Techniques for Fostering Mouse Pups (from the Jackson Laboratory)

Newborn litters are sometimes fostered onto nursing surrogate mothers for a variety of reasons. The following is a compilation of comments, suggestions, and proven techniques that can be used to foster a litter successfully. Approaches used to ensure that the foster mother will accept the new pups vary, and the method that works may depend upon individual preferences and the reason for fostering.

The first step in fostering is the selection of a suitable foster mother. Try to choose a mother that has weaned a litter successfully in the recent past. For best results, it is important to match the age of the litter to be fostered with the age of the foster mother's natural litter. The natural litter should be of a different coat color than the foster litter so the pups can be separated at weaning. If, however, the entire natural litter is removed and replaced with the foster litter, this is not necessary. Always be sure the foster mother has finished delivering her young before introducing the foster litter, because sometimes one or two pups may be born as long as six hours after the majority have been delivered.

It is critical that the size of the foster litter be as close as possible to the natural litter size. If the litter to be fostered is especially large (more than 10 pups), the pups may need to be divided between two foster mothers. A change in litter size of +/- 2 or more pups can affect the milk supply of the foster mother.

It is best to leave the foster mother in her cage, remove the natural pups, then add the foster pups to the cage. Remove the entire nest with the foster mother's pups in it. Place it under a heat lamp or another source of heat. Place the pups to be fostered in the nest and gently mingle the pups together to spread scent. Rub feces from the foster mom on the backs of all the pups; she'll lick them off and will accept the pups as her own. Return the nest to the cage of the foster female.

The mom and her foster litter can be transferred to a clean cage the next day. Disturb the foster mother and her new litter as little as possible for the first 24 hours, but do check during this time to be sure the litter is being cared for.