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Culling & Fostering

Lactation & Culling

The most important factor in the growth of baby mice is the amount of milk available to them. Thriving newborns will begin to nurse immediately after birth and within a matter of hours it is possible to clearly see the milk in their stomachs through their translucent bodies. When little or no milk is present by six hours after birth, it is likely that something is wrong with either the pup or the mother.

For litters of four or more pups, the amount of milk produced by a lactating mother will increase with the number of young, but the increase will not be proportional (Grüneberg, 1943). However, if there are two or more mature females in a cage, all may start to lactate in response to a single litter. When there is a large number of pups some breeders feel that a single mother will not be able to provide the nourishment required for the optimal growth and development of all. Thus many breeders feel it necessary to cull litters soon after birth. In some cases, it may be that the breeder will wish to select pups according to sex, coat colour or markings (at day two to four), or other factors such as size and type. To ensure optimal growth of selected animals, breeders recommend that litters should be culled to five to six pups during the first days after birth. A further reduction to three to four pups can then be carried out later if desired. Not all breeders use this method, and it is of course up to the individual as to what their breeding policies are.

With the common strains of mice, it is not often the case that only one or two pups will be born in a litter. However, this situation can arise more frequently with the breeding of mice with lethal mutations, and it is also problematic. Especially for first-time mothers, one to two pups may not provide the level of suckling stimulation required to effectively stimulate milk production.

Left: 9-day-old pups and Right: Doe standing guard over her nest on day 9

When newborn animals are not receiving sufficient amounts of milk and it is likely that the mother is the problem, one can consider fostering as a last resort. The foster mother should be an experienced female with her own newborn litter. After removing the mother from the cage (and any other adults present), the pups to be fostered should be rubbed in the dirty litter of their new cage and placed at the bottom of the nest underneath their foster siblings before replacing the mother.

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