Biomethodology of the Mouse - Written by laboratory personnel as guidelines and information for other staff working with mice. Remember that these guidelines are intended for use with mice in scientific research and not pets; however some of the information is still relevant.

General Biology, Genetic and Miocrobial Categories, Behaviour and Biological Characteristics and Data • Basic Biological Data and Basic Husbandry • Identification, Handling and Restraint • Transport, Sexing, Breeding and References

Genetic and Microbial Categories:
There are two common methods by which to characterise laboratory mice; Genetics and Microbial Flora. Common genetic categories are ‘random-bred’ mice which are managed to maintain genetic diversity by mating unrelated mice; ‘Inbred’ mice which are managed to maintain genetic homozygosity by breeding siblings; ‘F1 hybrid mice’ in which two inbred strains are crossbred for one generation; ‘transgenic mice’ in which specific genetic material has been introduced into the genome of another inbred mouse strain and ‘mutant’ mice which are inbred mice that have developed genetic mutations.

The microbial flora of mice can be used to group mice: Specific Pathogen Free Mice (SPF) mice are free from known bacterial, viral, and parasitic mouse pathogens, as opposed to ‘conventional’ mice, which are not known to be free of pathogens. To maintain the SPF microbial status of mice requires that animals be housed in more stringent conditions that prevent the introduction of rodent pathogens (i.e. barrier housing). Other less common microbial groups are axenic mice (i.e. free from all microbial organisms) and gnotobiotic mice, which have a known microbial flora.

Behaviour:
The laboratory mouse is a docile animal and can be easily handled. Animals that are grouped soon after weaning usually coexist peacefully. However, some strains of mice (i.e. BALB/cJ, SJL/J, HRS/J) will begin to fight even if grouped at weaning. Breeding males that have been removed from breeding cages and caged together will usually fight. Wounds on the tail are a good indicator of aggression between cage mates.

Biological Characteristics and Data:
Mice, like most species, have a circadian rhythm (a 24 hour cycle of waking and sleeping etc). The adult mouse weighs approximately 40 grams and this small size and resulting large surface area/body weight ratio makes them susceptible to changes in environmental conditions. The core body temperature is easily affected by small changes in temperature, which may modify the physiologic responses of the animal. The acute hearing of mice makes them highly sensitive to ultrasounds and high-pitched noises inducing a stress response that has been empirically related to cannibalism of pups by their dams. The well-developed sense of smell is used to detect pheromones used in social interactions. The poor vision of mice makes them unable to detect colour and red light is often used to observe animals during the dark cycle.

Basic Biological Data
Adult body weight: male 20-40gm
Adult body weight: female 20-40gm
Body surface area: 10.5(wt. in grams)
Life Span: 1.5-3 years
Food consumption: 15 gm/100 gm/day
Water consumption: 15 ml/100 gm/day
Breeding onset: male 50 days
Breeding onset: female 50-60 days
Gestation Period: 19-21 days
Body Temperature: 36-37C (adult), 36.5-38.0C (infant)
Heart rate: 500-600 beats per minute (adult), 325-780 beats per minute (infant)
Respiratory Rate: 84-230 per minute (adult), 60-220 per minute (infant)

Basic Husbandry:
Most mice are housed in shoebox cages composed of polypropylene (opaque) or polycarbonate material (clear) with a wire bar lid used to hold the water bottle and feed. Bedding is placed directly into the shoebox cage allowing the absorption of urine and the animal to burrow and/or nest. This type of cage will hold four to five adult mice depending on the size of the cage. When removing the lid from this type of cage it is important to remove the water bottle to prevent spillage. If the cage is to be transported the bottle should be turned sipper tube up to prevent spillage during transport. However, you should remember to turn the bottle back over to allow access to water after transport.

The care taking staff change the cages twice a week, thereby providing the animal a clean cage with new bedding, food and water. Water bottles and feed hoppers are checked daily by care takers to ensure the provision of water. Suspended cages are occasionally provided with automatic watering that is supplied by a nipple valve (i.e. lixit) located in the back of the cage that is operated by animal contact. When replacing a suspended cage that is provided with automatic watering it is important to push the cage fully into the rack care to insure that the lixit fully extends into the cage, allowing the animal access to water. Occasionally a mouse will jam the lixit open resulting in a constant dripping of water from the lixit, saturating the bedding material. Upon observation the bedding will appear much darker.

