Species specific considerations – housing and reproduction – mice

Mice are usually housed in either open-topped or individually ventilated (IVC) “shoebox” cages. These cages are usually constructed from plastic and have a metal lid which houses the food pellets and a water bottle. Some caging systems have automatic watering devices. Both food and water are generally provided ad-lib. Animals are usually transferred to a similar, cleaned and disinfected or sterilised cage on a regular basis.

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The frequency of cage changes depends upon the cage type, type of bedding, and the number of animals housed in the cage, but usually is carried out every 7-14 days. Cage changing has been shown to be stressful to rodents, although the majority of studies have been conducted in rats, rather than mice. The frequency of cage cleaning represents a compromise of avoiding disturbance to mice and, for example, decreasing litter size or increasing aggression (see Gonder and Laird, 20071 for review) and compromising the cage environment, resulting, for example, in a build-up of ammonia.

A recent study (Taylor et al, 20192) suggests that replacing the majority of bedding, without a complete change of cage may be a useful approach. Alternatively, a redesign of mouse caging to more effectively meet the needs of mice should be considered (Markowska et al, 20193).

To minimise the risk of exposing animals to rodent pathogens that could be present in the facility, cage changing may be undertaken in a cage-cleaning station.

Recommended floor areas for adults and breeding animals are specified as an annexe to the EU Directive. This Directive and the UK Home Office Code of Practice (Home Office, 2014) also specifies broad environmental conditions that should be appropriate to the species and age of animals housed. The underlying principals are that environmental conditions should not have adverse welfare consequences, and generally should be stable to contribute to producing consistent outcomes from research procedures. Home Office advice is that rodent holding rooms should be maintained within a range of 20-24oC, with relative humidity between 45-65%. The UK guidance is based on “performance” rather than “engineering” standards, with the environment meeting the needs of the animals housed in the facility.

It is important to note that environmental conditions in an animal room may differ considerably. Providing animals with the ability to modify their environment, particularly with regard to environmental temperature can be of considerable welfare benefit. Provision of nesting material, for example, can enable mice to adjust their environmental temperature to their preferred zone of 30-34oC (Gaskill and Garner, 20174).

Breeding programs

If breeding is not intended, mice should be housed in single-sex groups. It is easy to distinguish males from female mice when adult, from the appearance of the external genitalia. Sexing neonatal animals is more difficult, and requires an assessment of the ano-genital distance, which is larger in males than females (see Laboratory Mouse Biology).

Animals become sexually mature between 5 and 8 weeks, and females can become pregnant as early as 4 weeks of age. However, mice are not usually placed into breeding groups until females are 6-8 weeks and males 6 weeks of age, since very young females produce small litters. The breeding program used may vary depending upon the mouse strain. Mice are usually bred for 6-8 months, producing four or more litters, but some strains may have a shorter breeding life because of strain-specific health problems. For example, C3H/HeJ mice may stop breeding early because they have a high frequency of ovarian cysts and tumours.

To maintain inbred strains (see Mouse Strains), animals are bred from monogamous pairs (where a male and female are permanently paired) or in trios of one male and two females. Careful records need to be maintained of the pedigree and breeding performance of each pair, and animal facility staff will usually be able to assist with this.

Outbred strains are usually bred in small groups of 2-6 females with one male. To maximise genetic diversity and to avoid inadvertent inbreeding a scheme for selection of breeding stock is essential and a number of these schemes are available (e.g. Berry and Linder, 20075).

Pregnant females can either be allowed to give birth in the same cage or be transferred to a separate cage. If they give birth and the male is present in the cage, then mating will usually occur around 24h after parturition, so maximising the number of litters produced. Litter sizes range from only one or two pups to over twenty. Most mice have 10-16 pups in each litter, with outbred mice usually having larger litters than inbred strains. Mice continue to breed until they are 12-18 months old, but are usually maintained in breeding colonies only until they are about 6 months of age. Productive breeding age is determined by monitoring reproductive performance.

Given the rapid growth of mouse pups, breeding cages can rapidly become overcrowded and exceed recommended stocking densities. In addition, adoption of breeding systems which can result in mice giving birth before an older litter has been weaned can also have major impacts on the stocking density and welfare of the animals. If your research work involves maintaining breeding colonies of mice, it is always advisable to discuss colony management with animal facility staff. This can help both maintain good welfare standards, and maximise the efficiency of breeding programs, including minimising over-production of animals that cannot be used for research.

Details of managing breeding colonies, and how to calculate the number of breeding pairs needed to produce the required number of mice for studies can be found in The Jackson Laboratory Handbook on Genetically Standardized Mice available from the Jackson laboratory.

Updated on 12th May 2020

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