The urban evolution lab
Public release date: 19-Apr-2006
"IT'S the wild west of evolution and ecology," says Joel Brown, an
ecologist at the University of Illinois at Chicago. Evolution is
operating with a vengeance in the urban environment as animals struggle
to adapt to novel conditions and cope with "evolutionary illusions".
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 the
parents are unsuitable for nestlings, which need the more digestible
protein provided by insect larvae that will not emerge until later in
spring. As a result, suburban nestlings are more likely to starve or be
stunted, Bowman has found. He will report his results next week at a
meeting on urban birds at the American Museum of Natural History in New
The city's bright lights may also set a trap for animals. Sea turtles
that hatch on the beach usually head for the safety the sea, which in
the wild is always brighter than the land. Now they may head inland
towards lights from beachfront developments, where they are unlikely to
survive. Night-migrating songbirds may fly into brightly lit buildings
and radio towers, leading to deaths sometimes numbering in the tens of
Challenges of this sort make cities an ideal laboratory for
evolutionary biologists to watch adaptation happening before their
eyes. In 2003, for example, Hans Slabbekoorn of Leiden University in
the Netherlands showed that urban great tits sing at a higher pitch
than their country cousins so that their songs stand out better against
the city noise, which tends to be at lower frequency. Slabbekoorn is
now doing further experiments to see whether individual tits can learn
this response." Most of these species have just begun to adapt to human
environments," Brown says. "It's a cool natural experiment."
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