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Victim. Bulldog rats died out likely because of parasites transmitted by black rats.

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Island Invaders: Infect and Conquer

As if black rats didn't have a bad reputation already. Research published today in PLoS One suggests that a century ago these rodents, already blamed for the spread of the bubonic plague in Medieval Europe and considered one of the worst invasive species on the planet, carried a disease that killed off two rat species native to Christmas Island in the Indian Ocean.

Ocean islands are notorious extinction hot spots, because endemic species evolved with few predators or competition. When invading species (or people) arrive, the native animals are often as helpless as the flightless dodo, which was hunted to extinction in the 17th century.

The same fate awaited Maclear's and bulldog rats, which lived on Christmas Island. Both species were abundant at the time of the first scientific expedition to the island in 1887, but their populations started to decline in 1899, when black rats arrived with the SS Hindustan. By 1908, both species were declared extinct. Shortly before their final disappearance, Maclear's rats were reported sick and seen crawling along footpaths. Visiting naturalists at the time attributed the symptoms to a type of sleeping sickness (trypanosomiasis) transmitted by the invasive black rats, but many modern researchers have instead suggested crossbreeding with black rats as the main cause of the extinctions.

Alex Greenwood of Old Dominion University in Norfolk, Virginia, and colleagues set out to solve the mystery of the extinctions. They collected samples from 21 historical rat pelts from Christmas Island, stored at natural history museums in the United Kingdom. The 100-year-old specimens were then examined for genetic signs of crossbreeding and presence of sleeping sickness pathogens.

The researchers analyzed pelts from black rats, the two extinct species, and alleged crosses. The results showed "no consistent genetic differences" between black rats and the supposed hybrids, the authors say, indicating that the specimens cataloged as hybrids are in fact black rats. To Greenwood, the results eliminate hybridization as the main cause of extinction.

Some of the pelts also tested positive for Trypanosoma lewisi, a parasite belonging to the group of organisms responsible for the fatal sleeping sickness and Chagas disease in humans. The trypanosome is transmitted by the fleas carried by black rats, which are immune to its effects. But to the endemic rats that had no resistance, the parasite would likely have been fatal. Greenwood says that this study implies that invasive species and environmental destruction are not the only causes of extinction. "Disease may play a role as well," he adds.

Amy Pedersen, a parasitologist at the University of Sheffield in the U.K., says that despite the small number of samples, the "authors make a strong case for disease-mediated extinction." It is likely that infectious diseases could contribute to future extinctions, especially of species that are also facing other challenges, she adds.