The birth of a second child is a remarkable experience not only for the parents, but also for the older sibling. From human behavioral studies it is known that the change in the family constellation is a confusing and stressful time for the older child, frequently accompanied by clinginess, depressive conditions, and temper tantrums. So far, it was unknown to which extent this stress is also physiologically detectable. Verena Behringer, a scientist at the German Primate Center (DPZ)—Leibniz Institute for Primate Research in Göttingen, has investigated this question in one of our closest living relatives. The study is published in eLife.