Have you ever had to hold a newspaper at arm’s length to read it? If not, perhaps you’ve seen an older relative do it. Well, this long-sighted behaviour isn’t limited to humans – our primate closest relatives bonobos do it too (but to groom, not read).
Primatologists in Japan and Scotland examined the grooming distance of 14 wild bonobos from 11 to 45 years of age. In Current Biology, they found the older bonobos get, the longer they stretch their arms from the rest of their bodies as they groom.
And, like humans, bonobo age-related long-sightedness – called presbyopia – starts at around 40 years of age.
“This suggests that senescence of the eyes has not changed much from the [bonobo-human] common ancestor, even though the longevity of modern humans is far longer than that of chimpanzees and bonobos,” says study co-author Heungjin Ryu of the Primate Research Institute of Kyoto University.
While there have been some reports of presbyopia in ageing chimpanzees and bonobos, no one had taken the time to see if it manifested itself at the same time as humans and if it worsened at the same rate.
Ryu’s interest was piqued as he watched a male bonobo – TN – grooming another: “TN had to stretch his arm to groom JD, and only when he found something on JD’s body would he come close to remove it using his mouth.”
So Ryu and his colleagues used digital photographs to measured bonobos’ grooming distance and see how it varied in relation to age and sex.
Sure enough, grooming distance increased exponentially with age.
They were also able to get their hands on a video from 2009 of a bonobo called Ki, who was 35 years old at the time, and compared her grooming distance to today. As an older bonobo, she also groomed from afar.