The following was copied from a post on:
alt.pets.rodents.rats
and was posted on :
5/12/2003 by DataPacRat

Adult wild Norway rats weigh from 200 to 485 grams; are 12 to 18 inches

long; have a round abdomen, a head with a short muzzle, and thick ears

shorter than 3/4 of an inch. As a consequence of domestication, the

domestic rat attains puberty earlier than the wild Norway rat; displays

little evidence of a seasonal sexual cycle; has a larger body weight,

and increased litter size, and a more docile disposition. Wild rats live

as long as four or five years, whereas the domestic rat rarely exceeds a

life span of three years.

Teeth:

The most characteristic feature of rats is the four prominent, orange

incisor teeth. Rats are said to exert 24,000 pounds per square inch,

enough pressure to gnaw through wood, plaster, cinder block, or soft

metals such as lead.

The incisor teeth grow throughout the entire life span of the rat at a

rate of about 4 or 5 inches a year. The proper length is maintained by

the grinding of the upper incisors against the lower incisors. If the

teeth are fractured, they may not wear properly and subsequently

overgrow.

Rats have no second incisor, canine, or premolar teeth. There is a

distinct gap, called the diastema, between the incisor and the molar (or

cheek) teeth. The diastema allows the lips to withdraw into the mouth,

preventing ingestion of particles while the rodent is gnawing on

inedible materials. There are three upper and three lower molars on each

side of the jaw. All teeth are permanent teeth; there are no "baby"

teeth.

Biting:

When rats bite, it's usually without warning. Rats bite once or

momentarily and not repeatedly. Rats are not vicious.

Face:

The upper lip is split by a vertical cleft that extends between the

nares of the nose. The pointed snout is quite mobile, sensitive to

touch, and has a surface ridged in a pattern that is unique to each

animal, similar to human fingerprints.

The ears are covered by fine hairs.

Limbs:

Rats are quadrupeds with slightly larger hindlimbs than forelimbs. The

forelimbs are agile and able to hold and manipulate objects such as

seeds.

The front and hind feet all have five digits. All digits are clawed

except for the first digit on the forelimbs which is called the thumb or

pollex. The pollex has a small, flat nail and is also reduced to two

phalanges (finger bones). The other digits each have three phalanges.

Walking pads are present on the feet.

Tail:

The long, rasp-like tail is slightly shorter than the combined length

of the head and body.

The powerful tail is used for orientation on the ground and balance

during jumping. Young, healthy rats usually land on their feet after a

jump.

The naked tail is also important as the principal organ for losing

excessive body heat through the dilation of blood vessels.

Gait and Posture:

The normal gait is parallel to the floor with the head horizontal, the

abdomen just above the floor, and the anus near the floor. There is a

slight up-and-down movement with each step.

The normal postures are sitting upright on its haunches when awake or

lying on its side or curled up when asleep or resting. When curious, the

rat may rear, rising up on its hind legs and supporting itself using its

tail as a third leg.