The discovery that bonobos lose their close vision with age, as humans do, offers clues to the cause of this visual decline.
Heungjin Ryu of Kyoto University in Japan and his colleagues took photos of 14 wild bonobos ranging between 11 and 45 years of age as they groomed family and friends (pictured) — a behaviour that requires careful inspection. The team found that the distance from eyes to fingers was significantly longer in older animals than in younger ones. Moreover, a comparison with older images of grooming for one of the individuals, called Ki, showed that the distance had increased from about 12 centimetres in 2009 to roughly 17 centimetres in 2015. Bonobos tended to exhibit this 'long-sighted' condition, known as presbyopia, from about age 40.
The results suggest that presbyopia is caused not by human activities such as reading and computer work, but by natural ageing of the primate eye.