Frogs with fangs abound with astonishing diversity on a tropical island in Indonesia, a Canadian-led team has found.

Ben Evans, a zoologist at McMaster University in Hamilton, Ont., and his Indonesian and American colleagues found 13 species of fanged frogs on the island of Sulawesi — more than in all of the Philippines combined.

Among them were nine species that had never before been described by science. Evans and his team reported their findings in the August issue of The American Naturalist.

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Fanged frogs belong to the genus Limnonectes and are named for two bony protrusions in their lower jaws. They aren't real teeth, as they don't have any roots or other characteristics of teeth. (Rafe M. Brown)

Fanged frogs belong to the genus Limnonectes and are named for two bony protrusions in their lower jaws. They aren't real teeth, as they don't have any roots or other characteristics of teeth.

Evans said scientists aren't sure what the fangs are used for, but it's possible they're for fighting other males for territory; capturing prey such as fish, other frogs, tadpoles and insects; or defending against predators.

However, Evans said, "We've never seen them bite anybody."

The Sulawesi fanged frogs show a wide variety of adaptations to different lifestyles on a mountainous island with environments and microclimates that range from among the wettest to the driest in Indonesia, with varied vegetation to match, the paper reported.

Some species are large with highly webbed feet, well adapted to living in fast-moving rivers. Others are small with little webbing on their feet, better adapted to spending most of their lives on land. Some fertilize their eggs internally, lay eggs far from the water's edge and watch over them as the tadpoles develop in jelly-like capsules.

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In many cases, frog species with similar lifestyles living in completely different areas of the island showed similar adaptations.

"This discovery is a surprising example of how species may end up using similar tactics to survive and diversify if given the opportunity," the paper said.

The researchers credit the diversity to the fact that on Sulawesi, there are no other genuses of frogs competing with them, and suggest the frogs evolved to fill empty niches within the ecosystem.

The researchers spent many years walking up rivers in the jungles of Sulawesi at night, risking encounters with vipers and cobras, in order to catch fanged frogs by hand. In all, they caught 683. They analyzed and mapped the frogs' distributions and compared the frogs' characteristics to their environments.

Evans said the researchers tried sample frogs in areas untouched by the island's intense logging.

"There were many places that we sampled frogs in forests [where] when we visited in the next couple years, the forest was lost," he added.

Evans doesn't believe any of the frogs have gone extinct yet.

"But I think that essentially all of them have reduced their distributions really, really, profoundly."