Mammals of Mass Destruction

The Chinese Year of the Rat began Thursday, and the folks at People for the Ethical Treatment of Animals want us to treat the critters better. In a news release the rat-huggers claim that the vermin have gotten a bad rap. Black rats may have carried the plague that wiped out a third of Europe's population in the 14th century, and they have been a health hazard and overall nuisance as long as humans have walked the earth, but PETA insists that rattus rattus, as the species is known, has many redeeming qualities. They like to wrestle. They respond when called by name. They even make "chirping sounds that are strikingly similar to human laughter."

An international team of scientists has now found another bright side to the black rat. A new study is shedding light on how rat-borne diseases such as plague and typhus spread, potentially helping identify ways of heading off human illness. And the study of rats is providing intriguing clues about patterns of human migration from East and Southeast Asia. Rats are believed to have hitched rides on the ships of early colonizers and traders. Where humans went, rats scampered after.

The eight-year study, the first of its kind for black rats, has identified six distinct lineages—doubling the number previously known—and traces their migration. It turns out that disease-ridden black rats were one of Asia's early exports, riding an early wave of globalization to conquer new markets in the Middle East, Africa, and Europe. The plague-bearing lineage fanned out from southern India to the Middle East, and from there to Europe and Africa. Another major lineage hopped from its homeland in Southeast Asia and the southern Chinese hills to Java, and via Taiwan to Japan and the Philippines.

Indeed, if dog is man's best friend, man must be rat's. Originally adapted to forest habitats in Southeast Asia tens of thousands of years ago, when they were relatively rare, black rats began to boom in numbers when humans discovered agriculture. Rats feasted on spilled grain and, later, urban refuse. Rat populations always thrived after environmental disturbances like landslides or treefalls, says Ken Aplin of Australia's national science agency, the lead author of the study. Human encroachment was a disturbance on a grand scale, and the rats exploited it.

The global spread of rats followed patterns of human trade and colonization, first within Asia and then to the world. The new study is painting a clearer picture of when and where that migration occurred, says Aplin. For example, it suggests that merchants or colonizers likely came to Madagascar from the Middle East and India 1,000 to 2,000 years ago, rather than a few hundred years ago, as previously thought. And each of their rat-infested vessels carried a bonus cargo of diseases that are harmless to their rat hosts but can be deadly to humans.

In parts of the developing world, such as rural Bangladesh and Laos, black rats are more than a nuisance; they amount to a security threat, says Aplin. (He should know: since 2000 he has personally schlepped suitcases full of rat carcasses from vermin hot spots throughout Asia back Down Under for study, which made for some interesting moments at customs.) In poor rural areas, says Aplin, rats chronically destroy up to 20 percent of food crops in the field and another 20 percent of stored grain and other foodstuffs. They also gnaw through property; cause spontaneous abortion, low fertility and sickness in cattle and pigs; and make people too sick to work in the fields. "Rats probably consume, contaminate or destroy five or 10 percent of everything humans produce worldwide," says Aplin. "It's quite amazing how successful they've been at becoming parasites on human culture."