May 30, 2007 — 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 behavior 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 Thursday 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, 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 behaviors — 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 said.
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.
