Too bad chimpanzees can't buy sports cars. New research says it's not just humans who go through midlife crises: Chimps and orangutans also experience a dip in happiness around the middle of their lives.

"There may be different things going on at the surface, but underneath it all, there's something common in all three species that's leading to this," said study leader Alexander Weiss, a primate psychologist at the University of Edinburgh in Scotland.

The study team asked longtime caretakers of more than 500 chimpanzees and orangutans at zoos in five countries to fill out a questionnaire about the well-being of each animal they work with, including overall mood, how much the animals seemed to enjoy social interactions, and how successful they were in achieving goals (such as obtaining a desired item or spot within their enclosure).

The survey even asked the humans to imagine themselves as the animal and rate how happy they'd be.

Happiness Curve

When Weiss's team plotted the results on a graph, they saw a familiar curve, bottoming out in the middle of the animals' lives and rising again in old age. It's the same U-shape that has shown up in several studies about age and happiness in people.

"When you look at worldwide data, you see this U-shape," said National Geographic Fellow Dan Buettner, author of Thrive: Finding Happiness the Blue Zones Way. (National Geographic News is part of the National Geographic Society.)

"It's different for every country, but it's usually somewhere between age 45 and 55 that you hit the bottom of the curve, and it continues to go up with age. You see centenarians in good health reporting higher well-being than teenagers."

Social and economic hypotheses may partly explain this happiness curve in human lifetimes: Maybe it's tied to adjusting expectations, abandoning regret, or just getting more stuff as we grow older. But Weiss suspects there may be something more primal going on.

"We're saying, take a step back and look at the big picture: Is there any evidence that there's an evolutionary basis underlying this?" said Weiss, whose study was published today in the journal Proceedings of the National Academy of Sciences.

"Knowing that a similar phenomenon exists in human and nonhuman primates opens up the realm of possible explanations."

Strike While The Iron Is Hot

Although the stereotype of a midlife crisis is generally negative—feelings of depression or discontentment with one's life and where it's headed—Weiss believes such ennui may have an evolutionary upside.

By the middle of one's life, humans and apes often have access to more resources than when they were younger, which could make it easier to achieve goals. Feelings of discontentment may be nature's way of motivating us to "strike while the iron is hot," said Weiss.

"It may feel lousy, but your brain could be tricking you into improving your circumstances and situation, signaling you to get up and really start pushing while you're absolutely at your prime," he said. "And I think that's a really powerful and positive message."

Coping With Midlife Ruts

Knowing that a midlife dip in happiness is a natural—and temporary—part of life could make it easier for humans to cope with the experience, Weiss said. It could also help caretakers improve captive apes' quality of life, by identifying ages at which the animals might benefit from extra attention or enrichment.

"I don't think this totally subsumes other explanations for age-related changes in happiness, but it adds another layer," Weiss said.

Weiss has previously studied the correlation between personality and happiness in both chimpanzees and humans, and plans to look next at the impact of factors like sex and social groupings.