World's Only Tadpole-Birthing Frog Discovered in Indonesia

The newly described frog species (L. larvaepartus), with the male on the left and the female to the right. Jimmy McGuire

Here's how the vast majority of the world's frogs mate: The male clasps the female and the two release sperm and eggs into the water at roughly the same time. The eggs are fertilized outside the body, where they develop into tadpoles and then grow into little frogs.

But a scant few hop to a different beat, and they have adopted different ways of bring new froglets into the world. One, for example, is the newly discovered Limnonectes larvaepartus, a species from the Indonesian island of Sulawesi that gives birth to live tadpoles. The Sulawesi frog is the first ever found that spawns in this way.

The amphibian can be found in little pools aside streams on the Indonesian isle in the West Pacific. When University of California at Berkeley herpetologist Jimmy McGuire first encountered the species, he was taken aback to find a bunch of tadpoles inside its body.

"That was shocking to us," says McGuire, co-author of a study describing the species, published today in the journal PLOS ONE. "It was like Alien, when the monster comes out of the guy's stomach—we didn't expect to see tadpoles squirting out of this frog." (Except, presumably, it was much less violent and stomach-churning than in the movie.)

The little hopper was first originally discovered by Indonesian scientist Djoko Iskandar, a study co-author and McGuire's collaborator, more than 20 years ago, but hadn't been fully described until now, and it wasn't immediately known at first that it gave birth to live tadpoles.

It is part of a group called fanged frogs, so-named for the sharp, bony appendages that stick out of their lower jaws, McGuire says. The males presumably use these bone-spears to fight, and perhaps as a result, males in this taxonomic group are bigger than females— a reversal of the gender-size pattern in most other frogs.

Two tadpoles, each about 10 millimeters long, shortly after birth. Jimmy McGuire

There are only three other known frog species that practice internal fertilization: one in the Pacific Northwest (the coastal tailed frog, or Ascaphus truei) that lays fertilized eggs and two species (one extinct) that give birth to live froglets. But this is the first to deliver live tadpoles into the world, McGuire says.

There are also a number of other interesting reproductive strategies, such as that of the now extinct gastric-brooding frog, which swallowed fertilized eggs. Froglets then developed in the stomach of this species.

Unlike the coastal tailed frog, which has a modified tail that resembles a penis and allows for internal fertilization, this Sulawesian frog has no such anatomical blessing. It is not known how fertilization takes place exactly, but it is possibly accomplished when the males and females press their rear ends together. In birds, where the males of most species also lack penis-like structures, this maneuver is called a "cloacal kiss," McGuire says.

The frogs generally avoid the mainstreams, like little amphibian hipsters; can be found in somewhat-disturbed habitats where the forest has been cleared; and, given their wide distribution in Sulawesi, aren't considered at risk of extinction, McGuire adds.

Since they don't require pristine forests, they aren't likely to suffer as much as other species in Indonesia, where development is rapidly wiping out habitats for many creatures, like the more sensitive species of frogs that require thick jungles to live and larger animals like leopards and orangutans, he says.

Sulawesi is home to an astonishing array of biodiversity. It was formed when five or more islands collided millions of years ago, McGuire says, giving it its current shape, described by some as looking like "a drunk spider."