Urban Frogs Have Sexier Calls

A new study reports that city frogs sing more complex and attractive songs than their country cousins. Urban frogs can get away with producing more conspicuous mating calls, which are preferred by female frogs, because they have fewer predators and parasites eavesdropping than frogs living in forests.

At night in the Neotropics, male túngara frogs (Physalaemus pustulus) gather in ponds and puddles to call and attract females. Males can make their calls more complex, as well as more attractive to females, by adding vocal elements called chucks. However, female frogs aren’t the only ones listening in: Frog-eating bats and blood-sucking parasitic midges preferentially attack males that produce more complex calls.

Wouter Halfwerk of Vrije Universiteit, Amsterdam and colleagues knew from previous studies that calling male túngara frogs respond to changes in noise and light levels, but also that they can suffer tremendously from predation and parasitism. They decided to investigate how city life, with its noise and light pollution, has altered the calling behavior of túngara frogs.

First, Halfwerk and his colleagues recorded the mating calls of frogs living in both urban and forested areas near the Panama Canal. They found that urban males call more often and with greater call complexity than forest males.

Second, the researchers broadcast male calls and quantified the number of female frogs, frog-eating bats, and blood-sucking flies attracted by the calls. In urban areas, frog calls attracted fewer female frogs but also fewer predators and parasites than they did in forest environments.

Next, the researchers tested urban and forest females for their preferences. They broadcast the calls of urban males and forest males in the lab and discovered that females strongly preferred urban over forest males, irrespective of the female’s origin.

Finally, in a translocation experiment, the researchers moved both túngara urban frogs into forest habitats and forest túngara frogs into urban habitats. They found that urban frogs placed in a forest environment actively reduced the complexity of their calls. However, forest frogs did not increase their call complexity when moved to urban habitats.

“I was not surprised to find differences in the call or the predation pressure,” says Halfwerk. “I was surprised by the fact that an urban frog can call like a forest frog but not the other way around.”

Halfwerk and his colleagues say it could be that forest males are physically or physiologically constrained to produce simpler calls, perhaps because of smaller larynxes or lower levels of circulating testosterone.

It could also be that urban males are more flexible than forest males. High behavioral flexibility has been linked to successful colonization of urban areas in a variety of species. The urban environment seems to select for individuals that are more flexible and less risk aversive than their rural counterparts.

Halfwerk and his colleagues suggest that frogs in urban habitats, where the risks of being overheard by predators and parasites are lower, have evolved greater call flexibility.

“Because urban males are flexible in adjusting the complexity of their calls, they can match the forest males in the forest environment and outcompete them in the urban environment,” says Halfwerk. “Over long periods of time, if urban areas are replacing more and more natural areas, at some point you might expect forest males to disappear altogether.”

Differences in call complexity between urban and forest males reflect the differences in the selection pressures imposed on males in both environments: Urban males have to compete for a smaller number of females but they enjoy a lower risk of predation and parasitism. Increasing the complexity of their call is an adaptive response to city life for these frogs.