Early infant stress on rhesus monkeys produces long-lasting and irreversible biological and behavioral changes, according to a recently published study by Chinese researchers.

Seeking to determine if the deleterious effects of early adversity could be reversed by later experiences, scientists studied the physiology and behavior of monkeys who were separated at birth from their mothers but then reared in normal primate environments.

Importance of Motherhood

Two groups were evaluated for the study: a mother-reared (MR) group of 22 monkeys, and a peer-reared (PR) cohort consisting of 13 animals. The MR infants lived with their biological mothers in small primate groups for the first four months of life. They were each then relocated to an indoor cage with another MR mate of the same age.

The PR monkeys, separated from their mothers at birth, were raised alone in incubators for the first month, with a blanket to cling to and regular hand-feedings. The maternal separation occurred because of certain conditions that threatened the infants' survival. Those included having first-time mothers who could injure their babies from inexperienced handling; having mothers who produced insufficient breast milk; or being born during cold, rainy weather that would subject them to illness or death. After the first month of life, those in the PR group were paired in a cage with a fellow PR monkey of similar age.

After approximately seven months of age, both the MR and PR monkeys were relocated to connected indoor-outdoor colonies and reared together in mixed-sex social groups without adults. They were then evaluated at one-and-a-half, two, and three year intervals to assess biological and behavioral patterns.

In seeking to determine if the PR monkeys could recover from the trauma of being separated at birth from their mothers, the team measured the monkeys' hair cortisol levels. Blood cortisol levels were also taken immediately after the monkeys experienced the stress of being captured and restrained. This enabled researchers to investigate the monkeys' biological response to stress, which like in humans, should be indicated by a sudden surge of the cortisol hormone.

Monkeys without Mothers Showed Delay Stress Responses

Results revealed consistent and natural changes in basal cortisol levels in the MS monkeys, but in the PR monkeys, those same levels were significantly decreased. Also, the PR monkeys' blood cortisol response levels to the stressors were significantly delayed.

Abnormal behavior was also noted in the PR monkeys. Specifically, those in the PR group were far less social, initiating significantly less physical contact with their fellow primates. They also engaged in pacing, digit sucking and self-grasping behaviors at far greater frequencies than their non-traumatized counterparts. Researchers noted that these behaviors may indicate a state of high anxiety as subjects typically engage in such actions to alleviate stress.

The findings, just published in the journal Proceedings of the National Academy of Sciences, suggest that the effects of early trauma on monkeys may be irrevocable, even when later experiences are favorable. The team further concluded that maternal separation among primates mimics human adversity and is therefore a good model for examining the role of early trauma in the development of psychiatric disorders.

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