Bats hunt ballooning túngara frogs by echolocation

Bellowing male túngara frogs make easy prey for fringe-lipped bats thanks to echolocation, scientists report August 27 in the Journal of Experimental Biology. The bats’ sonar spies the Central American amphibians’ ballooning vocal sac, turning a sexual display into an invitation for lunch.

When inflated, the vocal sac nearly matches the full body size of the male túngara (Physalaemus pustulosus). Both the sac’s girth and a frog’s love song enchant female mates. Fringe-lipped bats (Trachops cirrhosus) hunt the males, but scientists wondered how senses besides hearing guide the pursuit.

To investigate, researchers placed 10 wild fringe-lipped bats into netted enclosures with two lifelike rubber imitations of male túngaras, dubbed robofrogs. Both robofrogs played mating calls, but only one had a billowing vocal sac. The other’s sac remained deflated. The researchers observed that the bats always attacked or hovered over the robofrog sporting a puffing sac.

Shutting off the lights didn’t stop bats from successfully hunting robofrogs, ruling out the involvement of vision. In tests where a plastic cup covered the frog’s vocal sac, however, the bats became flummoxed, indicating that the hunt requires echolocation.

An ultrasonic microphone situated in front of the fake frogs detected the signature sound waves of the bats’ echolocation chirps. The bats emitted at least two sonar bursts after each of the frog’s croaks, which overlapped with the sac’s expansion and deflation.

Constant sac movement was the key giveaway, says sensory ecologist and coauthor Wouter Halfwerk of the Smithsonian Tropical Research Institute in Panama. The bats build a mental scene with the contrasting echoes, spotting the moving vocal sac among rocks and other static ground clutter to home in on prey, Halfwerk says.