Stress at an early age can have important consequences on the behaviour and brain development of an animal. For example, in their 2000 publication in Primates, Drago and Thierry found that when baby Tonkean Macaques were separated from their mothers they became aggressive and depressed. Early adversity also has long-term effects on the hypothalamus–pituitary–adrenal axis; a neural system that, through the release of cortisol, prepares the body to deal with stressful situations. However, when adversity is sustained for prolonged periods of time the initial cortisol high fades and is followed by abnormally low levels of cortisol release. So, what happens when, after early misfortune, life resumes its normal course? Can the consequences of early adversity on behaviour and brain development be reversed? A study recently published in PNAS by a group of Chinese researchers led by Xiaoli Feng reveals that the behavioural and neurological consequences of mother separation at birth will last for at least 3 years, but probably for life, after the initial stress.
The scientists examined the behaviour and cortisol levels in 22 Rhesus monkeys that had been reared by their moms and 13 that had been separated from their mothers at birth. The babies were taken from their mothers because they were at risk if they had stayed with their moms. For example, some of the mothers were very inexperienced and incapable of properly caring for the baby, some others did not produce enough milk and in the case of a few babies, cold rainy weather at the time of birth threatened their health. Then, after a few months, all the mother-reared and mother-separated monkeys were moved to a communal indoor–outdoor facility with no adults simulating the primates' normal social environment.
To investigate the long-term effects of mother separation on cortisol secretion and behaviour, Feng and her team analysed the levels of cortisol from hair samples taken from the monkeys and evaluated different types of behaviour at 2 and 3.5 years of age. In addition, the researchers investigated cortisol secretion induced by sudden stress by analysing cortisol levels in the blood of the monkeys during the first 30 min after capture.
The cortisol levels in the monkeys that had been separated from their moms at birth were low even after 3 years of normal social life; this was consistent with the low cortisol production observed in animals exposed to prolonged stress. In addition, the mother-separated monkeys had a delayed response to sudden stress, in that the peak of cortisol present in the blood occurred later than in the mother-reared monkeys; this may be disadvantageous when the animal is trying to cope with imminent threat. The monkeys that had been separated from their mothers at birth also showed behavioural signs of anxiety, such as sucking on their digits, pacing and grasping parts of their own body, and were less prone to sit along side their peers. Furthermore, these monkeys were less likely to move than their mother-reared counterparts.
The results from this study are unique in that they show that the effects of stress at an early life stage cannot fully be reversed by subsequent exposure to a normal ‘unstressful’ life and that early adversity can have long-lasting consequences on the physiology and behaviour of an animal. The good news is, the better we understand how psychiatric problems develop, the better equipped we will be to deal with them.
