It is not just humans, but apes too seem to experience the "mid-life crisis", according to a new study.

Scientific evidence has earlier shown that the happiness of humans tends to follow a U-shaped curve, where their happiness increases at the age of 20, dips when they are in their 40s, and rises again in their old age.

The new study has found that the U-shaped curve related to well-being is not just unique to humans, but is also found in primates like chimpanzees and orangutans.

A team of international researchers led by University of Edinburgh, UK, wanted to assess the well-being and happiness of the apes. For their study, the research team asked zookeepers and volunteers who are familiar with individual apes at zoos to rate the well-being of more than 500 chimpanzees and orangutans using a questionnaire based on a human model, but suitably modified to measure the happiness of non-humans.

The zookeepers were asked questions whether the apes were in good moods and whether they enjoyed their social life. Researchers found that a similar U-shaped curve representing mid-life crises in humans emerged in apes. Apes in their mid-life were found to be more dissatisfied than the younger and older apes, reported LiveScience.

Although the mid-life crisis in humans might partly be explained by aspects of human life and society, experts suggest that there might be some biological and physiological reasons that humans share with their closest relatives - the apes.

"Biology and physiology have to be at the top of a list of possible explanations for the appearance of similar crises in both apes and humans," study author Andrew Oswald, a statistician at the University of Warwick in the United Kingdom, told LiveScience.

"That's what we share in common. Apes don't have mortgages and divorces and school fees to pay, and all of the paraphernalia of modern life," he said.

Until now, it was thought that the mid-life crisis existed only among humans. With the new study showing evidence that apes too experience a similar crisis, it opens the door to look deeper into the evolutionary past, said researchers.

"We have to look deeper into our evolutionary past and that of the common ancestors that we share with chimpanzees, orangutans and other apes," psychologist and lead author Dr. Alexander Weiss of the University of Edinburg, UK, told BBC.

The findings of the study, "Evidence for a midlife crisis in great apes consistent with the U-shape in human well-being," are published in the journal, Proceedings of the National Academy of Sciences.