Two new studies show how certain animals can adapt to the din of human activity in surprising ways.
Halfwerk had spent years in Gamboa, a town that houses people who work at the nearby Panama Canal. In that urban setting, túngara frogs find you. “They come up to your house, and if you approach them, they just keep on calling. You can just pick them up easily,” says Halfwerk. But in the forests of Darien, “it was like looking at a completely different species.” The forest individuals were shier, more elusive, and more easily disturbed than their urban cousins. And, as Halfwerk and his colleagues showed, they sound different, too.
Túngara frogs have some of the best-studied calls in the animal kingdom. Considering that the frog is just an inch long, its call is roughly as loud as a hair dryer, or a ringing phone. And while most frog calls consist of a single repeated chirp or ribbit, male túngara frogs have two elements: a downward whine, followed by one or more chucks. The more chucks a male adds, the more complex and extravagant his song becomes, and the more attractive he is to females. “We compare the call to a peacock’s tail,” Halfwerk says.
But animals can also adapt to urbanity in positive ways. As my colleague Paul Bisceglio reported, there’s increasing evidence that city life can make the animals within them smarter, or at the very least more flexible. When Halfwerk moved urban túngara frogs to forests, the males could dial back the complexity of their calls to avoid bats and bugs. But the reverse wasn’t true: Forest males don’t suddenly croon with more complexity when entering the big city.
In fact, there are now two such studies. A few years ago, Jennifer Tennessenfrom Pennsylvania State University found, through laboratory experiments, that wood frogs—a common North American species—are disturbed by traffic noise, which chronically raises levels of stress hormones in their bodies. But in the wild, “we were still seeing them in roadside ponds, and they were thriving,” says Tennessen. “It seemed like this mismatch.”
Such potentially rapid changes are encouraging, especially given how quickly humans are reshaping the world around us. But Tennessen cautions that there could be undocumented drawbacks to these seemingly positive adaptations. “If a population has evolved to mute its stress response to avoid the chronic costs associated with noise, they may not be able to respond to predators in an appropriate way,” she says.