In an effort to maximize production, minimize stress on the animals, and adhere to animal welfare policies and regulations, it is important to establish an appropriate breeding program. The objective(s) of the study, the peculiarities of the strain(s) involved, and the experience of the individual(s) responsible for maintaining the breeding program must be taken into consideration when developing a breeding program.
Several mating systems can be used, but cages may not house more than one adultmale and twoadult female mice that are left together continuously. Such a system facilitates production of offspring for lines that are difficult to breed and maintain. If not properly managed, this mating system can severely overcrowd the cage and can lead to changes in metabolic and physiologic processes, alterations in disease susceptibility (Broderson and others 1976, Schoeb and others 1982; Vesell and others 1976), and may adversely affect scientific research. In an ideal situation, each pregnant female should be moved to a separate cage to have her litter. If this is not possible, the next best method to help reduce overcrowding is to separate at least one of the moms and her pups immediately after birth. At any time, no more than one male and two females with litters can be housed in a cage. Occasionally, breeding trios will produce large litters that will strain the caging system. If at 14 days there are over 14 animals in a cage the two litters must be separated. If pups are not weaned in a timely manner, severe overcrowding may occur. This is due to postpartum breeding, which will produce another litter when it is time to wean the first litter.
Normal housing of mice allows five adults in a cage. One male and four females may be housed in breeding cages and plugged or pregnant females removed to separate cages. If this system is used no more than two females with litters are permitted to be housed together.
This policy applies to all breeding programs, including transgenic mouse production colonies. Any modifications to this policy must be made to the Animal Care and Use Committee and receive approval. An appropriate scientific justification will be required to obtain an exemption.
Accepted weaning age for most strains of mice and rats, regardless of mating system, is 21 days. However, some genetically altered animals benefit from weaning at a later age. Regardless of the system used or the approved weaning age, it is the PI’s responsibility to ensure prompt disposition of overcrowded cages. Details are provided below:
Investigators and their staff have the primary responsibility for weaning litters on time. The animal care staff is responsible for identifying overcrowded cages and following notification of the PI, separating animals in overcrowded cages, if not resolved by the investigative staff.