By Will Dunham

WASHINGTON (Reuters) - The sound made by a male big brown bat as it zeroes in on a bug that might make a tasty meal - if another bat doesn't get there first - is a sequence of chirps beyond the range of human ears.

But to another bat, the meaning is unmistakable: "Back off."

Scientists said on Thursday 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 honor the request, the researchers found.

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 said.

"Bats may be avoiding aggressive interactions with the other bat," said University of Maryland biologist 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 added.

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 13 inches.

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 analyzed 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 that 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 echolocation pulses, and is frequently followed by short, buzz-like calls, the researchers said.

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

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 said.

Wright said it is not entirely clear why the females did not make this sound. She said one possibility is that the males are more territorial or aggressive than the females.

She also noted that 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 said.

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

(Reporting by Will Dunham; Editing by Tom Brown)