Researchers from a range of scientific institutions, including the University of Texas at Austin, have established that bats prey on túngara frogs, principally using the water ripples cast during their mating calls. The findings of the latest study were published in the journal Science, and was entitled Risky Ripples Allow Bats and Frogs to Eavesdrop on a Multisensory Sexual Display.
The túngara frog (Engystomops pustulosus) is typically found across parts of Central and South America, including Costa Rica, Belize, Guatemala, Colombia and Venezuela. The frog typically inhabits tropical and subtropical dry forests and lowlands, freshwater marshlands and savannas. Túngara frogs nest along the surface of small ponds, with male members of the species constructing foam nests to support and protect eggs and hatched larvae.
As the túngara frogs commence their mating rituals from shallow waters, their activities are often monitored from high above. Predators – including the frog-eating bat Trachops cirrhosus – canvass the grounds from higher regions, ever-searching for their prey. However, the frogs are not oblivious to the dangers of predation and often cut short their mating calls, upon suspecting an eavesdropper is lurking in the distance. Unfortunately, for the túngara frog, the mating call initiates a “multisensory display,” which can be detected by enemies, as well as potential mating partners.
As the frog emits a series of “chucks” and “whines,” its vocal sac goes through a cycle of inflating and deflating, casting ripples that results in the frog inadvertently sabotaging its own position and alerting nearby predators. According to a researcher at the Smithsonian Tropical Research Institute, Rachel Page, the basic sound emitted by túngara frogs is a whine. The inclusion of chucks, however, can make the mating call seem more “attractive.”
In particular, bats can use their echolocation to more effectively detect the crooning túngara amphibians. Co-author Mike Ryan, who works in the Department of Integrative Biology at the University of Texas, Austin, explains that any method of communication creates a “… disturbance in the environment,” triggering an alteration in the air pressure around its original source.
To study this phenomenon, the researchers deployed a series of fake frogs in shallow waters and played the túngara frog’s distinctive mating calls. During some of the experiments, the researchers ensured the frog remained perfectly still; during other experiments, the researchers artificially created a series of ripples to coincide with the calls.
Ultimately, the researchers established that the frog-eating, fringe-lipped bats were much more likely to successfully hunt túngara frogs that were using mating calls, while ripples were radiating from the creatures’ bodies, compared to frogs that were only using mating calls; bats were 36.5 percent more likely to target the frogs that produced ripples. Intriguingly, this advantage was significantly impaired if the frog was sitting in shallow waters that were cluttered with “leaf litter.” The researchers conjecture the leaves to impede propagation of sound waves through the water, thereby reducing the magnitude of the ripples.
Meanwhile, lead author of the study Wouter Halfwerk recently explained that these frogs have developed a strategy to escape predation; he reflected upon this finding, during a recent press release:
“When a frog detects the shadow of a bat overhead, his first defense is to stop calling immediately. Unfortunately for the frog, the water ripples created by his call do not also stop immediately. The ripples continue to emanate out for several seconds, creating a watery bull’s-eye on the frog. Bats use the ripples, thereby beating the anti-predator strategy.”
Halfwerk claims he was inspired to investigate the potential link between the túngara frog’s mating call and the predatory instincts of bats after conversing with his colleagues; Halfwerk states they had mentioned that bats could use echolocation to detect fish breaching the water’s surface, causing them to swoop in and claim their prize.
The mating calls also generate different reactions in competing male frogs, depending upon the nature of the call. The study group established that other male frogs would respond to the mating call of a fellow male, by attempting to out-compete their rival’s amorous bellows. When a túngara frog’s mating call was accompanied by water ripples outside of the male’s “zone of defense,” – a circle with a diameter of approximately 15 centimeters – rival males would begin calling at twice the speed.
By James Fenner