Mice and Rats - General Care

Handling and Restraint

Housing

Hygiene

Food and Water

Breeding Considerations

Disease and Prevention

Handling and Restraint
As stated above, domestic mice and rats generally tolerate gentle handling, though both may bite if startled or handled roughly. Mice are more likely to bite than rats under these circumstances. In fact, mice housed alone are more likely to be aggressive with a handler than those housed in groups. Cage territoriality (possessiveness) may be exhibited by some pet rats. We have encountered a large number of pet rats, normally docile in nature, that attack the fingers or hands of a handler opening and entering its enclosure. This aggressive behavior is not noted when attempts to pick up these rats are made outside of their enclosures.

It is customary to pick up a mouse by gently lifting it up by the tail and placing it into a cupped hand. If a more secure hold is necessary (giving medications or food orally), the handler may grasp or pinch as much skin as possible over the neck, just behind the head. The mouse can then be picked up and turned over on its back by rotating the wrist. The tail can be restrained by gently grasping it between the fourth (ring) and fifty (pinkie) fingers of the same hand.

Rats can be lifted by their tail but great caution must be exercised in doing so The skin of a rat's tail can easily tear, so it is best to grasp only the base of the tail. Further, suspending the entire weight of a rat by its tail is, no doubt, painful for the rat. Therefore, this practice should be only momentary. Tail-lifting a rat that is grasping a fabric (wire mesh, etc.) may injure the tail and may also break or tear the toenails.

The best way to pick up a pet rat is to place one hand over the back, just behind the head, gently grasp it around the ribcage, and lift it upward. The rat can then be gently cradled against the handler's body, using minimal restraint.

Potentially aggressive or known vicious rats and mice can be captured and restrained using gloves and small towels to protect the hands of the handler. Cage-aggressive rodents should be allowed to come out of their enclosure before an attempt is made to pick them up.
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Housing
Proper housing is a major factor in the maintenance of healthy mice and rats. The psychosocial well-being of the animals must be a primary consideration. Mice and rats can be housed within enclosures made of wire, stainless steel, durable plastic or glass. The last 3 materials are preferred because they resist corrosion. Wood and similar materials should not be used in construction of enclosures because they are difficult to clean and cannot withstand the destructive gnawing of rodents. The construction and design of the enclosure must ensure that the resident(s) cannot escape. Further, the enclosure must be free of sharp edges and other potential hazards.

The enclosure must be roomy enough to allow the rodents to pursue normal movement and breeding activity, if the latter is desired. Visual security (a place into or under which the rodents can retreat for privacy) should be provided, as well as exercise wheels of optimum mental and physical health. Rats, in particular, ten to be burrowers and seem to enjoy hiding under things for extended periods.

Enclosures should be easy to clean, well lighted and adequately ventilated (see vital statistics for preferred temperature and relative humidity ranges). Bedding must be clean, nontoxic, absorbent, relatively dust-free and easy to replace. Shredded paper, wood shavings and processed corn cobs are preferred bedding materials. Cellu-Dri (Shepherd Specialty Paper, Kalamazoo, MI) is a pelletized cellulose bedding that is nontoxic and dust free. Tissue paper or cotton are often supplied to breeding rats for nest-building material.

Pet mice and rats seem most comfortable when they are spared exposure to excessive noise, needless excitement and confusion, and other similar or perceived stresses. Sudden environmental temperature changes should also be prevented because pet rodents do not tolerate them well.

Mice can be aggressive toward one another, so great care should be taken when housing more than one mouse within the same enclosure. Newly assembled male groups and new males entering established territories, in particular, are likely to fight, so it is wise to always house male mice separately. Domestic female mice seldom fight unless they are defending their nests.

Rats are more communal and, in contrast to mice, several males and females may be housed within the same enclosure, provided that it is roomy enough. In fact, young rats are raised by the group and nursing responsibilities are shared between females. These nursing females may fight among themselves, Males may occasionally bother the young, but aggression between rats is generally infrequent (in contrast to mice).

Every effort to prevent the escape of pet rodents should be made because they can be a tremendous nuisance when allowed the "run of the house." Escaped rats tend to eventually return to their enclosures, whereas escaped mice tend to fend for themselves within the home and do not return to their enclosures.
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Hygiene
The frequency with which the enclosure should be cleaned depends on its design, the materials out of which it is made, and the number of rodents within it. As a general rule of thumb, however, the enclosure and all cage "furniture" should be cleaned and disinfected once weekly. The food and water containers should be cleaned and disinfected once daily. More than one set of containers should be maintained, and the soiled set should be washed in a dishwasher, if possible.

Vigorous scrubbing of the enclosure and "furniture" with hot water and soap and a thorough rinse should be followed by the use of a disinfectant. Vinegar is often required to remove the scale deposited by rodent urine.
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Food and Water
Good-quality food and fresh, clean water must be readily available at all times. Laboratory rodent chows (milled pellets or blocks) are preferred. These foods are readily available from feed stores, pet hops, and suppliers or users of such commercial diets. Kibble-type kitten chows can be substituted. The rodent diets containing seeds and nuts are not recommended because they contain too many fats and oils, provide inadequate protein levels, and are not necessarily balanced. Obesity is a common problem with pet rodents (especially rats). Consequently, oil-rich and high-fat foods must be avoided. Healthy maintenance of small pet rodents depends upon their receiving foods with relatively high protein levels (16% or more), no more than 4% fat, and at least 18% fiber. Seed/nut-based diets generally fail to meet this requirement.

