When they feel threatened, they buzz like angry stinging insects: a study shows that our native mouse-eared bats use a clever trick to keep predatory owls at bay. This is the first documented case of acoustic mimicry in a mammal, the scientists say.
They only pretend to be dangerous: some harmless animals take advantage of the appearance of poisonous or defensive species in their habitat to deter predators. This concept is called Batesian mimicry. There are numerous examples of this, especially among insects: some species of stingless flies have the yellow-black markings of the defensive wasps and hornets in order to benefit from their deterrent “image”. But the international research team led by Danilo Russo from the University of Naples Federico II is now reporting on a case of mimicry that is unusual in two respects: the imitating creature is a mammal and it does not imitate the appearance but the sound of a threatening creature, um to protect yourself.
As the bat researchers report, their discovery was based on their experience of capturing greater mouse-eared bats (Myotis myotis) for research. “When we touched the bats to remove them from the web or to examine them, they sometimes made sounds that reminded us of the buzzing of wasps,” says Russo. It seemed possible that this was a kind of emergency call intended to warn other species. But then the researchers began to suspect that the acoustic similarity is no coincidence – the unusual sounds may be used in a special way to ward off enemies.
What’s with the strange hum?
To investigate this possibility, Russo and his colleagues first performed acoustic analyzes of the unusual bat calls and compared the sound patterns to those of biting insects. In this way, they were able to fundamentally show that there are in fact clear similarities to the typical “bzzz sounds” that hornets emit as a warning when they feel disturbed. But who could the bats scare off? As the researchers explain, barn owls and tawny owls are possible because mouse-eared bats are on their menu. To study the responses of these two nocturnal raptor species to the special bat calls, Russo and his colleagues played audio recordings over loudspeakers to captive specimens. Either the original sounds of the hornets, the suspected imitations and other sounds of the mouse-eared bats could be heard as controls.
This showed that the owls were attracted to the “normal” sounds of their prey – they approached the loudspeakers. If, on the other hand, the hornet calls or the unusual buzzing sounds of bats are heard, the birds of prey move away. This avoidance behavior was particularly pronounced in birds that had not grown up in captivity but had gained experience in the wild. Apparently, the aversion to hornet sounds is partly innate and is reinforced by negative experiences.
Acoustic mimicry in a mammal
Interestingly, the analysis of the sounds also revealed that the similarity between the buzzing sounds emitted by hornets and bats was clearest when acoustic parameters that the owls cannot hear were excluded from the analysis. In other words, the bats sound particularly like hornets when heard through the owls’ ears. According to the researchers, the results prove that this is actually a new case of acoustic mimicry: For the first time, a mammal appears as the mimic, insects are the model and birds of prey are the addressees. As Russo explains, the sounds could cause a small but potentially crucial hesitation in a charging owl, allowing the bat to escape.
One may ask, however, why owls should be so afraid of hornets. As the researchers explain, the special aversion could have something to do with nesting habits: both owls and hornets like to move into tree cavities. The stinging insects will probably not tolerate a visit from an owl and react aggressively. “It is surprising that the owls’ unpleasant experiences with stinging insects seem to be behind the development of the acoustic behavior of the bats. This is another example of the many fascinating processes in evolution!” says Russo.
As he and his colleagues finally report, further research potential is now emerging: because, according to them, there are indications that, in addition to other bat species, other vertebrates also make peculiar humming sounds when they are threatened. In future studies, the researchers want to clarify to what extent this could also be a form of acoustic mimicry.
Source: Cell Press, Article: Current Biology, doi: 10.1016/j.cub.2022.03.052