Pelleted natural ingredient diets used to feed all rodents are composed primarily of cereal grains that are supplemented with additional protein, vitamins and mineral. Due to the nature of this type of diet the exact composition can vary substantially from lot to lot. The water provided to animals is chlorinated tap water.

Some mice are housed to prevent the acquisition of rodent pathogens. Rodent pathogens often do not produce clinical signs in affected animals but often have an immunomodulating effect. Therefore, this type of housing is important for studies that involve the immune system. Immunocomprimised animals, which are sensitive to opportunistic agents, are also housed in this fashion to allow for their long-term survival. These animals are housed in sterilised cages and are provided sterile food and water. Access to this housing area is limited to prevent inadvertent fomite transmission of rodent pathogens by personnel.

Identification:
Cage cards are utilized to identify the strain of mouse, sex, number, principal investigator, and research protocol. Cage cards should not be removed from the cage to avoid misidentification of the animals. Temporary identification of individual mice can be accomplished by pen marks on the tail, hair clipping or dyeing the fur. Pen marks will only last one to two days whereas hair clipping may last up to 14 days. Permanent identification methods can be achieved by tail tattooing which will be performed by husbandry personnel upon request. Tail and toe clipping are not recommended. Ear punch identification can be utilised but may be obliterated by fighting between individuals.

Handling and Restraint:
When handling mice it is advisable to wear gloves to prevent the development of allergies due to direct contact with animal allergens. Mice are usually caught and lifted by the tail. The tail should be grasped between its midpoint and the mouse's body. The tail may be grasped with the thumb and forefinger. With this simple method of holding, they may be transferred to another cage, a balance, identified, examined casually or sex may be determined.

Pregnant mice or very obese mice may be handled by this method but they should be supported by use of the second hand placed under their feet. However, such restraint is not sufficient for treatment and close examination. For more effective control, the mouse may be held by the tail and placed on a table or other surface, (preferably one such as a wire cage lid that the mouse can grasp) and the loose skin over neck and shoulders grasped with thumb and fingers. It is necessary to perform this manoeuvre expeditiously, or the mouse may turn and bite. Once the mouse is grasped correctly, the head is adequately controlled. Restraint is improved if the tail or the tail and rear legs are held by the third and little fingers of the same hand or with the other hand. Mice should not be dropped into the cage, which may result in spinal fracture, but should be lowered into the cage and released upon contact with the bedding.

Mice less than two weeks of age can be handled by grasping the loose skin over the neck and shoulder with thumb and forefinger or smooth tipped forceps. Handling neonatal mice should be avoided especially during the first few days after birth to avoid cannibalism or litter abandonment. If it is necessary to handle the litter, remove the dam to a separate cage and handle the neonates using plastic gloves to avoid contamination with human scent. Multiparous females are less likely to cannibalise if they have been successful mothers and should be chosen if litter manipulation is necessary.

Numerous types of restraint devices are commercially available to restrain mice. Quality devices prevent the animal from turning around yet allow easy access to the tail or legs. Devices should also be easy to clean and provide adequate ventilation. For tail vein injections a wire box cage top can be turned over and the tail gently passed through the wire bars preventing the mouse from turning.

Transport:
Mice that are moved indoors can be transported in their cage after removing the water bottle and placing it upright in the cage lid to prevent spillage. It is recommended that a permeable drape be placed over the cages to darken the cage and prevent over arousal of the mice during transport. Mice cannot be transported out of doors or by vehicles by other than husbandry personnel.

Sexing:
Male and female mice can be differentiated by observing the distance from the anus and genital papilla, which is greater in males. This difference is also present in neonatal mice.

Breeding:
Mouse colonies tend to be very labour intensive and are discouraged for the production of commercially available strains of mice. The cost of breeding mice for research use far outweighs the cost of purchasing from commercial vendors. Also, it is very difficult to maintain the genetic heterozygosity required for outbred mice in small colonies and to prevent subline divergence when breeding inbred mice. For strains not available commercially or in instances where neonatal or foetal animals are required, breeding colonies are maintained. Investigators requiring the establishment of breeding colonies should consult with the Animal Care Unit to assure proper management of the colonies. Breeding animals have different nutritional requirements, which if not provided, will have a detrimental effect on reproductive performance. Light cycles are important in breeding and mice are provided with 12 hours of light and 12 hours of dark. Deviations from this cycle will effect reproductive performance.

Top of page