A tale of town frog and country frog

IN THE FABLE of the town mouse and the country mouse it is the country mouse who comes out on top. When he visits the city he discovers the fancy living enjoyed by his urban cousin comes at a heavy price of mortal danger. The countryside, though duller, is safer. It is all very moral. But research on some real animals that have living arrangements similar to those of Aesop’s pair of rodents suggests it is wrong.

Túngara frogs are tropical New World creatures that live both in the countryside and in towns. That made them attractive to Wouter Halfwerk of the Free University of Amsterdam, who wants to understand how urban life affects animal communications, and whether animals are evolving by natural selection to deal with the city, rather than just learning to do so.

Working in Panama, Dr Halfwerk and his colleagues began by recording the calls male frogs make to attract females. As they describe in a paper in Nature Ecology and Evolution, they collected 11 pairs of such calls, each being recordings of the frogs in an urban area and in nearby forestland. Comparing the pairs showed that the urban frogs had more complex songs and sang more often than those in the forest. Also, if approached by a human observer, the forest frogs stopped singing more quickly than did the urban ones.

Having established all this, the researchers went on to watch the effects of singing in each of their 22 sites. For a male túngara frog singing has effects both beneficial and baneful. The beneficial effect is to attract females. The baneful ones are to attract both predators (mainly in the form of bats) and parasites (mainly in the form of blood-sucking midges). Dr Halfwerk and his colleagues used video surveillance to count the numbers of females and bats who came looking, and deployed special insect traps to sample the midges. In the cases of all three, forest frogs attracted more interest than city ones.

Conversely, however, when researchers offered females the choice (in an experimental set-up using two loudspeakers), they found that the city-frog calls were much more attractive to individual females than were the forest-frog calls. Putting all these facts together, they came up with a hypothesis. This was that city frogs were at less danger than forest frogs of suffering predation or parasitism (presumably towns are a worse habitat for bats and midges than forests are), and could therefore afford flashier songs—and also risk singing for longer when approached by a threat, such as a human being. At the same time, because fewer females turn up at the pond, they need their flashy songs in order to win female attention.

That raised a question, though. Was each male adjusting his behaviour to his circumstances, or have town frogs evolved to be different from their forest confrères? One further experiment solved this. The researchers collected frogs and moved them around—sometimes to sites of the same type (urban to urban, rural to rural) and sometimes to sites of a different type.

The upshot was that urban frogs proved able to adjust their calls to the dangers of forest life, but forest frogs were unable to up their courtship game in the bright lights of town. They had not, apparently, evolved the capacity to do so. In Frogland, then, country frogs really are bumpkins and urban ones really are urbane. And the reason is that the countryside is more dangerous than the city. Aesop eat your heart out.