UT Study Finds That Frog’s Love Song Can Be Kiss of Death
Some say that love bites. This may be especially true for the túngara frog.
UT researchers have discovered that predatory bats eavesdrop not only on the túngara frog’s mating calls, but also on the lingering water ripples the calls create. So, even when a frog pauses its love song, bats are still able to swoop in and snag the frog for dinner.
“It’s like an animal running away from a predator and hiding behind a tree, and a predator following its footprints or odor,” says Mike Ryan, co-author of the study and professor in UT’s Department of Integrative Biology. “And once you’ve made a footprint, you can’t take it back.”
The discovery, published last week in Science, sheds light on how seemingly subtle secondary effects of behavior—such as ripples on the water’s surface—can be important signals within and across species.
Among the study’s contributing researchers is Longhorn alum Rachel Page, PhD ’08, now a staff scientist at the Smithsonian Tropical Research Institute in Panama.
Túngara frogs are inch-long amphibians native to Central and South America. They breed in shallow, temporary pools during the wet season. Males depend on their call, a series of “whines” and “chucks,” to attract mates. But the calls that are most attractive to female frogs are also the most appealing to frog-eating bats, says Ryan, who splits his time between studying frogs in his lab at UT and in the field in Panama.
And while the frogs instinctually stop their call when they sense movement from above, ceasing the call is not enough, Ryan and the other researchers found. The bats are able to zero in on the frogs by using echolocation to sense water ripples created by the frog’s vocal sac, a balloon-like structure that inflates and deflates as the frog calls.
Using recordings and wave-making machines, the researchers found that the bats preferred targets that included both the call and the ripples to a target that consisted only of a call.
The frogs may be developing a counter-strategy. Predation by bats and other aerial predators like owls has likely influenced the frog’s preference for spots shielded with vegetation, Ryan says. Besides serving as cover, the leaf litter in the surrounding water may help protect the frog by scrambling ripples; the research found that the bats’ preference for the target with ripples over a call-only target disappeared when the experiments took place in pools covered in leaf litter.
But ripples aren’t all bad. While they may help bats target frogs, they also help frogs communicate with one another.
The researchers found that male frogs use ripples to detect and respond to rival males in the surrounding area, with frog calls increased twofold when ripples were added to a recorded rival call. The ripples also help set and maintain the frog’s claimed space in the pool.
“They want the other males to know that they’re there, because like a lot of other animals, they maintain these distances between them,” Ryan says.
Being heard over a din of calls and having a space to yourself can make all the difference in the mating outcomes of the frog because it’s the females who pick and choose their mates.
The constant tension of sexual traits—like the túngara frog’s call and associated ripples—between wooing mates and attracting predators (or other adverse effects) is a central theme of biology, Ryan says. It’s the enabling force behind the cumbersome but beautiful tail feathers of peacocks, and bulky but impressive stag antlers. The tension may even help explain in part certain human behaviors, too, says Ryan, like the muscular but impotent anabolic steroid user.
“There’s a lots of examples of these kinds of trade-offs. And since we’re interested in these kinds of traits of sexual beauty, from an evolutionary point of view, you need to know what the advantage is, but you also need to know what the cost is,” Ryan says. “If there was no cost in beauty, all animals would be beautiful.”
Watch a video of the túngara frog’s call below:
Illustration by Melissa Reese
