Bonobo infants become highly stressed when they get a younger sibling and they don’t recover for seven months, according to a study that monitored levels of a stress marker in their urine.
In humans, many firstborn children struggle with the arrival of a sibling because “they’ve lived in a world where they have pretty unlimited access to parental time and attention, and now they’re having to share it”, says at the University of Queensland in Brisbane, Australia.
To explore whether this transition is also difficult for older siblings in related species, at the German Primate Center in Göttingen, Germany, and her colleagues studied bonobo infants when they gained a sibling. They picked bonobos because, similarly to us, additional offspring are often born while older siblings are still dependent on their mother for care.
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The researchers studied 20 female and six male bonobo offspring aged 2 to 8 years old living in the wild in Salonga National Park in the Democratic Republic of the Congo. They observed the infants’ behaviour when their mothers gave birth to younger siblings and also collected regular samples of their urine.
The bonobos made more body contact with their mothers for a short period after their siblings were born, possibly to try to regain attention or because they were curious to see the new infant, says Behringer.
The level of a stress hormone called cortisol in their urine also jumped to five times its normal level on average after the sibling was born, and stayed high for seven months before returning to the baseline.
This wasn’t because the older offspring were forced to stop nursing when their younger siblings were born, since most had already weaned.
“Instead, we speculate that the overall change in their environment, such as less attention by the mother, change in their social environment and other new situations, may be responsible for the increased stress levels,” says Behringer.
The researchers didn’t investigate whether higher levels of stress affected the long-term health of the older offspring, but they did notice that a marker of immunity in their urine called neopterin dropped sharply when their younger siblings were born.
Other studies in wild chimpanzees and baboons have found that older offspring tend to have and when they have younger siblings that are close in age to them, possibly related to similar stress responses.
Cortisol levels in human children haven’t been measured following sibling births, but observational studies have noted that they can become clingy, whiny, attention-seeking or withdrawn, which may be indicators of stress.
“You’re generally talking about a period of months for children getting used to having another sibling around,” says Sanders. “But it doesn’t end there because sibling influences are amongst the most important developmental influences in our lives and represent the longest relationships people are likely to have.”
Children may find the arrival of a sibling less stressful than bonobo infants because they don’t usually rely solely on their mothers to care for them – many also have a father, grandparents and others around them providing extra support, says Sanders.
To help older siblings adjust to a new baby, he recommends preparing them before the birth by “talking about the baby and where they’re going to sleep, buying them a baby doll, giving them opportunities to interact with friends’ babies, letting them touch and feel the baby bump and reading stories about having siblings”.
Although gaining a sibling can be stressful, it can also be “developmentally enhancing”, says Sanders. “It’s an opportunity to learn to wait, compromise, share, take turns and learn that you won’t always get what you want, and those are all incredibly important skills for life,” he says.
bioRxiv
