Bats buzz like hornets to scare away predators
The whining buzz of a wasp is enough to send many of us running for the hills. Now, it seems that one crafty species has used that aversion to its advantage. Researchers found greater mouse-eared bats mimic the buzzing sound of stinging insects like wasps, likely to scare off predators.
“This is a fascinating study,” says David Pfennig, an evolutionary biologist at the University of North Carolina, Chapel Hill, who studies animal mimicry but who was not involved with the work.
Nature is replete with examples of sneaky animals and plants imitating the traits of other organisms. The innocuous scarlet kingsnake (Lampropeltis elapsoides), for example, has adopted the red-and-black stripes of the dangerously venomous coral snake (Micrurus fulvius).
But there aren’t many noted instances of acoustic mimicry, Pfennig says, likely because they’re hard to study, not necessarily because they don’t exist. “We are a very visually oriented species, and there are a lot of sounds we can’t hear as humans.”
Danilo Russo happened upon one of these by accident. An ecologist at the University of Naples Federico II, he was conducting fieldwork in southeastern Italy more than 2 decades ago when he snagged some greater mouse-eared bats (Myotis myotis). The species is native to Europe and about the size of a house mouse. Every time Russo went to grab the animals and remove them from his nets, “they buzzed like wasps or hornets,” he says. It seemed like some sort of defense mechanism, he explains.
One of the mouse-eared bats’ biggest predators are owls, which commonly live in the tree nooks or rock crevices that wasps, hornets, and other buzzing, stinging insects hole up in. It occurred to Russo that the bats might be buzzing to mimic bees and send owls scurrying away. But it took him several years to find the right bat experts to help answer the question.
Once they teamed up, the researchers recorded the bats’ buzzing in the wild with microphones. They then used a computer program to compare the sounds with those made naturally by honey bees (Apis mellifera) and European hornets (Vespa crabro), which share habitat with both bats and owls. The program was only able to distinguish between the bats and the bugs about half of the time, a sign the buzzes are acoustically similar.
The scientists then set up an experiment in the lab to test predators’ responses to the buzz. They played both the bat and the insect recordings, as well as a control sound from a different nonbuzzing bat species, for eight barn owls (Tyto alba) and eight tawny owls (Strix aluco), which nest in the same crevices as the stinging insects. Half of the owls were raised in captivity, whereas half were wild-caught. The team classified the owls’ reactions to each sound, noting whether they tried to escape, attack, or inspect the speaker emitting the buzz playback.
The birds reacted consistently to both the bat and the bug sounds, darting away from the speaker in response to the buzzing. The wild owls had a stronger response to the sounds, possibly because of their prior exposure to stinging insects, Russo says. Some tried to fly away from the noise, whereas none of the captive owls tried to mount an actual escape. The finding is the first known case of a mammal mimicking a sound made by an insect species, the researchers report today in Current Biology.
Pfennig wonders how vulnerable the owls actually are to bee or hornet stings in the wild. “Owls are nocturnal, bees are not,” he says. The authors acknowledge they don’t know how often the birds get stung, but anecdotes point to an adversarial relationship. For example, when hornets colonize nest boxes set out for owls to live in, Russo says, “the birds do not even attempt to explore them, not to mention nest there.”
Despite this, the study is still “really cool,” Pfennig says. “Mimicry is one of the best examples of natural selection in action.”
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