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New research has revealed another close link between humans and chimpanzees.

Just like humans, as chimps get older they prefer to focus on a smaller social circle and with chimps they've spent the most time with developing meaningful and longer lasting friendships.

The study, published in the Science journal, was carried out by researchers at Harvard University's Department of Human Evolutionary Biology.

They spent 15 years studying 21 chimpanzees in the Kibale National Park in Uganda.

They were all male and aged between 15 and 58 years old - they chose to only focus on male chimps as they tend to form stronger bonds with other chimps than females do!

They found that the animals displayed much of the same behaviour as older humans do. As the chimps aged, they surrounded themselves with a smaller group of 'close' friends.

They also wanted to spend time with pals they have a positive relationship with - where it's much more of a case of you scratch my back, I'll scratch yours!

The chimps aged 40 and above would happily spend time grooming one another, taking it in turns.

Whereas the younger ones were more than happy to take a back scratch - but didn't always fancy giving one back!

The study showed that younger chimps would have lots of these one-sided friendships.

Whereas the over 40's kept to a small group of three chimps they know and interact with well.

Assistant professor of psychology and anthropology at the University of Michigan, Alexandra Rosati, was one of the lead authors of the study.

She said: "The really cool thing is that we found that chimpanzees are showing these patterns that mirror those of humans."

She added: "There's really a pressing need to understand the biology of ageing.

"More humans are living longer than in the past, which can change the dynamics of ageing."

The researchers decided who was friends with who by taking note of where they sat.

The older chimps would choose to be with the chimps they know better and they'd sit closer to one another and spend time in a small quiet group.

The younger ones did not.

The similarities with people don't just end with who is in a chimp's friendship circle.

The older chimps, much like older humans, would take time to be peaceful and quiet and preferred to stay out of any drama the younger ones might cause.

The researchers said that although chimps are clever, much like with human relationships, there is still lots to learn about why they behave this way.

They also highlighted how the study provides the first evidence that non-humans also show age-related social selectivity.

Professor Richard Wrangham, co-director of the Kibale Chimpanzee Project, said: "It raises the possibility that we are seeing behavioural systems that have been shared evolutionarily back to our common ancestor around seven or eight million years ago."