victim. Bulldog rats probably died out because of parasites transmitted by black rats.
P. Wynne / patriciawynne.com
As if black rats have a bad reputation already. A study published today in PLoS One suggests that a century ago, these rodents, already blamed for the spread of bubonic plague in medieval Europe and considered one of the worst invasive species on planet, wore a disease that killed off two species of native rats of Christmas island in the Indian ocean.
Ocean islands are notorious extinction hotspots, because endemic species evolved with few predators or competition. When invasive 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 of rats and Bulldog, who lived on Christmas Island. Both species were abundant at the time of the first scientific expedition to the island in 1887, but their populations began to decline in 1899, when black rats arrived with the SS Hindustan . In 108, the two species have been declared extinct. Shortly before their final disappearance of Maclear rats were reported sick and seen crawling along the trails. Visit naturalists of the time attributed the symptoms to a type of sickness (trypanosomiasis) transmitted by invasive black rats to sleep, but many modern researchers instead suggested the crossing with black rats as the main cause of extinctions.
Alex Greenwood of Old Dominion University in Norfolk, Virginia, and colleagues set out to solve the mystery of extinctions. They collected samples of 21 historic rat skins of Christmas Island, stored in natural history museums in the United Kingdom. 100 years specimens were then examined for genetic signs of interbreeding and the presence of pathogens sleeping sickness.
The researchers analyzed the skins of black rats, both extinct and alleged cross. The results showed "no consistent genetic difference" between the black rats and supposed hybrids, say the authors, that the specimens labeled as hybrids are actually black rats. Greenwood, results eliminate hybridization as the main cause of extinction.
Some also tested positive for skin Trypanosoma lewisi , a parasite from the group of organizations responsible for the fatal bedroom sickness and Chagas disease in humans. The trypanosome is transmitted by fleas carried by black rats, which are immune to its effects. But for endemic rats that had no resistance, the parasite would probably have been fatal. Greenwood says this study implies that invasive species and environmental destruction are not the only causes of extinction. "The disease can play a role as well," he added.
Amy Pedersen, parasitology at the University of Sheffield in the UK said that despite the small number of samples, " excellent arguments for authors extinction mediated disease ". It is likely that infectious diseases could contribute to future extinctions, especially species that are also facing other challenges, she added.
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