Rats 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. The frequency of cage changes depends upon the cage type, type of bedding, and number of animals housed in the cage, but usually is carried out once a week. To minimise the risk of exposing animals to rodent pathogens that could be present in the facility, the 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 and are given in table 2. This Directive also specifies environmental conditions, with room temperatures of between 20-24ºC, a relative humidity of 45-65% and a 12:12 light-dark cycle.
Breeding programs
If breeding is not intended, rats should be housed in single-sex groups. It is easy to distinguish males from female rats when adult, from the appearance of the external genitalia. Sexing neonatal animals is more difficult, and requires assessment of the ano-genital distance, which is larger in males than females (see Laboratory Rat Biology).
Animals can breed as early as 4 weeks but are not usually placed into breeding groups until females are 8 weeks and males 12 weeks of age. The breeding program used may vary depending upon the rat strain.
To maintain inbred strains (see Laboratory Rat Biology) 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 maximize genetic diversity and to avoid inadvertent inbreeding, a scheme for selection of breeding stock is essential and a number of these schemes are available.
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, maximizing the number of litters produced. Litter sizes range from only one or two to over twenty, but most rats have 10–16 pups in each litter, with outbred rats usually having larger litters than inbred strains. Rats continue to breed until they are 12–18 months old, but are usually maintained in breeding colonies only until they are about 9 months of age. Productive breeding age is determined by monitoring reproductive performance.