A nearly toothless shrew rat has been discovered on the island of Sulawesi in Indonesia by an international group of biologists led by Dr Jacob Esselstyn of McMaster University in Hamilton, Canada.

The rodent, called Paucidentomys vermidax, appears to eat soft, easy-to-chew earthworms. Because of its unique diet, the rat is lacking all of its molars, leaving it only with incisors. All of the planet’s other 2,200 known species of rat have most, if not all, of their teeth.

“Growing teeth is energetically expensive,” Dr Esselstyn said. “If a characteristic is not useful, natural selection should eventually eliminate it, or at least reduce it to a vestigial state (in which it has lost its ancestral function).”

“It probably took this rat hundreds of thousands of years to lose its teeth,” he said.

“This species illustrates how the process of evolution can lead to the reversal of previously successful traits when faced with new opportunities,” the team wrote in a paper published online in the journal Biology Letters.

The team’s work helps in the understanding of speciation – the evolutionary process by which new biological species arise.

“We live in a natural world,” Dr Esselstyn said. “We’re a small part of that world and we’re curious. We can’t understand how ecosystems work if we don’t even know how many species there are. It’s in our own self-interest to better understand the world we live in.”

The species was discovered in the mountainous rain forests of Mount Latimojong and Mount Gandangdewata, on the southern part of the island.

Sulawesi is the world’s eleventh-largest island, situated just west of the Indian Ocean. Its unique geology makes it a hotspot for scientists. While Sulawesi is one island now, a few million years ago it was an archipelago. When those islands eventually collided, they formed a single, large island with a very complex shape. The island, with its many peninsulas and large mountains, yields a great deal of potential for scientific research.

“We’d generally expect a large, isolated island with large mountains and a complex shape to produce a significant number of species,” Dr Esselstyn said. “There are certainly a lot of species on Sulawesi, but we only know of a very small subset of them.”

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Bibliographic information: Jacob A. Esselstyn et al. 2012. Evolutionary novelty in a rat with no molars. Biol. Lett. Published online before print August 22, 2012; doi: 10.1098/rsbl.2012.0574