Effects of litter age, litter size, and ambient temperature on the milk ejection reflex in lactating rats.

Abstract

Under a number of circumstances the duration of mother-young contact in the rat can be drastically reduced while pup growth is maintained. We investigated the possibility that under some of these conditions the timing of milk delivery is changed. We assessed the time of onset and pattern of milk delivery over a nest bout in litters of two different ages (Day 4 or Day 10) and three different sizes (4, 8, or 12 pups). Ambient temperature fluctuated over the course of the study and was found to correlate significantly with the latency to the first milk ejection (ME). Specifically, the warmer the ambient temperature, the quicker the first ME occurred. Durations between successive MEs correlated with ambient temperature in a similar fashion. Analysis of covariance revealed that the onset of the first ME occurred earlier in Day 10 litters than in Day 4 litters, but litter size had no effect. Durations between successive MEs were not affected by either litter age or litter size. These data indicate one possible mechanism by which adequate milk delivery can be maintained despite some reduction in mother-young contact.