Firstborns stressed for 7 months by siblings

The arrival of a brother or sister produces a profound long-lasting stress response in bonobo infants...
15 December 2022

Interview with 

Andreas Berghänel, Veterinarian University, Austria




Firstborns, parents often say, behave quite differently from their siblings lower down the birth order. Science also agrees with them and dubbs the phenomenon the firstborn effect. But why should this be? Something about the early years, perhaps as an "only child", must be influencing the subsequent development of that individual. No one knows what, yet, but perhaps a sustained surge of stress hormones when a brother or sister comes along could be part of the answer. Speaking with Chris Smith, that's what Andreas Berghänel, from the Veterinarian University in Austria, has been finding in our close relatives the Bonobos...

Andreas - In the moment when there as a, a new sibling born, the older child can show a lot of stress behaviour. But what we never have tested so far was physiological stress levels. And for this we measured cortisol levels in, in urine sems. Cortisol is also in humans the most important hormone that regulates stress. We wanted to know if this also happens right at a time in a new sibling is born.

Chris - Do Bonobos show the same stress behaviour that humans do? Then when a new one comes along and you've still got one that's relatively young, do they behave the same way as a truculent human infant does when it gets a sibling?

Andreas - They show at least some more stressful behaviour. They try to get closer to the mother, for example, and stay more around.

Chris - So how did you test this?

Andreas - What we primarily did here was really looking for this stress hormone levels to cortisol levels for this be collected urine samples when the new sibling is born so that we can really have a look to the cortisol levels.

Chris - So it's a measure before the sibling comes along so you know sort of what the baseline is for that infant. And then you're measuring after the sibling comes along and to see do the stress hormones change their levels?

Andreas - Exactly. And we really did that also for long time. So not only right before or after that, but really years before and years after that for a really long time. So we could also have a look how long these increased cortisol levels stay at high levels after that.

Chris - And, and I presume from what you've just said that they do go up then there is evidence of stress in the older infant when the new one is born.

Andreas - Yes, indeed. We find a very extreme increase. So we found a fivefold increase in cortisol levels, which is something if normally you would like to stress an animal, let's say in captivity for whatever reason, then you would never get this kind of increase. So it's a really very strong increase that you could see there. And you could even see that this really high cortisol levels stayed out for about seven months after the sibling was born. So for several months they show very high stress levels.

Chris - And is that stress manifest in any other way? Because one of the other things cortisol does is it dampens down the immune system, doesn't it? So do the animals succumb to any other effects when this is happening to them?

Andreas - What we could also see, we also measured nerine levels, which is a kind of marker for the immune system. And that showed a decrease. So it seems that right at the birth of a sibling, the immune system decreases a little bit.

Chris - Was this true for both male and female infants or was one more affected than the other?

Andreas - We could see that in both. So there was no difference between males and females.

Chris - And were there any examples where this didn't happen in the infants or was it a comprehensive effect? Every single infant got this response and it stayed up for seven months

Andreas - On what we have seen. It was really exactly like this and each single infant, which was particularly impressive because the age at which and new sibling was born was very different between the infants. So we had some infants that are about two years old. When the new sibling was born, we had other infants that were more than eight years old. And all these infants we could see exactly the same cortisol reaction.

Chris - Do you think there are any long-term consequences of them having this happen to them?

Andreas - This would be a really interesting question. Periods of very high cortisol levels early in life can have very strong effects on growth rates behaviour later in life. So that's really a question. If this also triggers some changes and development and also even adult behaviour.

Chris - So if you have the Bonobo equivalent of an only child and they're not gonna be susceptible or subject to this big cortisol surge for a prolonged period of time, seven months, might that have long-term health consequences or, or behavioral consequences for those only children? Has anyone looked at that?

Andreas - This will be very important questions. So one thing that we know also from humans, siblings are the strongest early answers that we can have throughout life to under creator, also the strongest competitors that we have throughout life. So it can be both. Both can be benefit from each other. So at least on this letter, we know that having a child can have strong impact on the photo development and life. What it means to have these kind of high cortisol levels. I think we have no ideas of far. We really have to find it out.


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