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These bats use stealth sonar, likely to evade rivals

Hoary bats are habitual squawkers. Sporting frosted brown fur á la Guy Fieri, the water balloon–size bats bark high-pitched yips to navigate the dark night sky by echolocation. But a new study reveals that as they fly, those cries often drop to a whisper, or even silence, suggesting the bats may steer themselves through the darkness with some of the quietest sonar on record.

To find out how hoary bats navigate, researchers used infrared cameras and ultrasonic microphones to record scores of them flying through a riverside corridor in California on five autumn nights. In about half of the nearly 80 flights, scientists captured a novel type of call. Shorter, faster, and quieter than their usual calls, the new “micro” calls use three orders of magnitude less sound energy than other bats’ yaps did, the researchers report today in the Proceedings of the Royal Society B. As bats approached objects, they would often quickly increase the volume of their calls. But in close to half the flights, researchers did not pick up any calls at all.

Listen to a regular hoary bat call here.
Listen to a microcall here.

This stealth flying mode may explain one sad fact of hoary bat life: They suffer more fatal run-ins with wind turbines than other bat species in North America. The microcalls are so quiet that they reduce the distance over which bats can detect large and small objects by more than three times. That also cuts bats’ reaction time by two-thirds, making them too slow to catch their insect prey.

So why risk starvation and fatal crashes? Making normal-intensity calls might attract unwanted aggression from potential rivals, say the researchers, who conducted their study during the bats’ mating season. Microcalls are much more discreet, slashing the distance that other bats can “eavesdrop” from about 92 meters to 12. So the stealth sonar might simply be part of a larger tactic to keep rival males out of earshot.