"Dethroned": Having siblings can be a stressful challenge for the first child—even for our closest relatives, the great apes. But until now it was unclear to what extent this stress can also be detected physiologically in them. Researchers have now been able to show for the first time using the urine of bonobo children: the birth of a younger sibling triggers a permanent stress reaction in the young primates and even weakens the immune system.
The birth of a second child is not only a special experience for the parents, but also for the older sibling. Because this usually experiences a profound change in the family constellation for the first time in his life, which is a confusing and exhausting time for the older child. Finally, the firstborn must now learn to share parental care with someone else. This transition from only child to sibling is seen as a very stressful time, often accompanied by behavioral changes such as over-clinging, depressed moods and tantrums. However, physiological data on the changes in stress levels have not yet been available.
Young bonobos examined
In order to better understand this special time in the life of newly minted siblings, Verena Behringer from the German Primate Center (DPZ) in Göttingen and her colleagues investigated this in great apes. They conducted their study on two groups of bonobos (Pan paniscus) that live near the LuiKotale research station in the Congolese rainforest. For more than 650 hours, the researchers observed the behavior of 17 cubs that were having a sibling for the first time and were between two and eight years old at birth.
These behavioral observations were crucial in order to exclude data bias caused by stress caused by other factors. As the young animals grow up, there are various processes of social weaning or dietary changes that can also trigger stress reactions, explains Behringer. "In order to separate the age-related weaning process from the birth of the sibling, we analyzed the urine samples and the behavioral observations before and after the birth of the second pup and related them to them".
Stress level up, immune system down
In order to get a picture of the physiological state of the young bonobos, Behringer and her colleagues collected 319 urine samples from the bonobos before and after the birth of the sibling pup and examined them for the concentrations of the three marker substances cortisol, neopterin and triiodothyronine (T3). Cortisol is a hormone released in response to a stressful situation, Neopterin is produced by the active defense cells of the immune system, and T3 is a thyroid hormone that regulates metabolic activity in the body.
The analyzes actually provided physiological evidence that the young Bonos suffer from increased stress after the birth of a sibling. The levels of the stress hormone cortisol in the urine of the firstborn increased fivefold when their younger sibling was born and remained at this level for up to seven months, regardless of whether the cub was two or eight years old when the sibling was born. "The five-fold increase in cortisone levels in our study is an unusually strong physiological response. For comparison, captive bonobos exposed to an experimental stress test only show a two-fold increase in cortisol levels,” Behringer and her team explain.
The measured values also showed that the immune system is also affected by the suddenly changing life situation. Urinary neopterin concentrations decreased significantly after the birth of the sibling and remained low for about five months, suggesting a reduced immune response. According to Behringer, this is a secondary reaction triggered by the permanently increased stress level of the older bonobo children. The thyroid hormone T3, on the other hand, showed no significant change, which, according to the scientists, suggests that the stress reaction is not triggered by energetic stress factors, such as an abrupt cessation of breastfeeding. This assumption was also confirmed by the recorded behavioral data. The researchers were able to confirm that all weaning processes that can act as additional stressors were either completed before the birth of the second cub or did not show any sudden change with birth.
Stress training for life
"Our study proves for the first time that the birth of a sibling is a really stressful thing for the older child," summarizes Behringer. "But you don't have to worry. It is very likely that this stress is tolerable and leads to a higher resistance to stress in the older children later in life. After all, younger siblings are not just competitors, but important social partners who have a positive influence on our development.”
Source: German Primate Center Göttingen; Specialist article: elifesciences, doi: 10.7554/eLife.77227