More laboratory mice are used in research every year than any other animal species. They are relatively inexpensive to purchase and maintain. They are efficient breeders for the most part that produce many generations of offspring in a short period of time. They are members of the order Rodentia, which have the feature of long front teeth known as incisors, which grow continuously and must be worn down by abrasion.
Inbred strains are the result of at least 20 generations of brother-sister mating. This causes the genes to become homozygous (same strain) at nearly all the loci on the chromosomes. There are over 400 documented inbred strains of mice. Different varieties are identified by a series of letters and numbers. Common inbred strains include BALB/c, C3H, C57BL/6 and DBA. Two strains may be bred together to produce a mixture of the genes of each strain. The result is called a hybrid.
Outbred mouse stocks are also used in research. Their mating is planned to maintain heterozygosity at the genetic loci. Common examples include CF1, ICR, S and Swiss Webster. Scientific technology has developed the technique of introducing genetic material (DNA) from one animal into the fertilized egg of a different animal. The resulting offspring are called transgenics. These animals are used to study the function of the various genes.
Another technique involves the removal or blocking instead of addition of a gene from the animal. These animals are referred to as knockouts. They are used to study the diseases associated with defective genes. A method of correcting these defective genes (gene therapy) is being developed in animals with hopes that prevention or a cure for humans will result. One naturally occurring genetic defect in mice results in an animal with no hair that is called a nude. These mice have deficiencies in their immune systems and thereby prove valuable in the study of immune diseases and cancer.
Identification
Permanent identification is meant to remain the life of the animal. Various methods can be used including ear tags, ear punch or ear notch. Cage cards must also be used on both individually and group-housed animals.
Handling and Restraint
The most commonly used methods for catching and picking up mice are grasping the animal near the tip of the tail, between thumb and forefinger, or by grasping the tail near the base or the neck between the ends of a smooth tipped forceps. These methods are suitable for cage changing or brief exams. Young mice ( pups) should be grasped by the cupping the hands around the whole body or by grasping the skin across the shoulder blades with a forceps.
For manipulations requiring more restraint the animal should initially be placed on a surface it can grasp, such as a cage top, while it is held near the tip of the tail by the dominant hand. The other hand is used to grab the loose skin at the back of the neck close to the head. The tail may then be held between the fourth and fifth fingers of the same hand. (Figure 1) If the skin is grasped too far from the head the mouse will be able to turn and bite the handler. The mouse must be held firmly but gently, so as not to restrict its breathing. Plastic restraint devices can be used to hold mice and other rodents for longer periods of time.
Using a tunnel or cupped hand to pick up mice causes less anxiety than traditional tail handling. Mice quickly habituate to tunnel handling and can subsequently be restrained by the scruff or tail base for procedures or health and welfare assessments without negating the positive impacts of the non-aversive capture.
Non-aversive methods do not add time to husbandry or procedures provided staff are adequately trained. The investment in training should be outweighed by the benefits observed with more reliable behavioral and physiological responses in the mice.
Physiological Data
Body temperature: 96.6° to 99.7° F (35.8° to 37.4°C )
Heart rate: 328-780 per minute
Respiration rate: 90-220 per minute
Weight: Adult 25 to 40 grams; newborn-1.0 gram
Water consumption: 4-7 ml, or 1.5 ml per 10 grams of body weight per day
Food consumption: 3-6 grams, or 1.5 grams per 10 grams of body weight per day
Feces: Firm, rice-sized, dark brown; amount of food and water consumption influence the amount of feces produced. Abnormal feces are soft and discolored.
Urine: Strong odor and is clear and yellow. Urinate in small amounts frequently.
Life span: 1 to 3 years
Sexing and Breeding
The ano-genital distance (the distance between the anus and genital papilla) is greater in the male than in the female. Sex determination in newborns is best done by comparing several animals at a time. In most adult rodents the testes of the adult males are readily observed between the tail and the penis. Two mating systems are commonly used for mice, monogamous pairing and polygamous or harem breedings. Males can be left with the females or separated prior to parturition.
Females come into estrous or heat, every four to five days and within 24 hours after parturition. The postpartum estrus usually results in a successful mating. Ovulation occurs spontaneously during estrus. Mating of mice is generally detected by the observation of vaginal plugs. These plugs are formed by a mixture of secretions from the accessory sex glands of the male.
Newborns are pink, hairless and helpless. Their eyelids are sealed and their ears are not fully developed. Shortly after birth they develop a distinct white spot visible through their transparent skin on the left side of the abdomen called the milk spot. This indicates that their stomach is full of milk and is a good indicator that they are healthy. By ten days of age the pups are fully furred and their eyes open.
Sexual maturity: 40 to 60 days old
Estrous cycle: 4-5 days; postpartum estrus
Gestation: 19-21 days
Litter size: 6-12
Cannibalism: Try not to disturb newborn litters for several days to prevent the possible destruction of the litter by the adults.
Weaning: 21 days, but may be longer for some litters and some strains.
Behavior
Normal posture is with all four feet on the cage floor, eyes and ears alert. When sleeping the mouse may curl its head under its body. When group housed they may sleep side by side or on top of each other for warmth. Mice are nocturnal so most of their eating and other activities take place when it is dark. They are constantly active except when sleeping due to their high metabolic rate.
Mice groom themselves almost constantly. Lack of grooming behavior or a dull hair coat, decreased activity and a hunched posture are early signs of stress or disease. A behaviorally dominant mouse sometimes bites or chews the fur of a more subordinate mouse, usually in the facial region, which is called barbering. Sometimes the more subordinate mouse may be denuded almost completely by the barbering of dominant cagemates.
Female mice do not usually fight. Group-housed males often fight, with some strains fighting more than others. Housing young male mice together from weaning may help prevent fighting as the animals mature. Aggressive mice should be housed individually to prevent injuries to others. Mice rare vocalize unless stressed. Then they emit a high-pitched squeal or squeak.
Husbandry and Diet
Mice can be housed on solid bottom cages. Solid bottom cages should be washed one to two times per week depending on the number of animals in the cage. Accessories should be washed at least once a week. Temperatures in the mouse room should range between 64 and 79 degrees F with humidity between 30 to 70%. However, mice are subject to heat stress so they should not be kept at high temperatures. Humidity should be kept at 45 to 55%, since too low a relative humidity may cause ringtail in young mice which can lead to tail sloughs. Twelve to fourteen hours of light per day is desirable.
Rodents are fed a laboratory grade feed which is nutritionally complete. With some slight variation, the requirements of the laboratory mouse are as follows: protein, 20%; fat, 6% ( for certain breeders 11%); and carbohydrates, 60%. The pellets are firm and require gnawing by the animals, which keeps their incisors worn down. Occasionally due to dietary, genetic or anatomical causes, these teeth may become overgrown and interfere with the animal’s ability to eat. They can be trimmed to temporarily correct the problem.
Mice are generally fed and watered ad libitum (meaning they are given a continuous supply of food and water). Mice are nocturnal animals and most of their eating and other activities take place when it is dark. Mice require more food per kg body weight than any other laboratory animal because their bodies use large amounts of energy to maintain body heat.