Young bats learn to discriminate while listening to the next meal

It's late at night and we're watching the bats silently through the night vision camera. From nearby speakers, there was a long rattling tremolo.

The rattled tremolo phone of cane toad.

The bat briefly cheered up and twisted his ears as it listened to the sound, then lowered his head downwards, not interested.

The speakers are next to a higher “complaint”, followed by “Chuck.”

Thungala's "complaint" call.

Bats shaking their ears violently as they fire from their habitat and dive down to attack the speakers, then spreading their wings.

Bats show huge differences in the food they survive. Some species specialize in fruits, others specialize in insects, while others specialize in nectar. Some species even catch fish with their feet.

Bats eat frogs — The phone calls used by male frogs to attract companions also attracted eavesdropping predators. Here, a consuming bat consumes the unfortunate male Thungala frog. Marcos Guerra, Smithsonian Tropical Institute

At the Smithsonian Tropical Institute in Panama, we have been studying a species, the marginal bat (cirrhosus), decades. This bat is a carnivore that specializes in feeding on frogs.

Male frogs from many species call to attract female frogs. Filled bats eavesdrop on those phones to find their next meal. But how do bats become companion voices and prey?

We are interested in learning how predators who eavesdrop on prey gain the ability to distinguish between delicious and dangerous meals. We combine expertise in animal behavior, bat cognition and frog communication.

How do bats know about delicious meals?

There are nearly 8,000 frog and toad species in the world, each with a unique appeal. For example, the first rattled call we played from the speaker came from a large and toxic cane toad. The second "embrace" comes from Túngara Frog, the first choice prey for these bats. Just as herpesologists can tell frog species through calls, carnivorous bats can use these calls to identify the best meals.

Over the years, our research team has learned a lot from carnivorous bats about how to use sound and echolocation to find prey, and the role of learning and memory in foraging success. In our newly published study, we highlight the association between the sounds heard by bats and the mass of prey generated over the lifespan of a single bat.

Bats catch frogs from the pond — Adult bats shown in the picture have extensive acoustic repertoire and remember specific frog phones year after year. Young bats must learn to respond to what calls through experience and to critically ignore them. Grant Maslowski

We considered whether the association between sound and delicious food is a natural evolutionary specialty for bats. However, this possibility seems unlikely, as the bat species we studied have extensive geographical distributions in both central and South America, and the frog species found in this range vary widely.

Instead, we assume that bats learn to associate different sounds with food when they grow up. But we have to test this idea.

First, we spent some time in forests and ponds with our collaborators to record mating calls from the most common frog and toad species in the Panama Research Area.

Researchers untie the bats from finely woven thin slices at night. — Rachel Page, one of the study's lead authors, took the bats from the mist net in Panama. Jorge Alemán of the Smithsonian Tropical Institute

We then built a mist net along the streams in Soberanía National Park to capture wild animals for research.

Frog phone call, bat respond

For testing, each bat was individually installed in a large outdoor flight room. From a speaker on the center ground, we played a phone call from a frog species in a loop and measured the behavior of the bats, which were inhabited from cloth. As we would expect, adult bats are often not interested in the sounds of impossible species, such as those sounds of toxins or species that are too large for bats.

But this is another story for young bats. Compared with adults, teenagers performed obvious predatory behaviors toward the venomous toads. Their response to the sound of the Túngara frog is weaker than that of adults, a delicious, rich prey that adult bats prefer.

Therefore, it seems that teenage bats must learn the association between sound and food in their lives. As they grow up, we believe they learn to ignore the call of frogs, which is not worth the trouble, and the call of frogs will be a good meal.

To better understand how sound drives prey associations, we measured the acoustic properties of different calls. We found that some of the most obvious features of the call were related to body shape: larger frogs produce low-frequency calls—that is, they sound deeper. Adult and juvenile bats respond more strongly to larger species, which will provide larger meals.

However, there is one distinct exception to the reactions of adults, where poisonous toads and very large frogs cause a much weaker reaction than expected for body size. This finding leads us to assume that bats have early biases to focus on louder sounds. Then they have to learn through experience that dietary quality is not only about size. Some meals are toxic or impossible to carry, making them impossible.

[embed]https://www.youtube.com/watch?v=mc3h4vjnrao[/embed]

Once the researchers studied each carnivorous bat for a few days, they released it safely to the place where it was initially captured. The footage is compiled by Caroline Rogan and courtesy of Léna De Framond-Bénard and Eric de Framond-Bénard.

After the bat stayed with us for a few days, we released everyone at its original capture site. The bats set out, carrying a small RFID tag, just like the people pet owners use to identify their dogs and cats, if we meet again as part of future research.

As bats live in the wild, we continue to seek to deepen our understanding of the subtleties of information discrimination. How can individuals weed with information overload to make meaningful and beneficial choices? This is a challenge we face every day.