Mimicry is widespread in the animal kingdom.
Some caterpillars can make themselves look like venomous snakes. The chicks of an Amazonian bird called the cinereous mourner shapeshift into poisonous larvae. Flower-loving hoverflies evolved to look just like stinging, unpalatable wasps.
These are all examples of Batesian mimicry, an evolutionary trick which leads a relatively harmless animal to copy a more dangerous species to scare off would-be predators.
But this specific type of mimicry is almost always visual in nature, so far as we know. And it’s most commonly found in insects, birds, and reptiles.
Now, for the first time, a type of acoustic mimicry has been observed in mammals. A study published May 9 in Current Biology found that a common European species, the greater mouse-eared bats, seems to imitate the buzzing sound of hornets—presumably to avoid being eaten by owls.
“We discovered that a mammal mimics the sound of an insect to scare a predatory bird,” says Danilo Russo, the lead author of the paper and an ecology professor at the Università degli Studi di Napoli Federico II, in Italy. “This is an amazing evolutionary interaction involving three species that are evolutionarily distant from one another.”
What’s the buzz?
Greater mouse-eared bats, also known as Myotis myotis, are a widespread European bat species that likes to munch on insects, especially beetles. They hang out in colonies in the woodlands and forest edges, roosting in caves underground for most of the year, or in buildings during the summer. They are often preyed upon by various birds, including barn owls (Tyto alba) and tawny owls (Strix aluco), especially when leaving or returning to their roosts.
Back in 1999, Russo was working to set up a call library for echolocation calls of European bats and collecting data about how various species communicate amongst themselves. While extracting a small mouse-eared bat from a mist-net, holding it in his hands, the creature started shivering and emitting a continuous, intense buzz, Russo says. Russo was surprised.
“My very first thought was… it sounds like hornets, or wasps!”
Initially, the researchers speculated that the buzzing was just an everyday distress call. But the sound was so obviously similar to an insect that a hypothesis originated almost immediately, Russo says, and, finally, years later, they decided to test it: Could it be that the bats were imitating hornets or bees?
Russo himself had collected pellets of barn owls in the past, at the entrance to a cave where these bats roost. “Believe it or not, the pellets contained a lot of bat skulls,” he says, so he felt it was not impossible these bats “may have, evolutionarily speaking, ‘made’ a very extreme attempt to deter [owls] to escape.”
Giving a hoot
In the current study, Russo and colleagues first compared the bat’s buzzing sounds with those of four different species of hymenopteran insects, including honeybees (Apis mellifera) and European hornets (Vespa crabro). They analyzed the sounds according to their wavelength, frequency, call duration and more, and they found that there was a large overlap in their structure.
Owls hear a wider spectrum of wavelengths than humans. So the researchers tweaked the sound parameters to fit what an owl would hear, removing the highest pitches. They realized that the bats sounded even more similar to buzzing insects to owl ears than for human ones. “The similarity was especially strong when variables undetected by the owls... were taken out,” Russo says.
Then, through speakers, the researchers played back two insect buzzing sounds. One was the sound of a buzzing bat, the other was a bat’s social call to some captive and wild owls from two different species, barn owls and tawny owls.
Although hearing recorded bat sounds made the owls move closer to the source of the sound, it seemed to mostly jar the owls. They attempted to escape or distance themselves from the speaker, or at least inspect what was going on.
During the experiment, wild owls, which might remember getting stung by some flying insect, acted more scared and likely to try to escape compared to captive-raised owls. Russo and his team speculate this is because the captives never had an encounter with a stinging insect. However, so far, there is little scientific data on how often owls are stung by bees, hornets, and wasps on a regular basis, and whether they encounter them often.
“They surely know it is a dangerous encounter,” says Russo. That’s also why he argues this type of Batesian mimicry is probably a technique deployed when a bat has been captured and wants to buy itself some time to buzz off.
As is always the case with such new findings, many questions remain.
Future work will have to replicate these findings in the wild, rather than in a lab, and with larger numbers of owls, in order to truly assert whether this is a type of Batesian mimicry, says Bruce Anderson, an entomology professor at Stellenbosch University, in South Africa, who was not involved in the study. Another question is whether the owls aren’t just scared by the volume of the bats’ buzzing, as they might by any other unexpected loud noise. “We may want to ask whether this is a case of mimicry or exploiting a sensory bias,” Anderson says.
It’s also still unclear whether, and to what degree, owls fear buzzing insects—although data seems to suggest that birds generally avoid nesting in cavities occupied by such insects. Researchers could also learn more about whether these buzzing sounds are unique to stinging insects or if other neutral insects can produce them. It would also be nice to test if owls who have been stung react with more fear than those who haven’t, according to David Pfennig, a biology professor at the University of North Carolina at Chapel Hill, who was not involved in the study.
While mimicry is common and some cases of Batesian mimicry are well-known, much about it remains mysterious and striking, says Pfennig. He says that’s why findings like this are important. “Batesian mimicry provides some of our best examples of how natural selection can produce remarkable adaptation, including between very distantly related groups of organisms,” Pfennig says. There are other examples of acoustic mimicry between different species, like how burrowing owls can make hissing sounds that resemble rattlesnakes, but a mammal copying an insect seems to be a real first.
In the future, the scientists would like to fine-tune and expand their research.
“While it is 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 over 1,400 bat species, as well as a handful of non-bat vertebrate species that buzz when disturbed, Russo guesses other species besides the one they studied may use the same trick.
The strategy of animals in cavities mimicking scary sounds to avoid predators could be, in fact, widespread, says Anastasia Helen Dalziell, an ornithology researcher at University of Wollongong, in Australia, who was not involved in the study.
“Most of what we know about mimicry has been gained from studies of visual mimicry, but in principle, mimetic signals could operate in any sensory [type],” says Dalziell. “It’s really great to have another example of acoustic mimicry… to help encourage a broader investigation of mimicry.”