"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," said 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. "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 behavior and temperament. The study found that infants whose mothers had higher levels of milk energy soon after their birth coped more effectively (moved around more, explored more, ate and drank) and showed greater confidence (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.

Rhesus macaques are found throughout mainland Asia: from Afghanistan to India and from Thailand to southern China. A free-ranging colony of rhesus macaques was established in 1938 on Cayo Santiago-a small island off of the east coast of Puerto Rico. The only primates with a broader geographic distribution than rhesus macaques are humans.

Source: Smithsonian