What Makes Some Animals Resilient to City Life?

An animal's ability to change its behaviour in response to a changing environment can make all the difference, Sih has found in his research. In some animals, this ability to alter their behaviour is the first step to more profound change.

"Step one is that they need to respond well behaviorally," Sih says. "If they do that well enough, they survive well enough to have a chance to later evolve [genetically] to adjust even better." 

In Illinois, armadillos have been spotted more frequently near major roads, which researchers concluded may be because the roads help the animals thermoregulate. Asphalt absorbs heat from sunlight and traps thermal energy, and in the region's winters – which can reach -30F (-34C) armadillos may benefit from the warmth the roads emit.

This is the type of behavioural change that could eventually buy an animal like the armadillo time to evolve in response to the roads that now crisscross its environment – though not all behavioural changes will end up influencing a species' evolution.

Urban birdlife

If you're lucky enough to hear birdsong in a city, it could well be a song shaped by the street you're strolling down. Many birds have altered their songs to be heard above traffic. Numerous studies have shown that birds actively shift their frequency, volume and timing of their songs in response to noise levels.

Take a closer look at urban bird life and you might see other changes. Consider the dark-eyed junco, for instance. Noise, human presence, food scarcity and nesting competition make cities a difficult environment for the junco. In the early 1980s, a group of juncos settled year-round on the campus of University of California, San Diego – giving researchers a chance to document the rapid changes in the birds.

They found the juncos became less aggressive – and even developed fewer white feathers on their tails (a trait used in aggressive displays) as a result. The birds also began breeding earlier, reared twice as many young, and altered their migration pattern.

Scientists studied another urban population that settled at University of California, Los Angeles, and found breeding pairs nested in trees and on buildings instead of their usual nests on the ground.

It's not just the juncos' behaviour that's changed. Researchers found urban juncos have smaller wings, an adaptation believed to come from the demands of flying through dense, built-up cities, rather than long-distance flight over open land.

North America has lost 168 million of the birds in the past 50 years. The juncos' ability to rapidly adapt may be the key to securing its future.

Beyond city limits

Animals aren't just adapting in urban environments, though. They're physiologically changing to survive living with humans. For example, in some places, there has been a rise in the number of elephants born without tusks. That's because poachers are more likely to kill those with tusks – while tuskless ones survive and pass on their genes, including the "tusklessness" trait. In other words, being tuskless has become an evolutionary advantage in places where poachers are a major threat.

Some fish are getting smaller as humans catch the big ones and thereby select them out of the gene pool, leaving the smaller ones free to survive and reproduce. Again, smallness – which may have been an evolutionary disadvantage when faced with non-human predators – can be an evolutionary advantage when faced with humans.

Other changes involve protective qualities. Atlantic killifish are rapidly evolving to become resistant to toxins and industrial chemicals. Through genetic alterations in the biochemical pathways that specifically respond to these chemicals, the fish are becoming less sensitive to them. 

The animal kingdom is changing rapidly to keep up with a rapidly changing planet.

But what happens to the animals who can't adapt quickly enough?

"Other animals – often in the same species – simply 'don't get it', and do not respond well behaviourally to these new challenges," says Sih. "If anything, I note that for many animals, they don't really have time to evolve, as things have changed too much too quickly."

Those animals who do adapt to survive amid humans, buildings, climate change and pollution, may still have a long road ahead, says Sih. "In the next decades, they may have time to evolve to adjust even better."

- Author: Lucy Sherriff, BBC

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