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.
For an animal, success is measured achieved through living long enough to reproduce, and individuals constantly gamble their current success against their future one. They could reproduce now, or defer it to a later time when resources are more abundant.
The crux of Wolf's theory is that those with a stable, assured future have more to lose by gambling, and are likely to be more risk-averse. Those with little to lose can afford to live fast and die young.
Wolf tested this idea by using a mathematical model to simulate these choices and their consequences. The protagonist of his model is a fictional animal that lives in an area with many territories, some rich in food and others lacking it.
The animal can choose how thoroughly it wants to explore its habitat. If it is adventurous, it could find a lush and bountiful territory, but it will have less energy to raise young, and must postpone this to the following year. That may not be so bad - its new home will give it a ripe, long life and it will have many opportunities for breeding.
He found that simulated animals picked one of two stable strategies. Some decided to explore thoroughly and hope for greater reproductive success in the future. Others decided to stay put, have young now and make the best of things, poor resources be damned.
Wolf then modelled how these two groups would react to decisions about risk, in a classic hawk-dove experiment. When faced with a predator or a rival, the animal could run away or back down (dove), which takes time and could lose it feeding opportunities or its territory. If it stood and fight (hawk), the likelihood of death or injury was greater but so were the rewards.
Sure enough, the explorers who were investing on future success, consistently evolved to be docile, timid and risk-averse, while those who reproduced immediately consistently became bold and aggressive. These patterns held up under a wide range of simulated conditions. Over time, they gave rise to stable individual differences and behaviour traits that were consistently linked with each other, the foundations of personality.
In New Scientist's coverage of this story, Judy Stamp from the University of California, Davis, criticises Wolf's work for only explaining extremes of personality. Obviously, animals are not always black hawks or white doves, but many shades in-between.
But Wolf's study answers this too. In the most advanced version of his model, he accounted for the fact that behaviours are governed by many heritable genes. This generated a much more realistic and continuous spectrum of personalities. Even with this more plausible model, the same principle applied - the more an animal had to lose, the fewer risks it was prepared to take.
Wolf is now keen to see his theory tested in the field. He suggests that many other behaviour traits could be linked to aggression or boldness. Individuals that invest heavily in the present may be more likely to guard nests, care for their young or woo mates with conspicuous and costly displays.
Reference: Wolf, van Doorn, Leimar & Weissing. 2007. Life-history trade-offs favour the evolution of animal personalities. Nature 447: 581-584.