The black rat first evolved in Asia, probably India, sometime before the last Ice Age. At a weight of four to twelve ounces, it is only half the size of its first cousin, the Norwegian brown rat—also an important vector in human plague—but Rattus more than makes up for its unprepossessing physical stature with incredible powers of reproduction. It has been estimated that two black rats breeding continuously for three years could produce 329 million offspring, as long as no offspring died and all were paired (fortunately, all very big ifs).
Rattus also has some other remarkable qualities that make it a formidable disease vector. One is great agility. A black rat can leap almost three feet from a standing position, fall from a height of fifty feet without injury, climb almost anything—including a sheer wall, squeeze through openings as narrow as a quarter of an inch, and penetrate almost any surface. The word “rodent” derives from the Latin verb rodere, which means “to gnaw,” and thanks to a powerful set of jaw muscles and the ability to draw its lips into its mouth (which allows the incisors, or cutting teeth, to work freely), Rattus can gnaw through lead pipe, unhardened concrete, and adobe brick.
A wary nature also makes Rattus a wily vector; the black rat usually travels by night, builds an escape route in its den, and reconnoiters carefully. This last behavior seems, at least in part, learned. During a foraging expedition, one young rat was observed taking a reconnaissance lesson from its mother. It would scamper ahead a few feet, stop until the mother caught up, then wait as she examined the floor ahead. Only after receiving a reassuring maternal nudge would the young rat advance. Rats also have another rather unusual, humanlike trait: they laugh. Young rats have been observed laughing—or purring, the rodent equivalent of laughter—when playing and being tickled. Rattus is, by nature, a very sedentary animal—usually. A city rat may wonder what lies on the other side of the street, but studies show it won’t cross the street to find out. Urban rats live their entire lives in a single city block. The rural rat’s range is a not much larger—a mile or so. However, if Rattus were phobic about long-distance travel, it would still be an obscure Asian oddity, like the Komodo dragon lizard. Rats do travel, and often for reasons that highlight the role of trade and ecological disaster in plague.
For example, on occasion an entire black rat community will abandon a home range and migrate hundreds of kilometers. Research suggests that what makes the rats override their sedentary impulses is a craving for grain germ—and perhaps more particularly, for the vitamin E in the grain germ. Under normal conditions, rat migrations are infrequent, but under conditions of ecological disaster one imagines that they might become quite common.
For distances beyond the multikilometer range, Rattus relies on its long-time companion, man. The stowaway rat is the original undocumented alien. In modern studies, it has been found in planes, in suit-jacket pockets, in the back of long-haul trailers, and in sacks carried by Javanese pack horses. Trade has also been a boon to Rattus in another, more subtle but very significant way. In the wild, when rat populations grow unstably large, nature can prune them back with a prolonged period of bad weather and scarce food. The advent of camel caravans, pack horses, ships—and, later, trains and planes and trucks—has weakened this pruning mechanism. Once commercial man appeared, the highly adaptable rat was able to escape to places where food was abundant.
SOURCE: The Great Mortality: An Intimate History of the Black Death, the Most Devastating Plague of All Time, by John Kelly (Harper Perennial, 2006), pp. 66-67