Biologists studying bonobos in Wamba in the Democratic Republic of Congo have spotted some struggling to focus during grooming. Older bonobos tend to lean back and try to groom at arm's length, the researchers say, showing behaviour that is reminiscent of older people struggling to read small print.

The results of the paper published in Current Biology suggest that bonobos' eye health deteriorates in a similar way to in humans.

While the behaviour is suggestive of presbyopia, or long-sightedness, the researchers can't get up close to the bonobos to find more definitive evidence.

"We cannot test anything on them because in the wild we have to be careful not to make any interaction with them. We can only observe and only take data," Heungjin Ryu of the Primate Research Institute at Kyoto University, Japan, told IBTimes UK.

Sight and social life

From monitoring the wild bonobos' behaviour from afar, Ryu and his colleagues found that a bonobos' sex and its friendships didn't affect this type of grooming behaviour. "Only age predicts the grooming distance," Ryu says. When bonobos get to about 45 or 50 years old they cannot really see much up close, he says.

Grooming is a huge part of bonobos' social life and an inability to clearly may be tough on the older animals. "Bonobos spend 10% of their time grooming. It is pretty big. It is mainly because they want to remove lice and but if they cannot remove it, well, maybe that will be very difficult for them."

Bonobo grooming
Ten, a 45-year-old male, grooms 21-year-old male JD. He needs 40-45cm between his fingers and eyes to focus Heungjin Ryu CC BY-NC 4.0

Old boys' club

Older males have a particularly bad time of it when it comes to grooming, Ryu says. "Old males really want to groom a lot but they are not popular. It is related with their grooming performance." Older males have particularly low status in bonobo social groups, Ryu says. Add this together with difficulty seeing and they become very unpopular lice-picking partners. The older males have to groom with each other, "within the 'old people' group", he says.

The eyesight of older female bonobos' may be just as bad, but they tend to do less grooming when they're older anyway. Older females have high social status and still get groomed a lot, Ryu says, but may only groom their own children.

Bonobos grooming
Fuku, a 17-year-old female, grooms 32-year-old female Hoshi. She needs only 5-10cm between her fingers and eyes to focus Heungjin Ryu CC BY-NC 4.0