Love song or dinner gong? Parasitic flies follow frog croaks to find blood meals

At night, during the rainy season in South and Central America, around ponds, lakes or ditches, you're likely to hear the peculiar high-pitched whines of male túngara frogs. Sometimes the amorous amphibians will spice up their songs with "chucks" that sound a bit like someone plucking at a rubber band. Like all frogs, they use these unique songs to attract the attention of possible mates, but the lady frogs aren't the only ones listening...

Flies of the family Corethrellidae are small blood-suckers whose taste for frogs is so infamous that they're actually called "frog-biting midges". But while their cousins, the mosquitoes, sniff out their targets using chemical cues, the midges that go after túngara frogs follow the croaks – an extremely rare case of invertebrates eavesdropping on vertebrates.

A frog-biting midge homes in on a male frog, landing on its back and walking to the nostrils to sample a blood meal. Video: XE Bernal

This unusual strategy was discovered through work done in large part by Ximena Bernal of the Smithsonian Tropical Research Institute. She and her colleagues set up speakers in Panama to mimic the frogs' calls, and they sure were popular with the midges, attracting as many as several hundred flies in just half an hour! And, like the female frogs, the bugs were more interested in the noisiest males, and the ones with the most complex songs.

For a frisky male frog, being harassed by insects during your sexy singing ritual is a pain (quite literally), and the frogs are often observed swatting at the flies while croaking. But there's another danger inherent in this situation: male túngara frogs are much more likely than females to end up with trypanosomes, parasites that are transmitted by blood-feeding insects.

It seems frog-biting midges are a musical species. During the mating season, they gather in swarms, buzzing alluringly to each other at the same frequencies as the frogs' calls. Bernal and her team suspect the flies' acute hearing first evolved as part of their own mating ritual, and turned out later to be a convenient sense for tracking tasty frogs.

And biting bugs aren't the only assailants drawn in by the vocals of the túngara frog. Fringe-lipped bats are also known to "listen" for their meals, following not only the sound of the croaks, but also the ripples that the frogs' croaking causes in the water around them. And, once again, the more complex croaking songs bring in the most bats.

This puts the male túngara frogs in a precarious position. Assertive, complex songs attract the most females – a big plus for breeding – but also make a frog more likely to end up covered in flies, infested by parasites or eaten by a bat. This fascinating conundrum makes the species an excellent case study for scientists interested in the crossroads between natural selection (survival of the fittest) and sexual selection (success of the sexiest).

"This system reveals the complex and intriguing ways that nature works," Bernal told The Scientist. "It shows that we cannot study animals in isolation. We have to take into account their enemies, as well."