Even Without Thanksgiving, Bonobos Find Family Life Stressful

Bonobos, like humans and other great apes, spend a long time in childhood dependent on their parents. And like humans, when a baby brother or sister bonobo arrives on the scene, there can be friction.

An international team of researchers led by the Max Planck Institute of Animal Behavior say they are “the first to investigate physiological changes in an animal as it transitions to siblinghood.” Their findings are published in the journal eLife.

How does one investigate transitions to siblinghood? You spend 650 hours observing 17 young bonobos between two and eight years old who had recently become first-time siblings. Then you collect their urine …

Skipping ahead to the urinalysis, where the researchers measured a marked increase in the stress hormone cortisol, up to five times above normal level that persisted for several months. Meanwhile they recorded a decline in neopterin, a marker that signals a change in the immune system.

“The birth of a sibling triggered high cortisol levels in all bonobos, regardless of whether the youngster was a highly dependent two-year-old or a mostly independent eight-year-old,” says lead author Verena Behringer.

It’s hard to know what evolutionary purpose elevated stress can have, if there is one. It’s possible the early onset of stress hormones can prep the animals for angsty challenges later in life, though that’s probably impossible to prove with observation or experiment.

Bonobos are an endangered ape that can be found only in forests south of the Congo River in the Democratic Republic of Congo. Like chimpanzees, bonobos share 98.7% of their DNA with humans—making the two species our closest cousins. One big family, but not always happy.

Photo credit: Christian Ziegler / MPI of Animal Behavior