'Then they deflate their vocal sack, hunch lower into the water, and finally dive under the surface and swim away.' Problematically, though, the rippling water can persist for a moment after they stop calling, a time lag that leads many an amorous frog to be eaten. 'When they see approaching bats, the first thing they do is stop calling,' Page says. The frogs are clearly aware that, in some cases, their calls give them away. Most bats are mainly sensitive to hearing their own echolocation calls, but this species has an additional sensitivity at a much lower frequency that matches the frog calls. The bats accomplish these feats, Page says, with a specially-adapted hearing system. 'This is the only species of bat in the world known to eavesdrop on the calls of frogs.' Separate research has shown that the bat species is even capable of interpreting the sound of other frogs' calls to determine if it's a palatable or poisonous species. Still, the ability to listen in on frog calls at all-both by hearing them and by detecting the ripples-is 'very unusual,' Page says. Interestingly, though, when they filled the pools with dead leaves-mimicking some natural forest pools, but also cutting off the movement of ripples-the bats' preference disappeared, indicating that in dirty pools, they can't track the ripples well enough to use them as a hunting cue. When they unleashed bats into the experiment, they found that they dove at the frogs next to rippling pools 36.5 percent more often than than the still ones. For some of the pools, they artificially generated ripples that resembled those made by calling frogs others, they left still. To test the hypothesis that the bats were picking up on these ripples, the researchers put fake plastic frogs next to shallow pools of water and played recordings of their mating calls.

'In addition, they possess a conspicuous vocal sac that's used to recycle air quickly for calling, and as air is shuttled back and forth into the vocal sac, the movement makes ripples on the water surface.' 'The frogs have developed a big larynx to produce very low and loud sounds,' says Wouter. The male túngara frogs, he realized, similarly disturbed the surface of the shallow pools of water on the forest floor when they made calls. Wouter was inspired to study the topic, he says, after conversations with colleagues about other bat species' potential ability to detect fish breaching the surface of water by echolocation, which would allow the predators to know when to swoop in for the kill. The finding emerged through a project led by Wouter Halfwerk that looked at the acoustic and tactile effects of the frogs' calls on fringe-lipped bats, as well as on other frogs competing for mates. Photo by Ryan Taylor/Salisbury University

Generating the call creates a circle of ripples in the water.

The frogs psychobilly