Mimicry is widespread in the animal kingdom.
Some caterpillars can make themselves look like poisonous snakes. The chicks of an Amazonian bird called the cinerous weeper metamorphose into poisonous larvae. Flower-loving hoverflies have evolved to resemble stinging, unpleasant-tasting wasps.
But this specific type of mimicry is almost always visual in nature, as far as we know. And it is most often found in insects, birds and reptiles.
Now, for the first time, a type of acoustic mimicry has been observed in mammals. A study published on May 9 in Current biology found that a common European species, the greater mouse-eared bats, seem to mimic the humming of hornets – likely to avoid being eaten by owls.
“We discovered that a mammal mimics the sound of an insect to scare away a predatory bird,” explains Danilo Russo, lead author of the paper and professor of ecology at the Università degli Studi di Napoli Federico II, Italy. “This is an amazing evolutionary interaction involving three species that are evolutionarily distant from each other.”
What’s the buzz?
Greater mouse-eared bats, also known as myotis myotis, are a widespread European bat species that enjoy nibbling on insects, especially beetles. They hang out in colonies in woods and forest edges, roosting in underground caves for most of the year or in buildings during the summer. They are often preyed upon by various birds, including barn owls (These albums) and tawny owls (Strix aluminum), especially when they leave or return to their dorms.
In 1999, Russo was working on creating a call library for the echolocation calls of European bats and collecting data on how various species communicate with each other. While extracting a small mouse-eared bat from a mist net, holding it in its hands, the creature began to shiver and emit a continuous, intense buzzing sound, Russo says. Russo was surprised.
“My very first thought was…that looks like hornets or wasps!”
Initially, researchers speculated that the buzz was just a daily distress call. But the sound was so obviously insect-like that a hypothesis was born almost immediately, Russo says, and ultimately, years later, they decided to test it: Could it be that bats mimic hornets or bees ?
Russo himself had once picked up pellets of barn owls at the entrance to a cave where these bats roost. “Believe it or not, the sinkers contained a lot of bat skulls,” he says, so he felt it was not impossible that these bats “have, during evolution, “made” a very extreme attempt at deterrence. [owls] to escape.”
give a hoot
In the current study, Russo and his colleagues first compared bat buzzes with those of four different species of hymenopteran insects, including bees (Apis mellifera) and European hornets (Vespa crabro). They analyzed the sounds based on their wavelength, frequency, duration of the call and more, and they found that there was a lot of overlap in their structure.
Owls hear a wider spectrum of wavelengths than humans. So the researchers changed the sound settings to match what an owl would hear, removing the higher-pitched sounds. They realized that bats looked even more like the buzzing insects in the ears of owls than they did humans. “The similarity was particularly strong when variables not detected by the owls…were removed,” says Russo.
Then, through loudspeakers, the researchers played back two insect buzzing sounds. One was the sound of a bat buzzing, the other was a bat’s social call to captive, wild owls of two different species, barn owls and tawny owls.
Although hearing recorded bat sounds brought the owls closer to the source of the sound, it mostly seemed to shake the owls. They tried to escape or get away from the speaker, or at least inspect what was going on.
During the experiment, wild owls, which could remember being bitten by a flying insect, acted more frightened and likely to try to escape compared to captive-bred owls. Russo and his team assume it’s because the captives have never had an encounter with a biting insect. However, so far there is little scientific data on how often owls are regularly stung by bees, hornets and wasps, and how often they encounter them.
“They surely know it’s a dangerous encounter,” Russo said. This is also why he argues that this kind of Batesian mimicry is probably a technique deployed when a bat has been caught and wants to buy itself time to buzz around.
As is always the case with such new findings, many questions remain.
Future work will need to replicate these findings in nature, rather than in the lab, and with larger numbers of owls, in order to really say whether this is a type of Batesian mimicry, says Bruce Anderson, professor of entomology at the University of Stellenbosch, South Africa, who was not involved in the study. Another question is whether owls aren’t just frightened by the volume of bat buzzing, as they might by any other unexpected loud noise. “We may want to ask if this is a case of mimicry or exploitation of a sensory bias,” Anderson says.
It is also unclear whether, and to what extent, owls fear buzzing insects, although the data seems to suggest that birds generally avoid nesting in cavities occupied by these insects. Researchers could also learn more about whether these buzzes are unique to biting insects or whether other neutral insects can produce them. It would also be nice to test whether owls that have been stung react with more fear than those that haven’t, according to David Pfennig, professor of biology at the University of North Carolina at Chapel Hill, who was not involved in the study.
Although mimicry is common and some instances of Batesian mimicry are well known, much remains mysterious and striking, says Pfennig. He says that’s why discoveries like this are important. “Batesian mimicry provides some of our best examples of how natural selection can produce remarkable adaptation, including between very distant groups of organisms,” says Pfennig. There are other examples of acoustic mimicry between different species, like how burrowing owls can make hissing noises that sound like rattlesnakes, but a mammal copying an insect seems like a real first.
In the future, scientists would like to refine and expand their research.
“While it’s always useful to validate observations in the field, our results were crystal clear,” Russo says. “It would be interesting to find similar strategies in other species.” With more than 1,400 species of bats, as well as a handful of vertebrate species other than bats that buzz when disturbed, Russo speculates that other species besides the one they have studied could use the same trick.
The strategy of animals in cavities mimicking scary sounds to avoid predators might actually be widespread, says Anastasia Helen Dalziell, an ornithology researcher at the University of Wollongong, Australia, who was not involved in the study.
“Most of what we know about mimicry comes from studies of visual mimicry, but in principle, mimic signals could work either way. [type]“says Dalziell. “It’s really great to have another example of acoustic mimicry…to help encourage wider inquiry into mimicry.”