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New ‘froggy-style’ mating position discovered in Bombay night frogs

  • While mating, most male frogs mount and hug their female partners in one of six known forms of embrace called amplexus.
  • But the secretive Bombay night frog, found only in the Western Ghats, has revealed a seventh, never-before-recorded form of amplexus, scientists write in a new paper published in PeerJ.
  • Unlike other frogs that typically use amplexus to get close to the female and fertilize her eggs as soon she releases them from her body, a male Bombay night frog loosely rests on the female, releases sperm over the female’s back and quickly moves away before the female has released her eggs.

In the forests of Western Ghats in India, scientists claim to have discovered a surprising new mating position in a group of frogs, one that has never been reported before.

While mating, most male frogs mount and hug their female partners. Scientists call their embrace amplexus, which a male typically uses to get close to the female and fertilize her eggs as soon she releases them from her body. A male frog can sometimes grasp the female by her waist, for example, or grab her in her armpits.

So far, scientists have recorded six forms of amplexus in all of the world’s more than 6,500 frog species. But the secretive Bombay night frog (Nyctibatrachus humayuni), found only in the Western Ghats, has revealed a seventh, never-before-recorded form of amplexus, scientists write in a new paper published in PeerJ.

Instead of grabbing the female tightly like males of other frog species do, a male Bombay night frog mounts and loosely rests on the female, holding on to a leaf or branch instead. The team has termed this new position the dorsal straddle.

The tiny 5-centimeter long male Bombay night frog then releases sperm over the female’s back and quickly moves away before the female has released her eggs. Unlike other species, there is no contact between the male and the female Bombay night frog while the eggs are being laid. So how do the eggs get fertilized?

The researchers posit that the sperm most likely trickles down the female’s back and then fertilizes the eggs.

Male Bombay Night frog found calling from a fallen tree trunk. Photo by SD Biju.

This discovery has turned out to be one of the most unexpected and exciting parts of the team’s research, SD Biju from University of Delhi, who led this study, told Mongabay. But precise timings are still unknown.

“My students are conducting further studies on the reproductive behavior of this frog, with specific focus on the exact moment of sperm release and egg fertilization,” he said.

The genus Nyctibatrachus, to which the Bombay night frog belongs, displays a vast variety of mating strategies. This could be because this genus is ancient, having evolved about 70-80 million years ago, Biju said. “Since these frogs are known to have an ancient origin, they might have evolved diverse reproductive strategies to help them survive through eons.”

A. Six previously known mating positions known among frog species worldwide. B. New mating position discovered in Bombay night frog. Illustrations from Peerj.

During field observations of the Bombay night frog, Biju and his team made another surprising discovery.

Among frogs, it is the males that usually call out to attract the attention of females. Female calls have rarely been observed in frogs and toads.

But the female Bombay night frog has a distinct call, the team observed.

“So far, female calls have been reported only in 25 anuran species, representing less than 0.5% of the total of 6583 anuran species that are currently recognized,” Biju said. “Even in our study, female calling in Nyctibatrachus humayuni was observed only briefly and on just four occasions over a total of 40 nights in the field, compared to the almost permanent presence of male advertisement calls.”

Biju’s team, however, is yet to determine the purpose of these calls. One possibility, according to the authors, is that females could be using their calls to help locate and identify their mates.

“During several of our observations, reproduction was interrupted by one or both frogs falling into the water,” Biju said. “Following this, male and female frogs were seen coming back to the same site to complete their mating sequence. Under such circumstances, females may be calling to facilitate mate location and recognition. This is just a possible interpretation.”

“However, more studies will be required to scientifically confirm the role of female calls in Bombay Night frogs,” he added.

According to the IUCN Red List, populations of the Bombay night frog are in decline. So any new information about these frogs would be crucial for their conservation, according to Biju.

“Natural history studies not only provides necessary information for planning effective conservation strategies, but also highlights unique frogs, such as the ancient Night frogs that exhibit highly diverse reproductive behaviors,” he said.

 

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