Merck Manual

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Description and Physical Characteristics of Rats


Katherine E. Quesenberry

, DVM, MPH, DABVP (Avian);

Kenneth R. Boschert

, DVM, DACLAM, Washington University

Last full review/revision Apr 2020 | Content last modified Apr 2020
Topic Resources

Rats at a Glance

Life span

1.5 to 3 years


Body: 9 to 11 in (23 to 28 cm)

Tail: 7 to 9 in (18 to 23 cm)


12 to 23 ounces (350 to 650 g)

Cage temperature range

64°F to 79°F (18°C to 26°C)

Relative humidity

30% to 70%

Breeding age

6 to 12 weeks

Estrous cycle

4 to 5 days

Gestation period

21 to 23 days

Litter size

8 to 18 young

Weaning age

21 days

Adult female rats typically weigh 12 to 16 ounces (350 to 450 g), and adult male rats usually weigh 16 to 23 ounces (450 to 650 g) (see Table: Rats at a Glance). They are about 9 to 11 inches long without the tail. Male rats are usually larger than females. Pet rats are available in several colors and coat patterns because of specialized breeding. The common color variations include brown, black, tan, gray, and white, with both lighter and darker shades as well. Some of the more exotic colors are Siamese, blue, silver black, silver fawn, lilac, cinnamon, black-eyed white, cinnamon pearl, lynx, silver agouti, silver lilac, and blaze. Rex rats have a curly coat. Tailless rats are born with no tails and hairless rats have no fur. The usual life span of rats is about 1.5 to 3 years.

For More Information

Also see professional content regarding rats as pets.

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