Ancient DNA extracted from the bones and teeth of giant lemurs that lived thousands of years ago in Madagascar may help explain why the giant lemurs went extinct. It also explains what factors make some surviving species more at risk today, says a study in the Journal of Human Evolution. Most scientists agree that humans played a role in the giant lemurs' demise by hunting them for food and forcing them out of habitats. But an analysis of their DNA suggests that the largest lemurs were more prone to extinction than smaller-bodied species because of their smaller population sizes, according to this team of American and Malagasy researchers.

By comparing the species that died out to those that survived, scientists hope to better predict which lemurs are most in need of protection in the future.

The African island of Madagascar has long been known as a treasure trove of unusual creatures. More than 80 percent of the island's plants and animals are found nowhere else. But not long ago, fossil evidence shows there were even more species on the island than there are today. Before humans arrived on the island some 2,000 years ago, Madagascar was home to 10-foot-tall elephant birds, pygmy hippos, monstrous tortoises, a horned crocodile, and at least 17 species of lemurs that are no longer living -- some of which tipped the scales at 350 pounds, as large as a male gorilla.

Using genetic material extracted from lemur bones and teeth dating back 550 to 5,600 years, an international team of researchers analyzed DNA from as many as 23 individuals from each of five extinct lemur species that died out after human arrival. They looked at a giant ruffed lemur, a baboon lemur, a koala lemur and two sloth lemurs -- all housed in the collections at the University of Antananarivo and the Duke Lemur Center at Duke University. The study also included genetic data from eight extant species, including the three largest lemur species still alive today.

The researchers found that the species that died out had lower genetic diversity than the ones that survived -- a hallmark of small population size.

The results aren't entirely surprising, said George Perry, a scientist from Penn State University who was part of the research team. "Larger-bodied species often need larger territories and are fewer in number than smaller-bodied species," he explained, so they would have been more susceptible to extinction as hunting, logging, farming and other human activities took their toll.