On a recent flight to visit family, my son loudly announced to the entire plane that he needed to pee. It wasn't his lack of modesty, but rather the fact that he was wearing headphones and had lost the reference of his own voice to modulate his volume. Although his amusing mistake was easily remedied, many animals face a similar problem due to encroaching urbanization. Traffic and people are noisy and so, to be heard, urban animals act like my son and make more noise themselves. This matters because animals use their calls to communicate: I'm here, I'm sexy, I'm tough, etc. And getting it wrong might mean not getting the girl or, even worse, getting eaten. But how capable are animals of adjusting their calls to match local conditions? In a fascinating new study by Wouter Halfwerk from the Vrije Universiteit Amsterdam in The Netherlands and his international colleagues, the answer is that ‘it depends’.
Túngara frogs in Panama face a trade-off between sex and death. While males use calls of varying levels of complexity to attract females to mate, these same calls attract the attention of predatory bats and blood-sucking midges. Over millennia, frogs have evolved strategies to balance this trade-off, but they haven't had millennia to deal with their shrinking habitat. Cities have grown where trees were harvested, and frogs have had to adapt to these changing conditions.
To quantify the differences that túngara frogs face between life in forests and urban areas, Halfwerk and his colleagues measured light and noise intensity in the two locations. As expected, cities are brighter and louder. But, rather than getting stressed out by city life, the urban frogs are apparently more relaxed. And for good reason, because cities have fewer bats and midges. Thus, released from the risks of death, city frogs can focus on sex! They call more, produce calls with greater complexity and they continue to call when they are approached by eavesdropping scientists, in contrast to forest frogs that clam up without much provocation. These changes are not only permissible but also necessary. The simple calls of forest males don't cut it in the city; urban females aren't impressed. By contrast, forest females swoon over the complex calls of urban males. City males are apparently sexier everywhere.
When the country mouse in Aesop's fable traveled to town to dine with his city cousin, he was frightened by dogs and returned to the safety of the country. No such luck for country frogs; the forest is a den of dangers. So how do city frogs fare when returned to the forest? Surprisingly, rather than getting eaten, city frogs dial down the complexity of their calls to match the increased risks of the forest. In other words, city living has selected for flexible frogs whose behavior is conditioned by risk.
It isn't known whether call flexibility has any downsides in the forest. But if not, Halfwerk and his team worry that forest frogs may be doomed if increasing urbanization brings túngara frogs from the forest and the city into more frequent contact. This, unfortunately, seems likely.
