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Why animals have attitude

Which seagull gets the food?

Differences in behaviour between members of the same species are part of a complex evolutionary strategy, scientists say (Image: iStockphoto)(Source: iStockphoto)


Baffling differences in behaviour within the same species are not just an accident of nature but an expression of animal personality and part of a complex evolutionary strategy, according to a new study.

A team of researchers argue their case in a theoretical study published today in the journal Nature.

Personality was once thought to be the exclusive domain of human beings. But scientists are increasingly seeing it as a trait common in wild animals, ranging from squids and spiders to mice and monkeys.

It also serves a purpose, they say.

Dozens of observational studies in recent years have shown that otherwise indistinguishable individuals - same size, age, habitat, sex - can consistently behave in very different ways, even when facing the same dangers or opportunities.

Whereas one stickleback fish or great tit bird will stand and fight when confronted with a predator or rival, another may be more inclined to cut and run.

From an evolutionary perspective, these behaviours - rigid in the individual, and varying within the species - are doubly perplexing.

Natural selection

The theory of natural selection would predict that being flexible is a much better strategy for survival and reproduction, says Max Wolf, lead author of the study and an evolutionary biologist at the University of Groningen in the Netherlands.

"In many situations, the evolutionary value of correlations like those between boldness and aggressiveness are poorly understood," he says.

The persistent variation between individuals is also puzzling, Wolf says.

Some researchers have attributed personalities as constraints imposed, for example, by hormones, or to insignificant 'noise' around an otherwise predictable response to critical situations.

But Wolf and his colleagues contend that animal personalities are part of an adaptive evolutionary strategy.

"Our argument is based on the insight that the more an individual stands to loose in terms of future reproduction the more cautiously it should behave in all kinds of situations," says Wolf.

Two paths in life

Young oyster catchers, a shore bird common in the Netherlands, for example, choose between two radically different paths in plotting their future.

"They can go into a territory of low quality with few resources right away, in which case they start breeding early in life," he says. Or they can wait their turn for access to 'high quality' territories.

The study, intended to stimulate empirical studies in the burgeoning field of behavioural syndromes research, would predict that the oyster catchers opting for early parenthood would be more prone to take risks because they have little to loose.

But their more patient brethren should be risk averse, staying healthy and safe until it is their turn to move into a choice piece of real estate and start a family.

In this interpretation, the birds' behaviour - aggressive and bold, in the first case, timid and shy in the second - is not simply a response to an immediate situation, but fits into a coherent 'life history' that will enhance the chances of surviving and producing offspring.

"Sometimes it pays to stick to certain behaviour," says Wolf.

Tags: animal-behaviour, zoology