Middle-Aged Bonobos Become Farsighted, Too

Evan Maclean via Wikimedia Commons // Public Domain
Evan Maclean via Wikimedia Commons // Public Domain / Evan Maclean via Wikimedia Commons // Public Domain

Apparently we’re not the only animals who could benefit from reading glasses; primatologists now report that older bonobos groom their kin from a distance, like a person holding a restaurant menu at arm’s length. Their study, published in the journal Current Biology, confirms that bonobos become farsighted as they age.

Along with the chimpanzee, the bonobo (Pan paniscus) is the closest living relative of humans; our three species share nearly 99 percent of our DNA. But unlike humans or chimpanzees, bonobos have a reputation as the hippies of the animal kingdom, resolving conflicts with sex rather than violence. They put a high value on group harmony, which they maintain through the aforementioned love-ins and by a near-constant social grooming regimen. 

It was during one of these grooming sessions that experts observed a very familiar behavior: An older male named Ten (abbreviated TN) was grooming a younger male, Jeudi (JD), from a somewhat ungainly distance. "TN had to stretch his arm to groom JD,” study co-author Heungjin Ryu of Kyoto University’s Primate Research Institute said in a statement, “and only when he found something on JD's body would he come close to remove it using his mouth."

Here's TN grooming JD:

Image Credit: Heungjin Ryu (CC BY-NC 4.0)

Compare that with this photo of 17-year-old Fuku grooming her friend Hoshi:

Image Credit: Heungjin Ryu (CC BY-NC 4.0)

Ryu and his colleagues suspected that TN was grooming at arm’s length to accommodate for presbyopia, or farsightedness. To find out if that was true, and if the strategy was widespread, they spent four months in 2015 monitoring wild bonobos in the Democratic Republic of the Congo. Each time they spotted the animals grooming, they snapped a photo, eventually capturing pictures of 14 different bonobos between the ages of 11 and 45.

The researchers then measured the distance in each photograph between groomer and groom-ee. They compared each animal’s grooming proximity with his or her age and, sure enough, a trend emerged. Whether male or female, each bonobo’s preferred grooming distance increased exponentially as he or she got older, especially after age 40.

The straightforwardness of the results was “very surprising even for us,” Ryu said. Bonobos have no corrective lenses, no reading glasses. When their vision fails, it fails—a dangerous proposition for animals who spend their lives in the forest canopy.

Ryu says his team’s findings also have some interesting implications for humans. If our bonobo cousins also lose their visual edge as they age, maybe our tech-centric lifestyle is less to blame for our midlife squinting than we previously believed.