Table scraps and alternative foods can be offered to pet mice and rats, but these should be limited to healthful items (whole-wheat bread, nonfat yogurt, fresh fruits and vegetables, lean protein sources such as tuna, chicken, etc.) and should not exceed 15% of what the pet consumes daily. Alfalfa hay, grass hay or hay blocks should be available in the enclosure at all times. If the above feeding recommendations are followed, malnutrition and related problems are very unlikely among pet rodents. Vitamin or vitamin/mineral preparations and salt blocks (licks) are generally unnecessary.

The food can be "dispensed" from a specially designed wire or mesh cage top that provides a generous depression into which the dry food is supplied and through which the food can be eaten by the rodent(s). This food-delivery system obviously depends upon the animals' ability to easily reach the food by standing on their hind legs. This type of arrangement is, therefore, not recommended when there are juvenile rodents within the enclosure preparing to wean. This food-delivery system is used most often in laboratory situations. It has 2 major advantages. One is that there is much less wasted and discarded food. The other is that there is little opportunity for fecal (stool) and urine contamination of food. Metal "hoppers" can be used for dispensing food or it can simply be placed in heavy ceramic crocks (preferred because they cannot be easily tipped over) or similar containers.

Water is most easily made available and kept free from contamination by providing it in water bottles equipped with "sipper" tubes. The tubes can become clogged with food debris, so they must be checked daily. The dispensing end of the tube must be accessible to the smallest rodent within the enclosure. Before juveniles are fully weaned, they begin drinking water and eating pelleted foods, so these essentials must be accessible to them at this time. Many deaths involving very young rodents of this age are due to starvation and dehydration.

Food consumption varies with the quality of the food(s) offered, age, health and breeding status of the individual, the environmental temperature, and the time of day. Both mice and rats tend to feed at night, though daytime feeding among both is quite common. Mice are voracious feeders and consume proportionately more food per day than rats. This is because of their smaller body size and relatively high metabolic rate. Rats tend to be more reserved in their feeding habits. In fact, rats show great caution and selectivity while eating and tend to avoid unfamiliar foods.
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Breeding Considerations
Sexing: Sexually mature mice and rats must be properly paired to breed successfully. A single male mouse may be included in an enclosure with one or more female mice without difficulty. Including more than one male mouse in this situation invites fighting between them. By contrast, more than one male and female rat may be housed together for breeding purposes within the same enclosure without aggressive displays.

Sexually mature male mice and rats usually exhibit a prominent scrotum. Sexually mature female mice and rats usually exhibit a prominent double row of nipples. Further, the distance between the rectal opening and the penis of the male is greater than the distance between the rectal opening and the urinary opening of the female in both mice and rats.

Mice: Female mice should not be bred before 50 days of age. They are continuously "polyestrous," which means that they come into heat at fairly regular intervals (every 4-5 days) throughout the entire year unless they are bred. The period during which the female is receptive to the male and allows breeding is about 12 hours and usually occurs at night. Female mice can come back into heat within 14-28 hours after giving birth to a litter. This is called a "postpartum estrus," which means that they can be nursing a litter and be pregnant at the same time!

Pregnancy lasts an average of 3 weeks but can be extended as much as 10 days longer if the pregnant female is suckling a previous litter. Litter sizes decrease as breeding females age. Though mutilation and cannibalism of the young are rare, it is wise not to disturb mice for the first 2-3 days after giving birth. Pups are usually weaned at about 3 weeks of age. The female resumes her breeding cycle 2-5 days after her pups have been weaned (unless she was bred during her postpartum estrus).

Rats: Female rats should not be bred before 65 days of age. They are continuously "polyestrous," which means that they come into heat at fairly regular intervals (every 4-5 days) throughout the entire year unless they are bred. The period during which the female is receptive to the male and allows breeding is about 12 hours and usually occurs at night. Female rats can come back into heat 48 hours after giving birth to a litter. This is called a "postpartum estrus." This period of receptivity is not used when breeding rats because they breeding male is removed from the enclosure just before the female delivers her litter because of the high probability of injury to the new pups by the male.

After mating, a white, waxy substance, called a "copulatory plug," is visible in the female's vulva for 12-24 hours. It is not uncommon to find these plugs in the enclosure after they have been discharged.

Pregnancy lasts an average of 3 weeks. Litter sizes average 6-12 pups, though it is not unusual for a female's first litter to be smaller. Litter sizes decrease as breeding females age. Female rats should not be disturbed for the first few days after delivery because stressed females may destroy their pups. Excessive handling, loud noises, and even insufficient nesting material have all been implicated with this destructive behavior. Pups are usually weaned at about 3 weeks of age. The female resumes her breeding cycle 2-5 days after her pups have been weaned.
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Disease and Prevention
Strict quarantine or isolation of all newly acquired rodents for at least 4 weeks greatly helps prevent disease among pet mice and rats. This recommendation is especially important for pet rodents because of the severity of certain diseases that they may harbor without showing signs of illness.

Mice and rats should be purchased from reputable sources. The prospective pet owner should never purchase an obviously or even suspiciously ill rodent. Further, it is never wise to purchase an animal that has been in contact with one appearing ill, even though the intended purchase appears perfectly healthy. These risky purchases never have happy endings and sometimes unnecessarily expose healthy pet rodents to serious and even life-threatening disease.

Laboratories associated with universities, colleges and research institutions most often purchase mice and rats from pathogen-free (disease-free) colonies. This is the preferred source for pet mice and rats, but purchases from these sources are not always practical or possible.

Rats and mice are especially sensitive to the irritating effects of ammonia. This chemical builds up quickly in the bedding from the relatively large volume of urine excreted by pet mice and rats. Bedding must be changed 2-3 times each week, or more often if necessary. Further, ventilation must be adequate to reduce or eliminate the irritating effects of ammonia on the respiratory lining of pet rodents.
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