The behaviour of females at all stages of lactation on finding a strange pup in her cage is described. Pups between seven and fourteen days old are most likely to be retrieved rather than attacked, but factors dependent on the time since the female gave birth also govern her response.

To separate these latter factors into those dependent on the age of the litter and those dependent on internal changes in the female, parts of the retrieving pattern were timed in mothers retrieving their own pups. Some of these animals were living with their own litters, others maintained foster litters of roughly constant age throughout lactation. It was found that the type of behaviour shown depended only on the age of the litter at the time.

Although the foster litters did not grow older and therefore change in their milk requirements, foster mothers did not go on lactating longer than normal. The time they spent away from the nestlings also changed in the normal way. It was concluded from this that the foster mothers were not affected physiologically by their litters.

The relevance of the concept of maternal drive in discussing these factors is considered, and it is concluded that it does not help to explain them.