Middle-aged bonobos need reading glasses to groom their friends

Older, long-sighted bonobos are forced to groom at arm’s length

Heungjin Ryu (CC BY-NC 4.0)

By Andy Coghlan

If only they had “grooming glasses” they’d be fine. But in the absence of a pair of specs, ageing bonobos have been found to compensate for dodgy eyesight by focusing on fur that’s further away.

The discovery of five cases of age-related long-sightedness at a bonobo colony in Wamba, Democratic Republic of the Congo, has demonstrated for the first time that ageing bonobos and humans develop poor eyesight at almost exactly the same rate.

This suggests that it might be an unavoidable throwback to a common ancestor of apes and humans, rather than a result of too much staring at books and computer screens. The team inferred deteriorating eyesight from the increasing distance between the eyes of a bonobo and their grooming target as they got older.

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“I didn’t expect age to be such a strong predictor of long-sightedness,” says lead researcher, Heung Jin Ryu of the Primate Research Institute at Kyoto University, Japan. Nor did he expect the compensatory increases in focusing distances to tally so closely with those of ageing humans.

“Young ones under 30 years old usually focus at 10 centimetres or less when they groom,” says Ryu. “But when they get to their 40s, it doubles to 20 cm, then continues, increasing rapidly. At 45 years old, an individual had a grooming distance of more than 40 cm.”

What’s more, archived video footage from 2009 of Ki, a bonobo then aged 35, suggested that within seven years, her grooming distance rose from 12 cm to 17 cm, evidence that her eyes were gradually getting worse with age, rather than poor to start with.

Deterioration seems to happen gradually

Heungjin Ryu (CC BY-NC 4.0)

Ryu says this was good evidence that the deterioration in humans is natural and unavoidable –probably through the hardening of lenses and weakening of eye muscles – rather than a modern phenomenon triggered by years of straining to focus on books and computer screens.

“The age-related decrease might be universal in mammals,” he says. “But I don’t think many mammal species will suffer from it because they probably die once their eyes start to malfunction.”

The species that cope best with weakening eyesight are likely to be those that are social and tolerant, such as great apes, elephants and whales, although Ryu says that there have been previous reports of eye deterioration in captive and wild rhesus macaques.

Ryu says that aside from problems with grooming, the older bonobos may find it more difficult to see clearly in the dark, a major survival factor in shady tropical forests.

The team now wants to explore what happens when the focusing distance required for grooming exceeds the bonobo’s reach. They also hope to find out whether the ageing of other tissues of the body is the same in bonobos as it is in humans – insights that may throw more light on how and why humans manage to live so long.

Other teams have already shown that chemical “epigenetic” marks on DNA that reflect ageing tally in humans and other primates. “Our own studies have shown that all primates age similarly on an epigenetic level, when looking at DNA methylation patterns in various organs,” says Steve Horvath of the University of California at Los Angeles. “I showed that the human epigenetic clock applies without change to chimpanzees and to a lesser extent to gorillas.”

Journal reference: Current Biology, DOI: 10.1016/j.cub.2016.09.019

Read more: Our eye sockets give us a wider field of view than other apes