An animal is said to be in an evolutionary illusion or trap when it does something it has evolved to do, but at the wrong time or in the wrong place. The concept may help explain why so many squirrels get squashed on city streets, says Brown. For millions of years, squirrels have evolved to cross open spaces as quickly as possible, without wasting time watching for predators that they would not be able to escape anyway. "Ordinarily, that was a very sensible thing to do," he says. "But as an urban squirrel crossing four lanes of traffic, that's a bad idea."
Though ecologists used to dismiss urban areas as unworthy of study, they have recently begun to realise that cities provide an ideal theatre in which to see behaviour evolving at a pace rarely seen in the wild. City environments tend to be less variable than the countryside. Urban heat islands mean that insects can be active longer or throughout the year, and human activity provides urban wildlife with more stable, predictable sources of food and water.
Surprisingly, this too can set an evolutionary trap, as an abundance of food is not necessarily a good thing because it may give animals the wrong signal. For example, the numerous bird feeders in Florida suburbs allow Florida scrub jays to live a well-fed and relatively stressfree life. This easy living has a cost, says Reed Bowman, a behavioural ecologist at the Archbold Biological Station in Lake Placid, Florida. By mimicking an unusually early and productive spring, the artificial abundance fools the suburban birds into breeding several weeks earlier than country birds, and laying larger clutches.
And here's the trap: the nuts and other plant foods that fatten