Rodent Breeding

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General Policy

Protocols approved by the IACUC may involve the breeding of mice and rats for research purposes. Rodent breeding activities are the responsibility of the Principal Investigator whose IACUC-approved protocol includes the establishment and maintenance of a breeding colony. Rodent breeding activities include selection of breeding pairs, mating, separation of breeders, and weaning of offspring.

1. Weaning Animals

Rodent pups are normally weaned by 21 days of age. In the case of some transgenic and knockout mice with less robust pups, weaning may be delayed until 28 days of age. Postponement of weaning after 28 days requires scientific justification and must be described in the investigators protocol and approved by the IACUC.

The Principal Investigator is responsible for weaning animals at appropriate times and setting up and labeling the cages for the weaned animals. These responsibilities require that the research staff check each cage as frequently as necessary to avoid overcrowding (see below). When weaning occurs, each new cage must have its own cage card. Information on cage cards must include the IACUC-approved protocol number, name of the Principal Investigator, date of birth and sex of the animals, as well as the name and telephone number of the most current contact person. Pre-printed cage cards are available on request.
Weaning of animals and extra cage set-ups are not included in the basic husbandry provided by the animal facility.

2. Overcrowded Cages

Rodents are housed in accordance with the Guide for the Care and Use of Laboratory Animals.

IACUC has adopted the minimum space dimensions in the Guide, as follows:

  • no more than five mice (the largest of which must weigh no more than 25g body weight) may be kept in a standard shoebox mouse cage (approximately 7.5 wide X 11 deep or 19.5 cm wide X 30 cm deep)
  • no more than four mice if the body weight of the largest mouse is over 25 g per standard shoebox mouse cage; and no more than one adult breeding pair per litter

The IACUC policy for housing rats is as follows:

  • 1 rat per standard shoebox rat cage if the body weight is over 500 g
  • 2 rats per standard shoebox rat cage if the body weight of the largest rat is between 400-500 g.; 3 rats per standard shoebox rat cage if the body weight of the largest rat is between 300 g - 400 g
  • 4 rats per standard shoebox rat cage if the body weight of the largest rat is between 200 g - 300 g;
  • and a maximum of 1 litter with a breeding pair of rats per standard shoebox rat cage. Protocols that involve breeding of animals may quickly result in overcrowding if the animals are not separated in a timely fashion. Overcrowded cages are a threat to the health and welfare of the animals, and may compromise intended research objectives. Therefore, overcrowding must be avoided

To avoid overcrowding of litters, animal care staff will document date of birth on cage cards for breeding animals, and will place an alert card on those cages when animals are within 3-4 calendar days of their required weaning date. If mice are not weaned on day 28 (or later, if approved by the IACUC), they will be separated into single sex groups of no more than five mice (at no more than 25g body weight each) per cage by animal care staff; rats will be separated according to the densities described above if not weaned on day 21 (or later, if approved by the IACUC). The newly generated cages will be labeled with information to locate the parent cage, date of separation, and the animal care technician who performed the separation.
The investigator will be charged a service fee per each additional cage set-up needed to separate animals in accordance with NIH space requirements.

For overcrowded cage situations not involving weaning, animal care staff will identify cages that exceed the maximum number of adult animals allowed per cage. When animal care staff identifies an overcrowded cage, an overcrowded cage card will be affixed to the overcrowded cage notifying the investigator that the occupants of the cage must be separated within three calendar days of the date noted on the card. Animal care staff will notify the Principal Investigator or the designated contact person, by both e-mail and telephone, of the overcrowding at the time of discovery.

If the overcrowding is not addressed within three calendar days following the initial documentation and notification by animal care staff, the animals will be separated into single sex groups at densities required by NIH space requirements. The newly generated cages will be labeled with information to locate the parent cage, date of separation, and the animal care technician who performed the separation. The investigator will be charged a service fee per each additional cage set-up.

When harem breeding (i.e., two or more females per male) is used, the pregnant females must be isolated before parturition. Separating the pregnant females before parturition is essential to limit cannibalism of the pups by the adults. If the sire is housed with the dam, it must be removed prior to birth of the first litter. Maintaining the sire in the cage with the dam is acceptable ONLY if the litter will be weaned at 21 days of age. Otherwise, the dam will become pregnant during post-partum estrus and deliver the second litter before the first litter reaches 28 days of age. This results in two litters of different ages in the same cage and an increased likelihood of deaths in the younger litter. Under no circumstances should any standard mouse cage have more than one litter at a time. Severely overcrowded cages (defined as more than one litter per cage, dam and litter with other adults present in the same cage, weanlings that have not been separated by sex) will be separated at the time of discovery, and the investigator will be notified afterwards and charged for each additional cage set-up.

If an investigator is found to repeatedly have overcrowded cages resulting from any of the causes described above, the IACUC may restrict approval to breed animals until appropriate oversight procedures have been established.

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