City Frogs Are the Sexiest Frogs

Two new studies show how certain animals can adapt to the din of human activity in surprising ways.  

A túngara frog
A túngara frog (Adam Dunn)

For years, the Darién Gap, a narrow strip of pristine rain forest connecting Central and South America, was occupied by the Revolutionary Armed Forces of Colombia–People’s Army, or FARC, making it far too dangerous for any scientist to visit. But in 2016, after FARC negotiated a cease-fire with the Colombian government, the gap became far safer, and Wouter Halfwerk, an ecologist from Vrije University Amsterdam, wasted no time in visiting. He started looking for the túngara frog, a small, warty species that he had been studying in Panama for years. And to his surprise, he couldn’t find any.

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.

Why don’t males add as many chucks as they possibly can? Because their audience doesn’t consist of just females. Hungry frog-eating bats and blood-sucking midges are listening, too, and they are also more attracted to males with more complex calls. For these frogs, courtship is a risky business, and the balance between lust and danger—between sexual and natural selection—determines how complex they’re willing to make their calls.

Cities change that calculus, because bats and midges both stay away from night-lit homes and streets. These sources of light pollution create a halo of safety, in which túngara frogs can be the best singers they can be, without risking their own lives. “There’s no limit anymore, so the urban males can go wild,” says Halfwerk. Indeed, his team found that in towns, males call more frequently than in forests, and with more chucks. These more complex tunes are more irresistible to females from any location. All else being equal, urban males just sound sexier.

It’s clear that the sights and sounds of cities are problematic for many animals. Bright lights can disrupt the flights of moths and migrating birds, and send turtle hatchlings scurrying away from the sea. Loud noises can drown out mating calls, mask the approach of predators, and drive animals away. One team of scientists demonstrated this in dramatic fashion by playing the sound of traffic through speakers lashed to trees. This “phantom road” drove away a third of the local birds, and suppressed the weight of those that stayed.

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.

Many studies have shown that “wildlife in urban areas are responding to human modifications,” says Danielle Lee, an urban ecologist from Southern Illinois University Edwardsville. “Birds are singing through the night because of light pollution, and raccoons and squirrels are becoming cleverer in overcoming barriers to access food.” But it’s unclear whether reptiles and amphibians would react in the same way, given that they’re more sensitive to environmental conditions than mammals and birds. To see at least one study in which some frogs are adapting to city life “is good news,” Lee says.

In fact, there are now two such studies. A few years ago, Jennifer Tennessen from 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.”

She and her colleagues collected wood-frog eggs from several quiet and noisy ponds, and raised them under the same laboratory conditions. Once the tadpoles had metamorphosed into adults, the team exposed them to recordings of either quiet ambient woodland noise or loud passing traffic, for eight continuous days. They found that frogs from quiet ponds showed higher levels of stress hormones and immune cells—two signs of stress—when exposed to traffic noise. But frogs that came from noisy ponds were unaffected. It seems that they had adapted to their cacophonous environments, perhaps in just a few dozen generations.

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.