Biology of the Mouse
Origin and Habitat
The common house mouse is native to Asia, India, and western Europe. This species is now found essentially worldwide due to introduction by humans. Mice exist as commensals, living inside during cold weather and moving outside into surrounding fields during the spring and summer, wild forms, living outside throughout their lives, and caged animals, kept for research, testing, teaching, pets and fancy. Although wild mice are nocturnal, commensal and caged mice have periods of activity during both day and night. Commensal mice feed on human food and household items but damage more than they eat. In the wild, mice feed on seeds, fleshy roots, leaves, stems, insects, and some meats, if available. Wild mice are burrowers and build nests wherever suitable cover and food exist.
Mice are occasionally kept as pets and for fancy. However, the vast majority of caged mice are obtained from laboratory animal breeders for use in biomedical research, testing, and education. In fact, seventy percent of all animals used in biomedical activities are mice. In excess of 1000 stocks and strains of mice have been developed, as well as hundreds of mutant stocks that are used as models of human diseases. In terms of genetics, the mouse is the most thoroughly characterized mammal.
A mouse is lifted by grasping the base of the tail with one’s fingers or a rubber-tipped thumb forceps. These are useful techniques for transferring mice from one enclosure to another. To manually restrain the mouse, the mouse is first lifted by the base of the tail, then the loose skin over the neck/shoulder area is grasped between one’s thumb and forefinger. This is made easier by lifting the mouse, allowing the mouse to grasp a wire cage top or other surface with the forelimb, then grasping the skin of the neck/should area. With a little practice, the mouse can be lifted and restrained with the one-handed technique. When hand holding, the mouse should be inverted so that the weight of the mouse rests in the palm of the hand. The caudal end of the mouse is restrained by placing the tail between the handler’s fourth and fifth fingers.
Grasping the tail other than at the base and lifting the mouse may result in slippage of the skin and subcutaneous tissues, and subsequent necrosis, infection, and sloughing of the caudal vertebrae.
Anatomy and Physiology
Adult body weight: female 25-40 g, male 20-40 g
Life span: 1.5-3 years
Respiratory rate 94-163 breaths/minute
Heart rate: 325-780 beats/minute
Normal average rectal temperature: 99.5 0F
The dental formula is 2 (I 1/1, M 3/3) = 16. The incisors are open-rooted and grow continuously. Mice will bite or "pinch" with their sharp incisors if mishandled.
The stomach is divided into a proximal nonglandular portion and a distal glandular portion. The two portions are grossly distinct. This is similar to the equine stomach.
The left lung consists of one lobe, while the right lung consists of four lobes.
The mouse 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.
Highly concentrated urine is produced; a large amount of protein is excreted in the urine.
The mouse has the narrowest thermoneutral zone of any mammal thus far measured. A mouse responds to decreases in ambient temperature by nonshivering thermogenesis, and to increases in ambient temperature by decreasing metabolic rate and increasing vascularization of the ears. Nonshivering thermogenesis can produce a threefold increase in basal metabolic rate, and for the most part occurs in brown fat. The highest concentration of brown fat is found in the subcutaneous tissues between the scapulae. Brown fat is also called the hibernating gland, although the mouse does not hibernate.
Bedding should be changed twice weekly. Ground corn cobs are most absorbent.
Mice should be fed a commercial pelleted mouse or rodent diet and water ad lib. These diets are nutritionally complete and do not require supplementation.
Food intake is approximately 15g/100g BW/day; water intake is approximately 15 ml/100g BW/day.
Breeding onset is at about 50 days of age in both females and males, although females may have their first estrus at 25-40 days. Mice 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 in timed matings to determine the stage of the estrous cycle. Mating is usually nocturnal and may be confirmed by the presence of a copulatory plug in the vagina up to 24 hours post-copulation. The presence of sperm on a vaginal smear is also a reliable indicator of mating.
Female mice housed together tend to go into anestrus and do not cycle. If exposed to a male mouse or the pheromones of a male mouse, most of the females will go into estrus in about 72 hours. This synchronization of the estrous cycle is known as the Whitten effect. The exposure of a recently bred mouse to the pheromones of a strange male mouse may prevent implantation (or pseudopregnancy), a phenomenon known as the Bruce effect.
The average gestation period is 20 days. A fertile postpartum estrus occurs 14-24 hours following parturition, and simultaneous lactation and gestation prolongs gestation 3-10 days due to delayed implantation. The average litter size is 10-12 during optimum production, but is highly strain dependent. As a general rule, inbred mice tend to have longer gestation periods and smaller litters than outbred and hybrid mice. The young are called pups and weight 0.5-1.5 grams at birth, are hairless, and have closed eyelids and ears. Cannibalism is uncommon, 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 10-12 grams. If the postpartum estrus is not utilized, the female resumes cycling 2-5 days postweaning.
Newborn male mice are distinguished from newborn females by noting the greater anogenital distance and larger genial papilla in the male. This is best accomplished by lifting the tails of litter mates and comparing perineums.
Pseudopregnancy of 1-3 weeks duration may follow sterile matings, but is rarely noticed.
Diseases of Mice
Bacterial, Mycoplasmal and Rickettsial Diseases