Minnesota Wild Animal Management | Rodents and Rats

April 26, 2012

 MN Rodents and Rats Control

Part 3 of 5

The Rat

The Norway rat (Rattus norvegicus) is strong, very aggressive, and able to adapt to cold climates.

  • Rats are 12-18 inches long, and weigh up to 16 ounces.
  • Rat muzzles are blunt, and their bodies look thick and heavy.
  • They have small eyes, naked ears, and coarse fur.
  • Rat tails are 6-9 inches long, scaly and nearly naked.
  • They have brown to dark gray fur, with scattered black hairs, and are gray, grayish brown or dirty white underneath.

A rat will shed over 500,000 body hairs each year. Left untouched, a rat’s incisor teeth would grow 4 inches in a year. So rats must chew continuously to wear down their incisors. Rats will chew on wood, aluminum siding, wallboard, plaster, paneling, frozen ground, concrete…anything but glass and most metals.

  • Their life expectancy is 9-12 months, although some rats have lived as long as 3 years.
  • They are good swimmers, jumpers and climbers, and they have keen senses of hearing, smell, taste and touch.
  • They are largely nocturnal, leaving their nests to forage at dusk.

Because they are so adept at hiding and scurrying, rats can exist in large numbers unbeknownst to their human neighbors. Unlike the mouse, rats are fearful of anything new. Even so, they quickly adapt to new places to live, new routines, new places to eat, and new kinds of food.

People see rats more often from April through June (spring breeding), and again in October and November as the season changes. But rats are active year-round. Outdoors, rats burrow in earth banks, along walls, under rubbish or concrete slabs, but they always locate close to sources of food and water. Rats follow the same routes as they make their rounds foraging for food each night, so they leave obvious runways in the grass. Rats often travel under objects like bales, planks, granaries and machinery to conceal their movement. A rat will normally travel no more than 150 feet from its nest, but at harvest time rats will travel much further to forage for corn, wheat or beans left in farm fields.

Outdoors or indoors, rats leave obvious oil stains on their trails and entrance holes. They can enter any opening larger than ½-inch in diameter, which means they can squeeze into your home through

  • The space around a pipe or conduit,
  • Under the door (especially garage doors that are left open, or do not seal properly),
  • Through a hole in the screen or floor, or through a gap between a window and its frame.

Rats will make holes in walls or floors soon after invasion. Rat holes are circular, average 2-3 inches in diameter, and are usually just inches off the floor. Holes in floors are generally close to walls. Rats leave behind a distinctive musk odor, particularly if they are confined to a small space.

Like the mouse, rats establish territories and colonies. The dominant male continuously guards his harem of females and aggressively prevents other males from mating. Females actively defend their group against strangers and often nest together. Their nests are constructed from leaves, paper, rags, twigs, or anything else they can find. Rats are nocturnal, but in areas having large rat populations some low ranked rats will forage during the day, because other rats have denied them access to food at night.

Rats are also profoundly prolific. If food and shelter are adequate, rats will breed throughout the year, although fewer litters are produced in winter.

  • Breeding occurs mostly in the spring and fall.
  • A female rat will begin breeding at 40-45 days of age.
  • Females often mate within 18 hours of giving birth.
  • Gestation is as short as 21 days, and a litter is 2-14 pups (the average is 7).
  • Females produce 8 or more litters per year.
  • Rats continue to breed until 18 to 24 months of age.
  • Newborn rats have no fur and are blind. But they grow rapidly.
  • In 2-3 weeks their eyes and ears are open.
  • Pups are weaned in 3 weeks, and are sexually mature in 3 months. However, male pups must develop sufficiently to challenge an adult male for supremacy.

It’s easy to see why rat populations can grow rapidly. Under ideal conditions, a pair of rats could produce 15,000 offspring in one year. Fortunately, breeding slows markedly as populations increase.

Part 3 of 5 Continued May 1, 2012

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