Biology of the Rat
Origin and Habitat
The Norway rat or brown rat, having no special connection with Norway, originated from temperate regions of central Asia. The rat is found worldwide (largely via ships). Historically, they have played a major role in disease transmission, a fact still important in today’s world.
In the wild, rats are largely nocturnal, live in burrows, and will eat just about anything including seeds, nuts, grains, vegetables, fruits, meats, and invertebrates. Rats are also known for their propensity to inhabit human dwellings and to destroy food supplies. As a result of their gnawing ability, far more food is destroyed than eaten. Further evidence of the damage caused by rats is illustrated by reports of gnawing the insulation from wire and even penetrating metal pipes.
The first domestic rats were originally raised by fanciers and to fight terriers (hence, rat terrier). Rats are occasionally kept as pets and still for fancy, but the vast majority of domestic rats are obtained from laboratory animal breeders for use in biomedical research, testing, and education. In fact, the rat is the second most commonly used animal in biomedical activities, exceeded only by its relative, the mouse. Several stocks, strains, and mutants are available, but there are much fewer than the mouse. Most laboratory rats are outbred albinos, such as the Wistar, Sprague-Dawley, and Holtzman. Another popular laboratory rat is the Long-Evans, also known as the "hooded rat", as it has darker hair over parts of the head and anterior body.
A rat can be lifted by the base of the tail, but this technique should be employed for only short periods, such as to move a rat from one enclosure to another. Grasping the tail other than at the base and lifting the rat may result in slippage of the skin and subcutaneous tissues, and subsequent necrosis, infection, and sloughing of the caudal vertebrae. It also affords the rat with a perfect opportunity to swing and bite.
A more suitable technique to lift and restrain the rat is to place the hand over the rat’s back, slipping the thumb ventrally between the forelimbs into the intermandibular space. The rat can then be inverted and held upside down in the palm of the hand.
Anatomy and Physiology
Adult body weight: female: 250-300g, male: 450-520g
Life span: 2.5-3.5 years
Respiratory rate: 70-115 breaths/minute
Heart rate: 250-450 beats/minute
Normal average rectal temperature: 100 0F
The dental formulae is 2 (I 1/1, M 3/3) = 16. The incisors are open-rooted and grow continuously. Rats will bite or "pinch" with their sharp incisors if mishandled.
The esophagus enters the stomach at the lesser curvature through a fold of tissue of the stomach. Because of this anatomical arrangement, the rat is unable to vomit. Like the mouse, the stomach is divided into a proximal, nonglandular portion and a distal, glandular portion. The two portions are grossly distinct.
Like the horse, the rat does not have a gall bladder.
The left lung consists of one lobe, the right lung consists of four lobes.
The rat has five pairs of mammary glands. Distribution of mammary tissue is diffuse, extending from the ventral midline over the flanks, thorax, and portions of the neck.
The urethra of the female rat does not communicate with the vagina or vulva; it exits separately just ventral to the vulva.
The deep gland of the nictitating membrane (Harderian gland) is a pigmented lacrimal gland located behind the eyeball, encircling the optic nerve. The secretion of this gland is rich in lipid and porphyrin. Although many species possess a Harderian gland, it assumes a special importance in the rat. During periods of stress and/or certain diseases, the tears overflow and stain the face around the eyes and nose. When the tears dry, the pigment has the appearance of dried blood. The pigment fluoresces under ultraviolet light and contains little or no blood.
The rat responds to decreases in ambient temperature by nonshivering thermogenesis, and to increases in ambient temperature by increasing the vascularization of its long tail, which may serve as a thermoregulatory organ. Nonshivering thermogenesis for the most part occurs in brown fat, the highest concentration of which is found in the subcutaneous tissues between the scapulae. Brown fat is also called the hibernating gland, although the rat does not hibernate.
Rats should be fed a commercial pelleted rat or rodent diet and water ad lib. These diets are nutritionally complete and do not require supplementation.
Food intake is approximately 5g/100g BW/day; water intake is approximately 10-12ml/100g BW/day.
Breeding onset is between 65-100 days of age in both females and males, although females may have their first estrus at 35 days. Rats are polyestrous and breed year round; ovulation is spontaneous. The duration of the estrous cycle is 4-5 days and estrus itself lasts about 12 hours, occurring in the evening. Vaginal smears are useful for determining the stage of the estrous cycle. Mating is usually nocturnal and may be confirmed by the presence of a copulatory plug in the vagina for 12-24 hours post-copulation, although this is not as reliable as indicator as in the mouse. The presence of sperm on a vaginal smear is a good indicator of mating.
The Whitten effect, the synchronization of the estrous cycles of females exposed to the pheromones of a male occurs in rats, but is not as pronounced as in mice. The Bruce effect (see Reproduction, Mouse) does not occur in rats.
The average gestation period is 22 days. A fertile postpartum estrus occurs within 48 hours of parturition, and simultaneous lactation and gestation may prolong gestation 3-5 days due to delayed implantation. The average litter size is 6-12. The young are called pups and weight 5-6 grams at birth, are hairless, and have closed eyelids and ears. Cannibalism is uncommon (except with new mothers), but females should not be disturbed during parturition and for at least 2 days postpartum. Pups are weaned at 3 weeks of age; weaning weight is 40-50 grams. If the postpartum estrus is not utilized, the female resumes cycling 2-4 days post-weaning.
Newborn male rats are distinguished from newborn females by noting the greater anogenital distance and larger genital papilla in the male. This is best accomplished by lifting the tails of littermates and comparing perineums.
Pseudopregnancy may follow sterile matings, but is uncommon.
Disease of the Rat
Bacterial, Mycoplasmal, and Rickettsial Diseases