Rodent Weaning

Last Review Date: January 6, 2021


I. Purpose

The purpose of this standard operating procedure is to define the Laboratory Animal Resources (LAR) rodent weaning policy and procedure.

II. Responsibility
It is the responsibility of all animal users, including investigators, their technicians, and LAR Staff to wean rodent pups at the proper time to avoid overcrowding in cages.

III. Weaning Policy

  • All rodent pups should be weaned by 21 days after birth (see special circumstances below).
    • The gestation period is 21 days for mice and about 22 days for rats.
    • The day pups are born (found) is day 0, the next day is day 1.
    • Due to post-partum (after-birth) estrus, female mice and rats can deliver their next litter at the same time the previous litter is ready for weaning.
  • A "Special Services Fee" will be assessed if the investigator chooses to have LAR staff perform rodent weans.
    • Service Fee forms are located on the clipboard on the door of each rodent room.
    • LAR staff will fill out the date, time, procedure, and their name on the form.
    • At the end of the month, the Service Fee form is given together with the cage census forms to the IACUC Coordinator and reviewed before being sent to Budge & Finance for billing.
  • If 21 days pass after the birth date of the last litter and LAR staff has not been notified by the PI of special circumstances, LAR will wean the pups on day 23 and charge the Special Services Fee.
    • A special circumstance in which weaning could be delayed by the PI or AV includes low birth/development weights of certain strains of rodents, large litters, or transgenic mice.  
    • Supplemental food and dry food on the bottom of the cage can help with slow pup growth.
  • If "pups on pups" occurs, LAR staff will wean the older rodent pups immediately and assess the Special Services Fee after notifying the PI.
    • This avoids cannibalism and overcrowding.
  • A PI should notify LAR staff in advance if he/she chooses to have LAR perform their weans.

IV. Weaning Procedure

  • Count pups to be weaned.
  • Have enough clean cage set-ups to house all weaned pups.
  • Separate pups by sex - males in one cage and females in another.
    • Place no more than 5 mice in one cage.
    • If there are more than 5 pups of one sex to be weaned, split the same-sex pups evenly between 2 clean cages.
  • Place a small handful of rodent chow (moisten with water if pups are tiny) in the bottom of the new cage and fill the hopper with dry food; add a water bottle.
  • Always put a sticky note on the cage stating "RE-SEX" if you have any doubt of sex.
  • Notify LAR staff to monitor small pups and put additional food pellets on the bottom of the cage.
  • Put a cage card on the cage.
    • Fill out appropriately: parents, date of birth, date of wean, sex, number of animals in the box, species, source, PI, and AUP number.
    • Use the appropriate cards for each investigator.
  • Put cages onto their specified stock rack.

V. Foster Care

  • Young animals, particularly valuable ones, will be given foster mothers under the following conditions and with PI approval:
    • Mother's death
    • Mother has no maternal instinct
    • Mother has a physical or genetic abnormality that prevents caring for young
    • Mother has low/no milk production
    • Mother may tend toward cannibalism
  • Early fostering increases the chances of raising healthy pups
  • Always check and confirm with P.I. before fostering, especially when dealing with a colony with multiple strains
  • Two important conditions for successful foster
    • litter size close to original birth litter size
    • close in age to foster mother's own pups - within 48 hr is best
  • Steps to introduce pups to foster mother (subject to change)
    • Wear clean gloves
    • Place pups in foster mother's nest on top of existing pups. Scent transfer is important for acceptance
    • Check back 2-3 hr later to see if the pups have been accepted and are nursing.  A different foster mother may be required if the pups are not accepted.