8 Interesting Facts About Rats

New York City might have pizza rat, but Chicago tops the list of the most rat-infested U.S. cities, according to a 2016 list compiled by Orkin. The Big Apple was second, followed by Washington, D.C. (make your own politician-rat joke). Globally, London and Paris also have a well-documented problem with rats. In fact, the City of Light was forced to close several parks in 2016 due to a rat crisis.

Think you’ve seen a really large rat? Probably not if you compare it with the Bosavi woolly rat, a species discovered in 2009. Believed to be the biggest rat, it is more than 32 inches (81 cm) long (including the tail) and weighs more than three pounds (1.36 kg). By comparison, the common house rat is about 20 inches (50 cm) long and weighs approximately 12 ounces (0.34 kg). Unfortunately, scientific studies have determined that common rats are getting larger. On the positive side, researchers think that common rats’ physiology prevents them from becoming gigantic rats. In New York City, for example, researchers determined that rats would likely never exceed 2 pounds (1 kg).

Part of the rat problem is their incredible fecundity. Rats begin breeding as soon as five weeks of age and continue until about age two. Females are fertile approximately every three weeks, and during this time they have been known to mate up to 500 times (!) in six hours. The resulting pregnancy usually lasts about three weeks, and the litter size typically ranges from 6 to 20 babies.

Although that is a misquote from the gangster film Taxi! (1931), rats have come to mean traitors or snitches. Why? Partly it’s due to their bad reputation as filthy and deadly creatures (see below) and also because rats will flee sinking ships and collapsing houses, giving the impression of abandonment.

Rats are well-known carriers of diseases. They have been implicated in the spread of some 40 diseases, including the bubonic plague, typhus, and leptospirosis. For years it was believed that rats with plague-infested fleas caused the Black Death (1347–51), which killed an estimated 25 million people in Europe. However, scientists now think that another rodent was to blame for the Black Death: gerbils. After examining the weather during that time period, the researchers determined that the wet conditions would have caused gerbils to flourish, not rats.

The answer is yes if you were born in 1924, 1936, 1948, 1960, 1972, 1984, 1996, or 2008. Those are the years of the rat according to the Chinese zodiac. People born under the rat are traditionally thought to be cunning, witty, and prosperous.

Rats are famous for being able to chew through things—soft concrete, wood, plastic, aluminum, and cinder blocks are no match for these rodents. Their love of chewing is due, in part, to the fact that their teeth never stop growing. In order to keep them from becoming too long, making eating impossible, rats must constantly gnaw to wear down their teeth.

Some people have proposed that a group of rats be called “a mischief.” (Shouldn’t it be more menacing? A flock of crows is known as a murder, after all.) In addition, male rats are bucks, while females are called does.