Humanity's closest living relative, the bonobo ape, can remember the voices of old friends for several years, just as people can, researchers have shown.
An international research team from the Universities of St Andrews and Saint-Etienne in France, made the discovery after recording the calls of individual bonobos and playing them to those they had known years before.
When it was a familiar voice in the recording, the bonobos became excited and would search for the individual, while the animals gave little reaction to hearing the calls of bonobos they had never known.
The team concluded that the primates are therefore capable of remembering the voice of a former group member, even after five years of separation.
Sumir Keenan, of the School of Psychology and Neuroscience at the University of St Andrews, said: "Members of a bonobo community separate regularly into small groups for hours or even days and often use loud calls to communicate with one another. Moreover, females leave their original community but may continue to interact with their old companions in subsequent meetings between communities. So, effective social navigation depends on the ability to recognise social partners past and present.
"It is fascinating to discover that this knowledge of familiar voices in the long term is another characteristic we share with our closest relatives."
The researchers were able to use recordings of bonobos from zoos across Europe, taking advantage of the fact that some bonobos had experienced several zoos and had formed links past and present with members of their species in different places.
Mimicking the events characteristic of the arrival of a new bonobo – the scientists played the recorded bonobo calls using carefully hidden speakers.
Bonobos, like many other primate species, including human beings, form complex social networks where remembering "who is who" is important, sometimes vital. These social associations need recognition between members of the groups – usually via faces and voices. Human beings are experts when it comes to recognising the voices of those closest to them and can recognise a voice many years after last hearing it.
The last common ancestors of bonobos, chimpanzees and humans lived 6-8 million years ago and shared numerous characteristics such as behaviours and genes.
In the natural environment, bonobos live in the Pacific Equatorial Forest in the centre of Africa, where they live in large communities with complex social networks.
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Sumir Keenan et al. Enduring voice recognition in bonobos, Scientific Reports (2016). DOI: 10.1038/srep22046