Spontaneous pup-killing by mice in response to large litters.

Primiparous lactating nice mice whose young were removed on the day of parturition were presented with foster litters of either 12 or 16 day-old young. The adults spontaneously killed young during Day 3-7 of the lactation period, killing more in the 16-pup than in the 12-pup litter. The lactating animal appeared to select the young to be killed on the basis of body weight. It killed significantly more young that were in the bottom half than in the top half of the weight range for its particular litter.