Plants, animals adapt

AUSTIN, TEXAS — City life can change the basic physical traits of its residents, down to what plant leaves taste like or how sticky a lizard’s toes are. The findings show that cities have become unintended experiments in evolution. That’s the process through which species change over time in ways that make them better suited to where they’re living.

Kenneth Thompson described how white clover (Trifolium repens) has adapted to urban living. Thompson is a graduate student at the University of Toronto Mississauga in Ontario, Canada. Leaf taste helps defend this clover against becoming lunch for grasshoppers and other predators, he noted on June 19.

Like all living things, the plant's genes carry the instructions for building its tissues. And small differences in two of those genes let clover booby-trap its leaves and stem. Now, when bitten, it releases a warning burst of cyanide, a toxic chemical. A tiny, tiny taste of clover doesn’t kill animals. But its bitter taste can repel the spitting predator away so that it dines on some other plant.

Clover’s cyanide-making genes are more common in warmer locations. (That’s because ice harms clovers that carry these genes). And cities tend to be warmer than other areas. So Thompson expected to find more bitter clover in downtown Toronto than toward the outskirts. But he found just the opposite. That was startling because snails and other clover-eaters did about as much damage in both areas.

The same surprising pattern showed up in Boston, Mass., and New York City. But it didn’t occur in Montreal. The explanation, Thompson and his colleagues now propose, lies in what happens during winter.