Why does one sparrow boldly grab crumbs from an outstretched hand while another hops about nervously at a safe distance, afraid to collect a free meal?
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, a team of researchers argue in a theoretical study published on May 30 in the British journal Nature.
Once thought to be the exclusive domain of human beings, personality is increasingly seen by scientists 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 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.
The theory of natural selection would predict that being flexible is a much better strategy for survival and reproduction, explained Max Wolf, lead author of the study and an evolutionary biologist at the University of Groningen.
"In many situations, the evolutionary value of correlations like those between boldness and aggressiveness are poorly understood," he told AFP.
The persistent variation between individuals is also puzzling, Wolf said.
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, including Franz Weissing, director of Centre for Ecological and Evolutionary Studies at Groningen, 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," said Wolf.
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 explained. 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.
Their more patient brethrern, however, 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 off-spring. "Sometimes it pays to stick to certain behaviour," said Wolf.
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