Brown rats likely originated on the Asian steppes, where they first learned they could eat well by hanging out with humans. They spread with trade along the Silk Road, and were established in parts of Europe by about 1500. (The misnomer “Norway rat” may have arisen when an infested ship that happened to be Norwegian docked in an English port.) They colonized today’s United States before it had that name, by the 1750s, and apparently from both the east and the west. Brown rats along the East Coast are descended mostly from European ancestors, but West Coast rats are a mix of European and Asian genetics.

Roof rats—Rattus rattus, also known as black rats—are a global species as well. They may have originated on the Indian subcontinent and adapted to human settlements millennia ago, when humans invented agriculture. They reached Europe by A.D. 300, in time for the decline of the Roman Empire.

Black and brown rats alike traveled with explorers and traders, then settled down to eat our trash and steal our food. Today in Africa the median farm still loses 15 percent of its yield to rats. In Asia rats and other rodents eat enough rice each year to feed 200 million people.

Pacific rats, a third species of Rattus, are a different story: Polynesian explorers sailing from Tahiti and other islands intentionally brought them along in their canoes—as food. They cooked them in their own fat to make rat confit; they made beautiful cloaks of the fur.

As the Polynesians colonized various Pacific islands, tiny rodent explorers settled with them. In fact the rats’ genetic family tree has been used to shed light on when and in what order various islands were discovered. Between 1200 and 1300, Polynesians and their companions reached New Zealand—which until then had no mammals at all other than bats.

Reproduction and life cycle

The brown rat can breed throughout the year if conditions are suitable, with a female producing up to five litters a year. The gestation period is only 21 days, and litters can number up to 14, although seven is common. They reach sexual maturity in about five weeks. Under ideal conditions (for the rat), this means that the population of females could increase by a factor of three and a half (half a litter of 7) in 8 weeks (5 weeks for sexual maturity and 3 weeks of gestation), corresponding to a population growing by a factor of 10 in just 15 weeks. As a result, the population can grow from 2 to 15 000 in a year. The maximum life span is three years, although most barely manage one. A yearly mortality rate of 95% is estimated, with predators and interspecies conflict as major causes.

When lactating, female rats display a 24-hour rhythm of maternal behavior, and will usually spend more time attending to smaller litters than large ones.

Brown rats live in large, hierarchical groups, either in burrows or subsurface places, such as sewers and cellars. When food is in short supply, the rats lower in social order are the first to die. If a large fraction of a rat population is exterminated, the remaining rats will increase their reproductive rate, and quickly restore the old population level.

Females are capable of becoming pregnant immediately after giving birth, and can nurse one litter while pregnant with another. Females are able to produce and raise two healthy litters of normal size and weight without significantly changing their own food intake. However, when food is restricted, females can extend pregnancy by over two weeks, and give birth to litters of normal number and weight.