Mice and Rats
Housing
Several types of cages are available which are suitable for housing small rodents. Many of these units come equipped with cage “furniture” such as exercise wheels, tunnels and nest boxes. These accessories contribute to the pet’s psychological well-being.
Cages should be constructed with rounded corners to discourage chewing. Rodents readily chew through wood and thin plastic. Recommended caging materials are wire, stainless steel, durable plastic and glass. Glass and plastic enclosures restrict ventilation and may lead to temperatures and humidity problems. These materials are acceptable when at least one side of the enclosure is open for air circulation.

These pets thrive in solid bottom cages with deep bedding and ample nesting material. Bedding must be clean, nontoxic, absorbent and relatively dust free. Shredded paper, pine shavings and processed corncob are acceptable beddings. Wood shavings and ground corncob must be free of mold, mildew or other contamination. Cedar chips or chlorophyll scented shavings should be avoided because of association with respiratory and liver disease. At least one inch of bedding should be provided to allow for normal burrowing behavior. Cotton and shredded tissue paper make excellent nesting materials.

Adult mice require a minimum floor area of 15 square inches and a cage height of 5 inches. Rats need at least 40 square inches of floor space and a minimum of 7 inches height. Breeder mice and rats require much larger areas. Optimal temperature range for these pets is between 65 to 80 degrees Fahrenheit, with a relative humidity of 40 to 70%. Twelve hour light cycles are preferred, with most rodents being more active during the night.

Pet rats and mice can be housed singly or in groups. These rodents are colony oriented by nature. However, occasionally an overly aggressive mouse or rat may have to be caged individually. Territorial disputes also develop when the cages are overcrowded or when they lack food or water. Group cages should be provided with multiple food and water sources.

As a rule of thumb, the cage and accessories should be thoroughly cleaned at least once weekly. An exception to this schedule is when newborn babies are present, then wait until they are at least 10 days old. Other factors that may require increased frequency of cleaning are the number of animals in the cage, the type of bedding material provided and the cage design and size. Cages should be sanitized with hot water and nontoxic disinfectant or detergent then thoroughly rinsed. Water bottles and food dishes should be cleaned and disinfected daily.

Breeding
Sex determination is the first step to breeding success. Fortunately, mice and rats are fairly easy to sex. Neonatal male rodents can be distinguished from females by a greater distance between the anus and the urinary opening. Males have a one and a half to two times greater distance between the anal and urogenital openings. Sexually mature male rodents also exhibit a prominent scrotum. Females can be identified by their prominent bilateral rows of nipples.

Sexually mature mice and rats need to be properly paired to avoid fighting and permit successful mating. Adult male mice often fight when caged together, especially in the presence of females. Therefore, a single male mouse should be housed with one or more mature females. Rats, on the other hand, usually can be caged in mixed groups of males and females without aggression.

Female mice become sexually mature at about 50 days of age. They have an estrous (“heat”) cycle of about every 4 to 5 days throughout the year, unless they are bred. Female mice are usually receptive to males for about 12 hours of this cycle, typically at night. They also have a fertile postpartum estrous, which means that they can be bred within 24 hours after giving birth.

Gestation in mice lasts approximately three weeks, but can be up to ten days longer if the pregnant female is also nursing a litter. Litter size averages 10 to 12 young. Small litter size is common with a female’s first litter or older females. New litters should not be disturbed for the first few days in order to minimize injury or abandonment by the mother. Baby mice are weaned at about three weeks of age.

Female rats become sexually mature at about 65 days of age. They have an estrous cycle of about every 4 to 5 days throughout the year. Breeding usually occurs at night during a 12-hour period of receptiveness to the male. They also have a fertile postpartum estrous and can be bred within 48 hours after giving birth. However, unlike mice, this usually does not occur because the male should be removed from the cage prior to the female giving birth, in order to avoid injury to the pups by the male.

Pregnancy lasts about three weeks in rats as well. Litter size averages 6 to 12, but smaller litters are common as described for mice. The female and her litter should not be disturbed for the first few days after birth, because a stressed female rat may injure or destroy her pups. Rats are weaned at about three weeks of age. The female will resume her normal estrous cycle within 2 to 5 days after the young are weaned.
© 2004 Midwest Bird and Exotic Animal Hospital