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City critters: The rat

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We may not usually think of it as "wildlife," but one wild animal that can be seen easily in New York City is that familiar subway-track scurrier, the brown, or Norway, rat. It is also known as the gray rat, wharf rat, water rat, sewer rat, alley rat, or house rat. The exact number of rats in the city is unknown, but it is estimated that there are at least as many rats as people--and possibly as many as four times the human population! A survey by Straphangers Campaign in fall 2013 found rats on 13% of subway platforms. While rats are not native to North America, they immigrated along with the first European settlers and have been present in New York City since early colonial times.

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The brown rat is a marvelous example of natural selection and adaptation, having perfectly adapted to living around humans in urban environments. Rats can squeeze through holes as small as three-quarters of an inch, climb straight up brick walls, fall five stories and live, walk on overhead wires, and climb up drain pipes into toilets. They have exceptionally keen senses of smell, touch, and hearing, and excellent night vision, although they cannot see well in daylight. Rats can make sounds too high-pitched for human hearing and detect motion thirty feet away in almost total darkness.

Like the more popular city resident, the gray squirrel, rats belong to the largest group of mammals, the rodents. Their nearest relatives are mice. A common feature of all rodents is teeth that never stop growing, and this is the reason why rats and other rodents must gnaw on things to wear their teeth down. In fact, rats can bite with a force of 7,000 pounds per square inch, and gnaw through bone, wood, metal, electrical wiring, and even concrete!

The main reason rats have adapted to living around humans is that they can eat all the same things people do, as well as a lot of things people don't. This means they can survive when food is scarce and thrive when it is plentiful. The rat's preferred foods are grains, fruits, meats, fish, and eggs. (Their fondness for eggs is a problem where they have been introduced to fragile environments; rats are responsible for wiping out 18 species of birds.) A rat can eat up to a tenth of its body weight every day--the equivalent of an average adult person eating 16 pounds of food! Rats famously feed on garbage, and when good food is scarce, they have been known to eat everything from human corpses to elephant toenails. Rats are also eaten by people in many cultures, such as China and Mexico. In New York City, they are preyed on by raptors such as hawks and falcons or feral cats. They are probably the reason cats were originally domesticated. In Chicago, they are a favorite food of coyotes.

Rats are highly social animals and live in family groups that may have hundreds of members. That means that if you see one rat, others are probably nearby. Family members sleep together in nests lined with soft materials such as gnawed plastic shopping bags. Members of a rat family care for each other and may even rescue each other from traps or feed a crippled relative for its whole life. However, they will defend their territory against outsiders and even kill invaders. Female rats can breed at two months old, producing large litters of from six to 22 pups after a three-week pregnancy. They can mate again immediately, producing more than 300 pups in one year. However, their life expectancy is only about 15 months, although in captivity they have been know to live more than five years. Because of their rapid life cycle, they can develop immunity to poisons used against them.

Rats are intelligent and will work together to obtain food. They can pass on things they learn to each other; if one rat eats poisoned bait and dies, or even if one rat is suspicious of a food, the rest of its family will avoid that food. Pet rats and lab rats can be trained in a variety of ways.

Wild rats can carry several serious diseases that affect people, including rat-bite fever, Lassa fever, typhus, polio, meningitis, trichinosis, and bubonic plague. They can also bite, so it is best to avoid any contact with wild rats. If rats are a problem in your community, the city government maintains an online Rat Information Portal where you can find out what to do and where to get help. Property owners are responsible for eliminating rat infestations and maintaining property to avoid them, but tenants are also responsible for proper storage of food and disposal of garbage. By correcting conditions conducive to rats, problems can be addressed at their source and unnecessary pesticide use can be avoided.

To learn more about rats, look for the book Oh, Rats! by Albert Marin at your local library.

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