Few pet owners would admit that their pooches or other furry friends lack personality. But some dogs are more aggressive than others. The question is why. So far, the answer has been as much a mystery as why animals ranging from ants to apes behave consistently over time. For example, fish that act aggressively toward other fish also tend to be bold around predators—swimming right up to check them out. Now researchers say animal personalities may stem from what they have to lose—at least in a simplified model. "If they differ in their expectations for reproduction, then these individuals should differ in personality," says Max Wolf, a theoretical biologist at the University of Groningen in the Netherlands.

He says, for instance, that birds, such as oystercatchers in the Netherlands, have a choice of reproductive strategies: They can either breed early and forgo acquiring prime territory or they can hold off mating until they find good patches of land to mine for food like mollusks. Both tactics can work, but the choice each bird makes suggests they will develop different personalities, according to a study by Wolf and colleagues published this week in Nature.

Using mathematical model and computer simulations, the scientists assumed that a population of animals made specific reproductive choices; some chose to breed early and some set their early sights on acquiring territory. Their finding: the animals that opted to breed first tended to be bold and aggressive and more willing than their patient brethren to take risks, because they had already reproduced and therefore had less to lose. Their real estate savvy peers, on the other hand, were generally more timid, because they stood to lose more in a confrontation.

"If you have little to lose, then you should be consistently [more] risk prone, in all kinds of different situations," Wolf says. "But we expect that the ones who go for high quality territory, they should be consistently avoiding risks."

The theory has yet to be tested in the real world but similar experiments have demonstrated consistent results. Behavioral ecologist Alison Bell of the University of Illinois at Urbana-Champaign, for instance, subjected sticklebacks (a small fish) that were never before hunted to predators, in this case trout. The survivors developed stable personality traits to deal with the change. "Predators are imposing a life history trade-off that is causing them to behave consistently," in this case, less aggressively and more cautiously, Bell explains.

The role of genetics in personality remains unclear, though it is clear that personalities can be inherited, as does why animals react as they do to environmental changes. In fact, Bell adds, it is unclear why animals, including humans, even have personalities. Wolf notes: "Our theory is good as long as the individuals differ [consistently] in what they have to lose."