The birth of a second child is not only a special experience for the parents, but also for the older brother or sister. Behavioral studies in humans have shown that the change in family setting is a confusing and stressful time for the older child, often accompanied by clingy, depressive states, and tantrums. Until now it was not known to what extent this stress is also physiologically demonstrable.
Verena Behringer, a scientist at the German Primate Center (DPZ) — Leibniz Institute for Primate Research in Göttingen, has explored this question in one of our closest living relatives. In a study she conducted with Andreas Berghänel, Konrad Lorenz Institute for Comparative Behavioral Research, University of Veterinary Medicine, Vienna, and an international research team, she examined several markers in the urine of wild bonobos (Pan paniscus). The researchers found that giving birth to a second young animal resulted in a five-fold increase in the stress hormone cortisol and a reduced immune response from the older sibling. The physiological changes were observable up to seven months after birth and were independent of the usual weaning processes experienced by the young animals with age (eLife).
The research was conducted at the LuiKotale research station in the Congolese rainforest. Two habituated bonobo groups live close to the field station. For more than 650 hours, the researchers observed the behavior of 17 young animals who had become siblings for the first time and who were between two and eight years old when a sibling was born. At the same time, they collected 319 urine samples from the bonobos before and after the sibling’s birth.
“As the young animals grow up, there are several processes of social weaning or food change that can also stimulate stress responses,” said Verena Behringer, a scientist in the Endocrinology Laboratory at the German Primate Center and lead author of the study. “This includes the fact that after a certain point the youngsters stop suckling or are less carried. To untangle weaning with age from the birth of a sibling, we analyzed urine samples and behavioral observations before and after the birth of the sibling. sister in the older bonobo and puts them in perspective.”
Verena Behringer analyzed the urine samples in the laboratory for the concentrations of three different substances: cortisol, neopterin and triiodothyronine (T3). Cortisol is a hormone secreted in response to a stressor, neopterin is produced by the immune system’s activated defense cells, and T3 is a thyroid hormone that regulates metabolic activity in the body. The concentration of these markers in the urine provides insight into the physiological state of young bonobos. The study showed that urinary cortisol levels in the older siblings increased fivefold when their younger sibling was born and remained at that level for up to seven months. At the same time, neopterin concentrations decreased, indicating a reduced immune response. The thyroid hormone T3, on the other hand, showed no significant change.
“The young bonobos suddenly experience an extreme state of stress at the birth of their sibling,” Verena Behringer explains these results. “Cortisol levels were unusually high for a long period of time, regardless of whether the youngster was two or eight years old when they gave birth to a sibling. This sustained stress response has a negative effect on the immune defenses. Since the concentration of thyroid hormone showed no change See, we can assume that the stress response is not stimulated by energetic stressors, such as suddenly stopping suckling.”
This assumption was also confirmed by the recorded behavioral data. For example, the researchers looked at the extent to which the older sibling suckled, how much body contact they still had with their mother, and how often they were carried. All weaning processes, which can act as additional stressors, were either completed before the birth of a sibling, showed no sudden change at birth, or were only significant in young individuals and disappeared as the young bonobos got older.
“For the first time, our research shows that the birth of a sibling is a truly stressful event for the older sibling,” summarizes Verena Behringer. “Don’t worry, though. It’s very likely that this stress is tolerable and perhaps leads to a higher stress tolerance in the older siblings later in life. After all, younger siblings are not only competitors, but they are also important social partners that positively influence our development.”