Distributed by Tribune Content Agency.