Ripple Effect Used By Bats When Hunting Frogs During Mating Season
January 24, 2014

Bats And Male Frogs Sense Mating Call Waves Through Ripples

Brett Smith for - Your Universe Online

As male túngara frogs call from their puddles to attract females, they create ripples that spread across the water. According to researchers at the Smithsonian Tropical Research Institute (STRI) in Panama, these ripples are used by other male frogs to assess their competition – and also by bats looking for their next meal.

If it sees a bat flying overhead, a male túngara will stop calling – yet ripples continue moving for several seconds after the calls cease, the researchers said. Inside the their report, published by the journal Science, Smithsonian scientists said the bats use echolocation – a natural kind of sonar – to recognize these ripples and zero in on the frog. The findings reveal new details regarding a continuous evolutionary competition between bats and frogs.

"A general theme of this research is that the way we communicate with any kind of a signal is by creating a disturbance in the environment," said study author Mike Ryan, a biology professor at the University of Texas, Austin. "When we vocalize, we're causing changes in the air pressure around us and that's what our ears hear. When we use visual signals, light bounces off whatever pigments we're using and is transmitted to the receiver.”

“Anything we do disturbs the environment, whether it's intended as a communication signal or not,” he added.

The study team discovered that frog-eating bats were much more likely to attack a target that had both frog calls and ripples radiating from it than a target with frog calls and no ripples. This suggested that they can detect these ripples, most likely using echolocation, the researchers said. However, bats seem to lose this predatory advantage if the area around the frog is cluttered with leaves or other debris, which may stop the tiny waves from spreading.

The “tún-gara” noise of the very small rainforest frog is analogous to a peacock's train. The ripples generated by the male’s call seem to increase the response of rivals to the first caller. Female túngara frogs are enticed by the nightly calls of their male suitors. All this activity makes it simpler for the bats to discover their victims, according to the study team.

"It's comparable to the use of lip reading," said Wouter Halfwerk, a post-doctoral fellow at STRI from the University of Leiden in the Netherlands. "While sound is the most obvious component of the frogs' communication, the call-induced ripples alter the behavior of competing males that sense them. Bats perceive the ripples too, using echolocation, which shows that the costs associated with communication can be imposed through a sensory domain that is fundamentally different than the intended receiver of the frog's call."

The team discovered that contending male frogs more than doubled their call rate when presented with rivals’ ripples and sound compared to sound alone. However, male frogs did not respond to ripples by themselves. Male frogs stopped calling when they were within about 3 inches of a wave-making rival, an indication that ripples are competitive interactions.

“This study shows how important it is to look at the full picture—perception not only of signals but also of their by-products by different receivers through different sensory channels can generate both costs and benefits,” said co-author Rachel Page, an STRI staff scientist.