A new study suggests that apes like orangutans and chimps may all experience a mid-life crisis in much the same way as humans, potentially giving an important insight into mid-life anxiety and depression.
Scientists have for years been puzzled as to why when approaching mid-life, humans exhibit signs of erratic behavior, depression and anxiety that does not seem to rationally tie to any one direct cause. Now, researchers have found that captive orangutans and chimps all appear to go through a similar mid-life slump.
The study, published on Monday in the journal Proceedings of the National Academy of Sciences, was undertaken by researchers from the U.S., Japan, Germany and the UK. It asked zookeepers, carers and others charged with tending to apes of various ages to complete a questionnaire on the behavior of the animals in their care.
More than 500 apes were included in the study from three separate groups of animals from zoos in Japan, the U.S., Canada, Australia and Singapore. The first two groups were chimpanzees, while the third was comprised of orangutans.
The questions ranged from inquiring about the apes’ general mood, to the enjoyment they appeared to gain from socializing, and even their success at achieving certain things within their groups (for instance, their place in the social strata).
When researchers went back over the results of the questionnaire,s they found a few interesting trends.
One was that, based on what their carers had written, the apes’ demonstrated sense of wellbeing appeared to fall as they reached middle age. A second trend showed an upward curve in wellbeing toward old age.
This matches the observed “U” shape or “happiness curve” that has been shown in research into self-reported human emotions throughout an average lifespan.
“Our results imply that human wellbeing’s curved shape is not uniquely human and that, although it may be partly explained by aspects of human life and society, its origins may lie partly in the biology we share with great apes,” the study says.
Professor Oswald said that changes in human physiology with age, such as hormonal changes, may contribute to the mid-life crisis, although the menopause in women has been ruled out as a cause.
The researchers have suggested that the mid-life dip may in fact be a motivating factor that leads to the upward curve in later life; that in effect there is a dawning sense of impetus to use a now established status, which may itself be a reason for the lull, in order to achieve greater things that were previously out of reach.
If this is sounding a little far-fetched, there are some important caveats to this research.
The obvious is that the apes were all captive and therefore have markedly longer lives, some up to fifty years or more, than their wild counterparts. As such, the research applies only to captive apes and can tell us little about the wider population.
More importantly, while great apes and humans share common ancestry and have demonstrated comparable faculties, it is problematic to define wider animal behavior in human animal terms because it can cloud our understanding. This is something that critics of this study have noted may have occurred in asking the apes’ carers to respond in anthropomorphic ways.
Critics also say they have yet to be convinced that the very human paradigm of “mid-life” has any relevance to the lives of apes because it is intrinsically tied to human constructs that simply are not present in the ape’s world.
Robin Dunbar, professor of evolutionary psychology at Oxford University, was dubious about the findings. “What can produce a sense of wellbeing or contentedness that varies across the lifespan like this? It’s hard to see anything in an ape’s life that would have that sort of pattern, that they would cogitate about. They’re not particularly good at seeing far ahead into the future, that’s one of the big differences between them and us.”
Of course, there are other critics who deny there is any real evidence for the mid-life crisis at all. They contend the subjective odd behavior of a few has given a false impression that a wider phenomenon is at work when the evidence shows very little support for a conclusion that there is any kind of general mid-life crisis at work.
However, if the research does later translate into wider findings that pin down a genetic or at least biological marker for the happiness curve, this would help scientists to advise on the best course for addressing and moving past the depression phase, potentially helping both humans and their animal cousins to enjoy a sustained and more consistent level of happiness.