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Bat's call: back away from my bug


Bug off: Big brown bats have a special call to keep rivals away from their dinner (Source: Jessica Nelson/University of Maryland)

Call of the wild I'm bad, brown and a hungry bat, so leave that bug alone.

While the human ear may not be able to hear it, that is the message male big brown bats send out to rivals as they home in a tasty meal.

Scientists today say they have identified a previously unknown call made by these bats - different from the sonar-like echolocation used for mid-air navigation and hunting - that tells another foraging bat to keep away from their prey.

The call is made exclusively by the males of this species for reasons that are not entirely clear. And the other foraging bats seem to honour the request, the researchers find.

The discovery indicates that acoustic communication in these flying mammals may be more sophisticated than previously thought and underscores the importance of vocal social communication for these nocturnal insect-eating animals, the researchers say.

"Bats may be avoiding aggressive interactions with the other bat," says University of Maryland biologist Dr Genevieve Spanjer Wright, who led the study published in the journal Current Biology.

"Chasing and even occasional physical contact have been observed in this species during foraging flight, so use of - and response to - social calls could be a way to limit the need for aggressive interactions or even injury to the responding bat," Wright says.

Different call

The big brown bat, whose scientific name is Eptesicus fuscus, ranges from Canada through the United States, Mexico, Central America and northern South America.

The fur of this medium-sized bat is brown, its muzzle, ears and wing membranes are black, and it has rounded ears and a broad, blunt snout. Its wingspan is roughly 33 centimetres.

The researchers, as they examined audio recordings of bats doing mid-air foraging together, detected some calls that seemed different from the usual echolocation - bouncing sound waves off objects - sounds made by the animals as they fly.

Intrigued, they analysed audio and video recordings of flight paths and calls during an experiment inside a flight chamber at the University of Maryland as male and female bats flew alone or in pairs in search of mealworms that the researchers dangled from thin threads attached to the ceiling.

They found males made an individually distinct, ultrasonic call dubbed a "frequency-modulated bout" or FMB, that warned other bats to stay away from the prey.

It is a series of three to four sounds, longer in duration and lower in frequency than echo-location pulses, and is frequently followed by short, buzz-like calls, the researchers say.

"When slowed down, it sounds a bit like a sequence of chirps," Wright says.

When males made the sound, other bats generally moved away from that bat and from the prey, helping to ensure that the bat making the sound snagged the meal, the researchers say.

"When two males flew together in a trial, it was not uncommon for each bat to emit FMBs," says Wright. "We found that the bat emitting the greatest number of FMBs was more likely to capture the mealworm."

Wright says it is not entirely clear why the females did not make this sound.

She says one possibility is the males are more territorial or aggressive than the females.

She also noted while the females roost in maternity colonies during the summer months, males tend to roost alone or in smaller "bachelor" colonies.

This might make the males less familiar with the other bats out foraging for bugs, she says.

Wright says it is possible other species of bats use similar calls but further research is needed on that.

Tags: animals, animal-behaviour