The research, conducted by scientists from the Smithsonian Institution and the University of California, Davis, points out that this signal may program the infant's behaviour and temperament according to expectations of available resources and discourages temperaments that prove risky when food is scarce.
For the study, researchers used large groups of rhesus macaques living in an outdoor enclosure at the California National Primate Research Center at UC Davis. Researchers collected milk two different times from 59 mothers: once when their infants were 1 month old and again when the infants were 3 1/2 months old. They recorded the quantity of milk produced by each mother, and the energy value of each one's milk was analysed for its sugar, protein and fat content. These figures were combined to calculate the available milk energy generated by each mother.
While all the monkeys were fed the same diet, the scientists discovered natural variation in the quantity and richness of the milk produced by the 59 mothers. Milk from mothers who weighed more and had had previous pregnancies contained higher available energy when their infants were 1 month old than the milk of lighter, less experienced mothers.
Katie Hinde, the study's lead author and anthropologist at the California National Primate Research Center and the nutrition laboratory at the Smithsonian's National Zoo, said: "This is the first study for any mammal that presents evidence that natural variation in available milk energy from the mother is associated with later variation in infant behavior and temperament.
"Our results suggest that the milk energy available soon after birth may be a nutritional cue that calibrates the infant's behavior to environmental or maternal conditions."
At 3 to 4 months old, each infant was temporarily separated from its mother and assessed according to its behaviour and temperament. It was seen that infants whose mothers had higher levels of milk energy soon after their birth moved around more, explored more, ate and drank, and were more playful, curious and active. Infants whose mothers had lower milk energy had lower activity levels and were less confident when separated from their mother. Mothers and infants were reunited immediately after the experiment.
The study has appeared in the American Journal of Primatology Feb. 16. (ANI)