1. The confounding effects of undernourishment and body cooling, resulting from maternal separation, were investigated by separating food and warmth deprivation. Rat pups aged 3–16 d were deprived of food for alternate 24-h periods by removal from the lactating mother. Some of the pups were placed with a foster mother, who kept them warm, whereas others were put in an empty cage at 22° which resulted in a sharp drop in body temperature.
2. Pups which were kept warm showed great fluctuations in weight between periods of starvation and feeding. The cooled pups lost less weight during deprivation but also recovered less on refeeding.
3. The resultant growth rate was much lower in non-fostered (i.e. cooled) than in fostered pups.
4. Up to the age of 8 d, cooled pups failed to raise their body temperature above that of the surroundings and did not digest the milk in their stomachs. Although, thereafter, they were able to raise their temperature to 26° and to digest stomach contents, the extra energy expended resulted in more severe growth restriction.
5. One-third of the pups died at 16 d but the rest were quickly rehabilitated by ad lib. feeding and showed a normal growth rate, although they remained smaller than the controls.
6. The development of nipples, hair, eye opening and vaginal opening was related more to chronological age than to weight.
7. A side effect of cooling was an almost complete abolition of active (REM) sleep, which is normally very high in infants; a slight rebound increase in active sleep was seen at 21 d. Direct as well as side effects of cooling may thus be responsible for some of the observed consequences of maternal separation.