In further evidence of just how similar chimpanzees are to humans, an international team of researchers from Scotland, Japan, Germany and America studied 336 chimpanzees and 172 orang-utans in zoos. They found the U-shaped wellbeing pattern - which describes a positive outlook in youth before a dip in middle age and a resurgence in old age - also played out with primates.

Led by Alexander Weiss from the Scottish Primate Research Group and University of Edinburgh, the researchers said the findings had implications for both enhancing human and ape wellbeing and better understanding the evolutionary origins.

Keepers from participating zoos were asked to answer questions based on observations of their apes, which they had to have known for at least two years. Three studies were carried out: the first among 155 chimpanzees from Japanese zoos, which found the average age the chimpanzees reached their wellbeing low point was 28.3 years. In the second study, 181 chimpanzees from Australian and American zoos reached their low point about 27.2 years. The third study looked at 172 orang-utans from Australian and Singaporean zoos and found the low point struck about 35.4 years of age.

"The curves ... are comparable to human wellbeing, which ranges from 45 to 50 years," the researchers concluded.

Melbourne Zoo's Jan Steele said while the findings were consistent with her own observations, she had concerns with the study's methodology, which she felt was subjective. But the former primate keeper said the results still struck a chord with her.

She said young chimpanzees and orang-utans were enthralled with their world, but in middle age they are jockeying for status. And by the time old age comes around, they have both status and respect.