Stress at the birth of younger brothers went to people from monkeys: what scientists have learned

Psychologists note that the birth of a new child in a family is often stressful for children. Appearance

younger brothers and sisters (siblings) provokes a wide range of behavioral changes in older ones.

Humans are not the only species thatlong time dependent on maternal attention, similar behavior is also characteristic of anthropoid apes. However, no studies have yet been conducted that describe how, in this environment, cubs react to the appearance of “competitors” for maternal attention.

In a paper published in eLife,An international team of researchers reports for the first time a study of physiological and behavioral changes in primates that have not yet reached adulthood when siblings are introduced.

Who are bonobos?

Primates are different from other socialmammals have a remarkably slow life cycle. Immature individuals grow slowly, social maturation continues into adulthood. However, unlike other animals, in which the dependence and bonds between parents and cubs weaken after the cessation of feeding, the relationship between mother and offspring in primates can last a lifetime.

Bonobos (Pan paniscus) are humanoidmonkeys that live only in the Congolese rainforests. Like other great apes, they have a long juvenile development and late adulthood. Until 4–6 years old, they depend on mother's milk, and the offspring fully grow only after 12–15 years.

But even after this, young monkeys oftendepend on maternal attention: sons remain in their birth group, and throughout their lives there is a close bond between mother and child. In most animal species, offspring are weaned long before the mother gives birth to another child.

But in bonobos and other great apes, development is much slower, and co-growing and even feeding with siblings is the norm.

Baby bonobos. Image: Christian Ziegler/Max-Planck-Institut für Verhaltensbiologie

How did the study go?

To understand how monkeys deal withWith the advent of new family members, scientists observed the behavior of 17 young animals aged two to eight years for 650 hours, which became brothers and sisters for the first time. Urine samples were taken from all monkeys to measure the levels of various hormones: cortisol (shows the response to stress), neopterin (characterizes cellular immunity) and total triiodothyronine (responsible for energy metabolism and metabolic rate).

In addition, the researchers noted howbehavior of animals in new conditions changed. For example, the duration and frequency of breastfeeding of children by the mother, as well as a decrease in the desire of the mother to take care of older children.

Researchers note that as they get olderAnimals go through different stages. For example, the process of "weaning" occurs at a certain age, regardless of the presence of a sibling. To separate the two different effects (weaning and having younger siblings), the scientists compared hormonal data with observations of the animals' behavior, age, and developmental level.

What have scientists learned?

The study confirmed an obvious hunch:the birth of a younger brother or sister caused an increase in the level of the stress hormone cortisol. What was surprising was that it increased five times at once and remained at such a high level for the next seven months. At the same time, the researchers recorded a decrease in the level of neopterin, a marker that signals the activation of the immune system.

At the same time, the stress reaction occurred in olderchildren, regardless of their age: both at two years old and at eight years old. This suggests that the stress response is not related to reduced breastfeeding and weaning. At the same time, animal observations also showed that all the stages associated with normal maturation and cessation of feeding were either already completed before the birth of the youngest child, or did not undergo significant changes.

The researchers note that the older childoften begins to show aggression and rage after the birth of a brother or sister, and this has nothing to do with a change in attitude from the mother or nutrition, but only with a change in hormone levels.

What does it mean?

The results show that both people andthe great apes seem to have inherited the stress response to sibling birth from a common ancestor. Since the birth of younger children is the norm, animals could develop natural mechanisms for adaptation and adaptation to increased stress over a long period of time.

The fact that this did not happen suggests thatthat they may have evolutionary significance, scientists say. In future research, they will try to understand why older brothers and sisters need so much cortisol, because they are not only competitors, but also important social partners who can have a positive impact on each other's development.

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