It’s a familiar sight: Your mom or grandmother picks up a document and immediately holds it out at arm’s length to make out the small letters on the page, while simultaneously reaching for her reading glasses. As people age, their ability to see things close up often fades, a condition known as presbyopia. The eye can no longer focus light on the retina, focusing it instead just behind and causing poor close-up vision.
Many have thought that presbyopia was a consequence of living in an era in which people are overburdened by tasks that require frequently focusing in the near-field of vision. But perhaps not: A new study finds that if bonobos could read, they too would need glasses as they age.
Bonobos aren’t burdened with having to read tiny newsprint or letters on a mobile phone screen, but they do perform one task that regularly requires close focus: grooming. This behavior not only removes tiny bits of dirt and ectoparasites from the animals but also promotes social relationships. Most of the time, a bonobo grooms by putting its face within 20 centimeters of its partner and picking off the offending debris with its fingers. But older bonobos place their partner at arm’s length, probably because they can no longer see close up, researchers report November 7 in Current Biology.
For more than 40 years, researchers from Japan’s Kyoto University have studied wild bonobos in the Luo Scientific Reserve near a village called Wamba. Some researchers had noticed that older bonobos groomed differently than young ones, keeping their companions at arm’s length. Then in 2015, Heunglin Ryu of Kyoto University and colleagues decided to try to quantify this. Was it all older bonobos, or just some? They photographed 14 bonobos as they groomed, using the bonobos’ ear length and a ruler to determine grooming distance. Then they plotted out their data.
Grooming distance increased exponentially after a bonobo hit age 35, with the oldest bonobos, at age 45, keeping their partners around five times as far away as did young bonobos. A video of one bonobo taken in 2009 showed how her vision changed. When Ki was 29 years old, she placed his face 11.9 centimeters from her fingers as she plucked away at her partner. But at age 35, that had increased to 16.9 centimeters.
Scientists had reported anecdotes of older female chimpanzees (male chimps generally do not reach old age) that developed presbyopia. The bonobo finding combined with our own bad eyesight may indicate that presbyopia is a condition that dates to at least our most recent common ancestor.
As in humans, bad eyesight may come with a price for older bonobos. People who are farsighted often have trouble seeing in the dark. If that is also true for bonobos, they may have difficulty seeing in the low light of the rainforest canopy. Plus, if they are not able to groom others well, that may affect their social lives. Maybe they would benefit from reading glasses.