Bats that buzz

Danilo Russo first noticed something rather odd about bats when out in the field, catching them for his PhD thesis. Picking them out of the net, he noticed that many would vibrate and hum. In fact, the sound was uncannily like a wasp or hornet. Could they, he wondered, be doing it to fool a predator into thinking they’re a stinging insect and to steer clear? Now, 20 years later, he’s followed up on his hunch and provoked quite a buzz by showing these bats do indeed do just that, as Harry Lewis has been hearing…

Danilo - Among animals we have a lot of situations where unarmed animals imitate dangerous or unpalatable species mimicking their colours or morphology or whatever, so that they can actually deter predators for the time needed to escape. In this particular case, these bats actually use sound rather than colour, which is very unusual. And you know, it's the very first time that this kind of strategy is found in mammals. We have no other situations where mammals mimic insects.

Harry - We've got a clip. So let's, um, let's take a listen. How did you figure out that this sound could be used to scare away predators?

Danilo - What we did first was to record bats, then we recorded insects which might be potentially mimicked. And we recorded four species: buff-tailed bumblebees, European Hornets, domestic bees and European paper Wass. All these guys actually may be encountered by owls for example, when owls explore cavities, where they roost in caves or whatever, because the main predators of these guys are birds of prey, especially owls. So the idea is that owls can sometimes experience the stinging defense of these insects and the outcome was that especially if you take out of your data set the variables that cannot be heard by the owls. These kind of buzzes are especially convincing, for owls because they don't detect the higher frequency. They just sound like insects to them.

Harry - Right. So it doesn't need to be a precise mimicry. It just needs to fool the owl. It just needs to sit within the owls hearing threshold.

Danilo - That's exactly the point. Yeah. You know, I was just surprised to see that owls actually have a sensitivity to higher frequencies, which is quite similar to that of the human ear. And this is why when these bats buzz, they really sound like wasps or hornets to us as well. I mean it's quite convincing.

Harry - Okay. So here is a comparison. This is the sound a domestic bee makes. And this is that bat mimicry again. I mean, it still strikes me as strange though that this sound alone is capable of scaring your birds of prey.

Danilo - Owls that have not been raised in captivity, that have been out there for say at least one year or more, they are especially reactive. So the idea is that they may have encountered buzzing insects and know that they're dangerous. While if you use naive owls, they're sort of less reactive.

Harry - Why is it then that this mimicry is so hard to find in other mammals? Why is it that the bat is the only specialist?

Danilo - I don't think that bats are the only specialists. We know that there are some rodents, for example, that buzz when disturbed, and that's gonna be our next step, because I'm pretty convinced that this kind of mechanism may actually be more widespread than thought.

Harry - This is like opening the scientific curiosity door. There's a lot of research that could be built upon your findings, that stem all the way back to your PhD.

Danilo - Yeah, I really think so. I really think so. It's quite exciting. And you know what a nice part of the story is that I actually thought about that first time over 20 years ago. And it stayed there in corner of my mind, waiting for the right people to start doing the experiments.