Nature

Frogs Fertilize Eggs With Sperm Dripping Down the Female’s Back After Unusual Sex

June 14, 2016 | Erica Tennenhouse

Photo credit: SD Biju

A new frog sex act: the dorsal straddle

The amphibian Kama Sutra (or Kermit Sutra) is a rather short read consisting of only six mating positions known among the almost 7,000 species of anurans (frogs and toads) found worldwide. Scientists are now adding a new leaf to the book with a seventh sex position that they recently observed being performed by the Bombay night frog. They call it the dorsal straddle.

Diagram of the seven mating positions in frog species

^{The seven known mating positions among frogs and toads, including the newly discovered dorsal straddle. Credit: SD Biju}

This newly described position is as interesting as it sounds. It begins when a male straddles a female over her back. But this is no embrace — the male grabs on to a leaf or branch, hastily releases sperm on the female’s back, and then hops off into the night. Following this very brief encounter, the female lays her eggs just in time for the sperm trickling down her back to fertilize them.

But why read about it when you can watch frogs performing the dorsal straddle here:

In most frogs, the female lays eggs at the same moment that the male releases sperm to fertilize the eggs. All of this normally happens while the male and female are locked in their embrace.

The dorsal straddle is thus a highly unusual mode of sex for frogs. "This is a remarkable frog with an unprecedented reproductive behavior, which is unique for a number of reasons. This discovery is fundamental for understanding the evolutionary ecology and behavior in anuran amphibians," said S. D. Biju from University of Delhi, lead author of the study published in the journal PeerJ, in a press release.

The uniqueness of the Bombay night frog does not end there. Females of the Bombay night frog emit calls during mating season. While male frogs always call to attract mates, female calling is rare and known to occur in only 25 frog species. The research team also observed eggs of a Bombay night frog being eaten by a snake — the first documented observation of snakes eating frog eggs in India.