A study has found chimpanzees and orangutans can experience a mid-life crisis just like humans.
The study set out to test the theory that the pattern of human well-being over a lifespan might have evolved in the common ancestors of humans and great apes.
An international team of researchers, including economist Professor Andrew Oswald from the University of Warwick and psychologist Dr Alex Weiss from the University of Edinburgh, discovered that, as in humans, chimpanzee and orangutan well-being (or happiness) follows a U shape and is high in youth, falls in middle age, and rises again into old age.
The authors studied 508 great apes housed in zoos and sanctuaries in the United States, Japan, Canada, Australia and Singapore.
The apes' well-being was assessed by keepers, volunteers, researchers and caretakers who knew the apes well.
Their happiness was scored with a series of measures adapted from human subjective well-being measures.
"We hoped to understand a famous scientific puzzle: why does human happiness follow an approximate U-shape through life? We ended up showing that it cannot be because of mortgages, marital breakup, mobile phones, or any of the other paraphernalia of modern life. Apes also have a pronounced midlife low, and they have none of those," Professor Oswald said.
The study is the first of its kind and the authors knew their work was likely to be unconventional.
"Based on all of the other behavioural and developmental similarities between humans, chimpanzees, and orang-utans, we predicted that there would be similarities when looking at happiness over the lifespan, too," Dr Weiss said.
"However, one never knows how these things will turn out, so it's wonderful when they are consistent with findings from so many other areas," he added.
The team included primatologists and psychologists from Japan and the United States.
In the paper the team point out that their findings do not rule out the possibility that economic events or social and cultural forces contribute part of the reason for the well-being U shape in humans.
The study is published in the Proceedings of the National Academy of Sciences of the USA.