July 2007, Volume 5, Issue 7

Listening for Tasty Frogs

The fringe-lipped bat eats a lot of things, from insects to small vertebrates, and sometimes even other bats. But it is most noted for feeding on frogs. It finds those frogs by listening for the loud, conspicuous calls the males produce at night to attract mates. The calls also tell these bats whether the noisy frog is poisonous or edible.
Graduate student Rachel Page of the University of Texas at Austin wanted to determine how bats learn to identify which frog calls promise a tasty dinner – and which signal danger. Are they born knowing what the calls mean? Or do they learn it over the course of their lifetimes?
Previous experiments she conducted with Michael Ryan of the University of Texas showed that the bats are extremely flexible and can quickly learn and relearn associations between prey cues and prey quality. Her next question was: Can these associations be transferred from bat to bat by social learning?
To find the answer, Page and her team captured fringe-lipped bats at the Smithsonian Tropical Research Institute in Panama and took them to an outdoor flight cage for behavioral tests. She decided to use a call that bats would probably never approach in the wild, that of the cane toad, which is not only highly toxic, but also several times the size of a fringe-lipped bat. Initial tests found that none of the bats responded to the cane toad calls.
Then they took one bat and offered food rewards for responding to calls that sounded – initially – like those of the túngara frog, which fringe-lipped bats eat frequently. The bat flew to the food. Then, over several trials, they gradually “morphed” the frog call until it sounded like the call of the poisonous cane toad. In a very short time, the bat learned to respond to the toad calls to get its food.
Next, Page put this newly trained bat and an untrained test bat together in a flight room. A food reward was placed on a concealed audio speaker, which broadcast the toad call. The trained “tutor” bat promptly flew down to the speaker and retrieved its food reward. Would the untrained bat follow suit?
They counted the number of times the tutor bat retrieved its reward before the test bat learned that, in this environment, cane toad calls mean palatable prey. Learning was extremely rapid. On average, the test bats watched a tutor bat just over five times, before they, too, responded to toad calls by flying to the speaker and collecting food.
At that point, Page removed the tutor bat to see whether the test bat had really learned the lesson or was just copying what the tutor did. In every case, the test bats had indeed associated the calls of a toxic toad with palatable food.
To further confirm the results, the team placed single, untrained bats in the flight cage, set up the speaker and food and played the toad call a total of 100 times over five nights. They repeated the tests with sets of two untrained bats in the cage. Only one bat in each control group ever figured out the link between toad calls and food, and each needed more than 80 trials. The others never responded.
Once the tests were completed, each bat was carefully reconditioned to avoid toad calls in the wild, then released where it had been caught.
These experiments clearly show that bats can learn socially from other bats. All that is required is that one bat observe another foraging. Fringe-lipped bats are social animals that roost in groups of four to fifty or more individuals. Multiple bats are often found at foraging sites, such as ponds that host chorusing frogs. So it is probable that individuals eavesdrop on the foraging activity of other bats, leading to the transfer of novel behavior from bat to bat.
BCI members can read the whole story of how fringe-lipped bats learn from each other in the Summer 2007 issue of BATS magazine.

BCI members can read the whole story of how fringe-lipped bats learn from each other in the Summer 2007 issue of BATS magazine.

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