Of mice and men

Living in the city gives animals amazing evolutionary advantages

WHITE-footed mice were living in Manhattan long before human settlers showed up, back when Times Square was home to a redmaple swamp.

Today, they’re found in just a few parks across the city. And because each colony is isolated from the others, they’re evolving in different ways.

All city mice, for example, are better equipped than their country cousins to deal with toxic metals in the urban soil. But the white-footed mice in Central Park are equipped with slightly stronger genes for processing fatty foods — likely spurred by exposure to leftovers from human snacks.

In his new book, “Darwin Comes to Town: How the Urban Jungle Drives Evolution” (Picador), Dutch scientist MennoSchilthuizen reveals how our cities, rather than being intrusions, are actually just another part of the landscape that nature learns to take in its stride.

Schilthuizen has plenty of stories about animals adapting to city life in remarkable ways, from the Japanese crows who’ve learned to let passing cars crack walnut shells under their wheels to bullfinches in Barbados who come to outdoor cafés, grab paper sugar packets, then rip them open and help themselves to a snack.

There’s even a species of catfish that, 35 years after being introduced to the city of Albi in southern France, has learned to supplement its diet of smaller fishes by jumping onto the gravelly shoreline and dragging pigeons back into the river with them.

Most forms of evolutionary adaptation, though, are decidedly less flashy. If you were paying close at- tention, for example, you might notice some city birds lining their nests with discarded cigarette butts, and you might chalk that up to an ugly consequence of human littering. But it turns out that nicotine is a natural insect repellent — as a result, the birds with butts in their nests suffer fewer infestations of blood-sucking parasites.

Or think about how we line our streets and sidewalks with salt after a winter snowfall. There are some plants that grow along the seacoast that have developed a higher tolerance for salt; now, when their seeds make their way inland — blown in by the wind, maybe, or tracked in on our boots — they have a better chance of flourishing in patches of soil along the road.

As far as Schilthuizen’s concerned, that’s just how natural selection works. We should accept the fact that rapid urbanization is creating one of the most powerful, widespread spurs to evolution in the plant and animal kingdoms in millennia and run with it rather than try to suppress it. If anything, cities may turn out to be faster breeding grounds than nature, where relatively stable conditions mean there hasn’t been much need for significant changes. Look at the starling: It wasn’t introduced to North America until 1890, when several breeding pairs were released in Central Park. It only took 70 years for the population to spread to the West Coast, and now there are as many starlings on the continent as there are people. Yet, in little more than a century, the starling’s wings have become noticeably more rounded compared to their ancestors. Nobody’s sure why, but some researchers think the birds might have developed the new wing shape to quickly take off when cars or predatory cats get too close. As humans trade and travel between cities, though, they inevitably bring other life forms with them on purpose or by accident. If a species has made its way to a city and can thrive there without threatening the local ecosystem’s survival, perhaps it’s earned the right to stay. But we should do our best to preserve undeveloped pockets of land in and around the city, parks, forests and beaches where nature can continue to evolve with minimal interference from humanity. And, Schilthuizen says, we should space those natural habitats out, to promote true diversity. Separation is what enabled the mice in Central Park to distinguish themselves from their counterparts in Brooklyn and Queens. Just as human neighborhoods can lead to exciting pockets of culture, small ecosystems are essential to a flourishing natural world.

Whitefooted mice in Central Park have slightly stronger genes for processing fatty foods — likely because of their exposure to human snacks.