As a rule, frog teeth are nothing special; they resemble small, sharp spikes along the upper jaw. However, a group of frogs living in the rivers of Southeast Asia have a peculiar adaptation: two bony “teeth” protruding from their lower jaws. They use these teeth to fight each other for territory and mates, and sometimes even to hunt hard-shelled prey such as large centipedes and crabs. In a new study published in the journal PLOS ONE, researchers describe a new species of “toothed” frog: the smallest frog ever discovered.

“This new species is very small, about a quarter the size, compared to other toothy frogs on the island where it was found,” said Jeff Frederick, a postdoctoral researcher at the Field Museum in Chicago and lead author of the study. The study was conducted as a doctoral degree at the University of California at Berkeley. “Most frogs in this genus are very large, weighing up to two pounds. At the large end, this new species weighs about the size of a dime.”

Sulawesi’s unique environment

A team from the McGwire Laboratory at Berkeley, in collaboration with the Bogor Museum of Zoology, found the frogs on Sulawesi, a rugged, mountainous island that is part of Indonesia. “This is a huge island with a vast network of mountains, volcanoes, lowland rainforests and montane cloud forests. Having all these different environments means that the magnitude of biodiversity among many of the plants and animals we find there rivals places like the Amazon,” says Frederick.

While hiking in the forest, members of a joint American-Indonesian team of amphibian and reptile researchers noticed something unexpected in the leaves of tree saplings and mossy rocks: frog egg nests.

Frogs are amphibians and lay eggs covered in jelly rather than a hard protective shell. To prevent the eggs from drying out, most amphibians lay their eggs in water. To the research team’s surprise, they continued to notice mounds of terrestrial eggs on leaves and moss-covered rocks several feet above the ground. Soon they began to see the little brown frogs themselves.

“Usually when we’re looking for frogs, we scan stream banks or wade through streams to spot them in the water,” says Frederick. “But after repeated monitoring of the nests, the team began to find visiting frogs sitting on leaves and cuddling up in their little nests.” This close contact with the egg allows the frog parents to coat the egg with compounds that keep it moist and free of bacteria and fungi.

Relationship between breeding behavior and dog size

Closer examination of the amphibian parents revealed not only that they were tiny members of the dog frog family with pale teeth, but also that all the frogs handling the egg clutches were males. “The male’s egg-protecting behavior is completely unknown to all frogs, but it is quite rare,” says Frederick.

Frederick and his colleagues suggest that the frogs’ unusual reproductive behavior may also be related to their smaller canines. Some of their frog relatives have larger teeth, which help them fend off competition along rivers to lay their eggs in the water. As these frogs evolved to lay their eggs far from water, their need for such large teeth may have been lost. (Scientific name of the new species Limnonectes filofolia ; filofolia stands for “nest page”.)

“It is surprising that with each subsequent expedition to Sulawesi we still discover new and different breeding methods,” says Frederick. “Our findings also highlight the importance of protecting these special tropical habitats. Many of the animals living in places like Sulawesi are quite unique, and habitat destruction is a conservation issue that poses a constant threat to preserving the extreme diversity of species we find there. About animals like these frogs, found nowhere else in the world “By gaining knowledge, we help protect these valuable ecosystems.”