A fringe-lipped bat, Trachops cirrhosus, approaches a Fitzinger’s robber frog, Craugastor fitzingeri, in Panama. This species of bat eavesdrops on the mating calls that male frogs produce to attract females, and uses these frog calls to detect, assess and localize its prey. Young bats do not share the same responses to frog mating calls that adults do; juveniles must acquire the ability to discriminate which frog calls signify palatable and poisonous prey. Credit: © Merlin Tuttle.

Data collected on juvenile and adult fringe-lipped bats reveal the first clues into the development of eavesdropping behaviour in predators.

Scientists at the Smithsonian Tropical Research Institute (STRI) found that the fringe-lipped bat, known to eavesdrop on frog and toad mating calls to find its prey, learns to distinguish between palatable and unpalatable frogs and toads through experience.

The findings provide the first evidence that eavesdropping predators fine-tune their hunting cues over the course of their development. 

To source their food, some predators eavesdrop on calls emitted by prey.

Fringe-lipped bats, which range from Panama to Brazil, are some of the most skilled eavesdroppers in the world: they are attuned to the sexual advertisement calls of over a dozen frog and toad species that live in their habitat.

If a fringe-lipped bat hears a call, it will fly toward the sound within seconds.

However, just like some incoming calls on people’s cell phones originate from scammers, not every frog or toad call guarantees a safe and healthy meal – if a frog is too large or emits a toxin, the prey can pose a danger to the bat. 

Caller ID

Fringe-lipped bats have adapted to this risk by developing their own caller ID; if they hear a call from an unpalatable frog or toad, they save their time and energy by not responding.

But frog and toad calls do not come with automatic ‘spam’ warnings like phone calls do, and until now, scientists did not know where the fringe-lipped bat’s ability to distinguish between palatable and unpalatable frogs came from. 

Lead author Logan James, who is an STRI postdoctoral researcher, said: “It’s truly remarkable that these bats hunt using the calls of an entirely different group of animals in the first place, and we have wondered for a long time how these bats acquire this unusual skill.

“We knew from previous research that these bats are very clever and can learn during experiments, but we had not tested whether their aptitude for learning had a role in fine-tuning their responses to sound cues from their natural prey.”

The research team tested how individual, wild-caught adult and juvenile fringe-lipped bats responded to mating call recordings of 15 local frog and toad species.

These species included frogs known to be palatable, toxic or too large to handle.

First, the team confirmed previous studies showing that adult bats responded more strongly to palatable versus unpalatable frogs and toads.

Yet, as the team discovered, juvenile bats did not make the same distinctions; on average, juvenile bats did not respond differently to frogs and toads based on their palatability. 

Looking more closely, the team found that juveniles could identify larger prey by their calls, just like adults can, but they could not distinguish the toxic species.

This indicates that juveniles tend to respond to body size early on, but they learn to identify toxic species over time. 

Time and experience

STRI staff scientist Rachel Page, one of the study’s senior authors, said: “We have studied this fascinating species for years, and in many aspects, we understand its behaviour very well, but this was the first time we had ever tested juvenile bats.

“It was so interesting to see that, like human children, young bats needed time and experience to hone their discrimination skills.” 

This study is the first to explicitly compare eavesdropping responses of generalist predators at different ages.

But it is likely that this strategy is widespread across the animal kingdom, and fringe-lipped bats are not the only predators that need to learn how to distinguish palatable prey.

Ultimately, this study highlights the critical role of experience in early life in shaping predatory behaviours in the wild. 

Senior author Ximena Bernal, research associate at STRI and professor at Purdue University, said: “This study highlights the power of development and learning to shape eavesdropping behaviour, an insight that may extend far beyond bats to other predators also navigating complex sensory environments.

“We hope it will inspire other scientists to examine how early experience modulates predator-foraging decisions.” 

The findings are published in the Proceedings of the Royal Society B.