A romantic serenade at sunset: this is the favourite mating strategy of the little Túngara frog (Engystomops pustulosus) in the Panamanian rainforest. When all is dark, after the evening rain, the 3 cm male frogs gather in puddles to utter their mating calls in a concerto that quickly escalates into a fierce calling competition. In towns and cities, however, where noise covers up the little frogs’ orchestra, Túngara males have had to change song style. The new urban frog repertoire consists of more complex calls uttered at a faster tempo, which seems to attract females. But is it enough to switch genre in the distracting and misleading urban conditions to stay attractive? That's the question Judith Smit and colleagues from the Amsterdam Institute for Life and Environment, The Netherlands, and the Smithsonian Tropical Research Institute, Panama, tried to answer.
To measure how urban noise and light pollution affected the rivalry between males, the researchers arranged frog call battles in the lab. They set up an arena with synthesised distracting city noises and bright light to recreate the conditions frogs encounter in towns, or softer sounds with dim light to simulate the forest. Then, they caught male frogs from a town and the nearby forest and recorded vocal contests between pairs that had originated from the same location in the artificial town situation and in the simulated forest. In the urban conditions, all of the frogs ended the competition relatively quickly. The urban males also called louder, their calls were more complex in urban conditions and the calls of rivals were more similar. They seemed to take advantage of the safer conditions in the town with fewer predators to produce more attractive calls. In contrast, the forest males were less competitive in the urban context, as though they were distracted by the unfamiliar conditions, which might seem more risky to them. However, the forest rivals’ call intensity and complexity were more different in the urban setting. Large differences between rivals’ calls typically helped to settle the contest faster, and helped the female decide on the best mate. But can females perceive the changes in the males’ performances caused by light and noise pollution?
To answer this second question, the researchers decided to ask the female frogs directly. They caught female frogs from the town and the forest and took them to the artificial town and forest in the lab. They then broadcast the male call battles, which they had recorded previously, from a pair of loudspeakers located on either side of the arena, with each speaker playing the voice of a single contestant to the female placed in between, allowing her to select the voice she found more attractive. The females had stronger preferences when the differences between the calls of the two performers were more pronounced and when the contests were short. They were also less distracted and had stronger preferences when the conditions were more similar to those of their home. Finally, the females’ preferences were stronger when forest males were duelling in the noisy urban conditions. This shows that females can perceive how males adapt their calls to light and noise pollution, and this influences their preference.
The moral of this frog story is that city-dwelling animals have to adapt their communication strategies to both the distracting noise and lights, which drown out calls and impact female choice directly. How they adjust to city conditions can also indirectly influence the decisions of females seeking a suitor.
