Animals have distinct personalities and temperaments, but why would evolution favour these over more flexible and adaptible mindsets? New game theory models show that animal personalities are a natural progression from the choices they make over how to live and reproduce.
Any pet owner, wildlife photographer or zookeeper will tell you that animals have distinct personalities. Some are aggressive, others are docile; some are bold, others are timid.
In some circles, ascribing personalities to animals is still a cardinal sin of biology and warrants being branded with a scarlet A (for anthropomorphism). Nonetheless, scientists have consistently found evidence of personality traits in species as closely related to us as chimpanzees, and as distant as squid, ants and spiders.
These traits may exist, but they pose an evolutionary puzzle because consistent behaviour is not always a good thing. The consistently bold animal could well become a meal if it stands up to the wrong predator, or seriously injured if it confronts a stronger rival. The ideal animal is a flexible one that can continuously adjust its behaviour in the face of new situations.
And yet, not only do personality types exist but certain traits are related across the entire animal kingdom. Aggression and boldness toward predators are part of a general ‘risk-taking’ personality that scientists have found in fish, birds and mammals.
Max Wolf and colleagues from The University of Groningen, Netherlands, have found a way to explain this discrepancy. Using game theory models, they have shown that personalities arise because of the way animals live their lives and decide when to reproduce.
You could argue that life is all about cheating death and having enough sex to pass on your genes to the next generation, as many times as possible. From this dispassionate viewpoint, human reproduction is very perplexing for our reproductive potential has an early expiry date. At an average age of 38, women start becoming rapidly less fertile only to permanently lose the ability to have children some 10 years later during menopause.
From an evolutionary point of view, this decline is bizarre. Other long-lived animals stay fertile until close to the end of their lives, with elephants breeding until their 60s and the great whales doing so in their 90s. In comparison, a human woman is exceptional in losing her child-bearing potential years or decades before losing her life. Even in hunter-gatherer societies that lack our access to modern medicine and technology, women who pass through menopause can expect to live well into their sixties.
Now, a pair of scientists have proposed a new model to explain the origins of menopause. Michael Cant from the University of Exeter and Rufus Johnstone from the University of Cambridge suggest that the loss of fertility helps to lessen reproductive conflicts between successive generations of women.
A few theories have already been put forward to resolve this conundrum. I’ve previously blogged about one of these, which suggests that the menopause reduces the health risks that repeated childbirth brings to both mother and child. This idea complements the most popular theory, known as the “grandmother hypothesis“, which suggests that older, infertile women can still boost their reproductive legacy by feeding, teaching and caring for their existing children and grandchildren.
The basic idea makes sense and while some studies have backed it up, it’s clearly not the whole story. Some analyses of hunter-gatherer populations have found that the indirect advantages of helping your family don’t outweigh the potential benefits of having more children yourself. Alone, the grandmother hypothesis can explain why women continue to live past the menopause, but not why they go through it in the first place.