Kalina Davies: Supersenses - The evolution of bat echolocation
Kalina Davies, Photo courtesy of Kalina Davies
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Kalina Davies: Supersenses - the evolution of bat echolocation
Kat: What you’re hearing now is the sound of what’s probably a pipistrelle bat using ultrasonic echolocation to navigate and hunt, converted into audible sound thanks to a clever bat detecting device. It’s the same principle as sonar - bats send out high-pitched ultrasonic noises, which reflect back off objects and are detected and interpreted to build up a picture of what’s going on.
Kat: Although this ability to sense the world through sound is not unique to bats - and not all bats can echolocate - they have certainly taken it to another level. So, how do they do it? And how did this ability evolve? Georgia Mills spoke with Dr Kalina Davies at Queen Mary University of London, who’s been trying to find out.
Kalina: There's many different kinds of echolocation, but to describe it sort of in the simplest form, the animal that's undertaking it produces sounds that can either be produced from the larynx, So they're producing kind of like vocalisations, or in other ways they can make clicking noises with their tongues or some maybe even will make clicking noises with their wing beats. And so then the animals use the sounds that they've actively produced and listened to the echoes as they bounce back from objects around them to work out if there are objects there or where there's empty spaces. And then that's how they can create this kind of sound landscape around them, which they can use to help them orientate themselves or find food and move around.
Georgia: And so this is quite a specialised system. Not only do you need the special squeaks, which can come from a variety of places on the bat, you also need to get them back to receive them and then interpret them.
Kalina: Yeah, exactly. So it's kind of a very complex process altogether. So you need specific adaptations in the way that you're going to make the sound, how you're going to hear it when it comes back and then also process it. So it requires adaptations in many different systems, not just in the, in the ears of the animals.
Georgia: So do we know anything about how this evolved and the genes behind it?
Kalina: So we don't really know exactly when it evolved or how many times even it's evolved. And it's very much an active area of research. We can hypothesise that either it's evolved once in all bats and was then lost in the non-echolocating fruit bats, or otherwise it's evolved multiple times, at least twice in different groups of bats, but we're still not really clear. And so one approach that people have used to try and help them understand this is looking at the genes behind it and the different molecular adaptations, but obviously because it's such a complicated system that requires so many different aspects of the biology, there's not like a simple marker that we can go to and look at and try and understand what's happening. It's still very much an open question I think.
Georgia: So this is something you do, right? You're investigating the inner ears of bats so how do you go about that?
Kalina: Yeah. So this is something that I'm really interested in, I've been working on for a number of years. So I've been looking at the molecular aspects of it as well as the morphology. So I've been previously looking at different rates of evolution in protein coding genes, non-coding elements, and then also looking at the gross morphology of the cochlear or the inner ear of the bat species to try and look for changes that have happened.
Georgia: How do you investigate the areas of bats? Do you have living bats or what's your method there?
Kalina: It would be nice to have living bats in the lab. Obviously it's very difficult to do that. And also if we're looking at just the inner ear because obviously bats are tiny their inner ears are even much smaller. So it's very, very difficult to look at the inner ears of living bats so instead we've used x-ray technology. So microcomputer tomography to try and look inside the skulls of the bats where we can look at the really complicated morphology that's shown by the different inner ears of different species. So it's possible to use museum specimens, which allowed us to look at a really interesting range of bats. So I was able to use different specimens from different museums. And some of them were several hundred years old. Obviously the outer features of the skulls were a little bit damaged, but inside they're quite protected and obviously the specimens are very well looked after in the museum so it's still possible to see them obviously. So, whereas the cochlea and living bats is actually the fluid filled gap where all the hair cells are, but I was able to look at what's left of the bony labyrinth, which surrounds this space. And then from that, you can reconstruct the inner space. So I'm not directly looking at the cochlear, but looking at the bone that surrounds it, and you can use that to reconstruct the shape of what would have been there in the living bat.
Georgia: Amazing. And is there a visual representation of echolocation? Can you see like a marker in the skeleton of the inner ear thats like, oh, this bat echolocates.
Kalina: Yeah. It really jumps out at you, which one's echolocating and which ones aren't, because the cochlear is so much bigger compared to the non-echolocating fruit bats so you immediately see, even though the shape varies a lot between different echolocating species that use different forms of echolocation, there's still kind of this overriding similarity. So you can really see immediately which ones are likely to echolocate and which ones aren't.
Georgia: And so from your investigations of bat ears, have you got to the stage yet where you've made some findings and what are you specifically looking for?
Kalina: So we've had, we've made a number of studies on this in terms of either the genes or the morphology. If I kind of summarise, it seems to me that it's more likely that echolocation probably did evolve multiple times or at least was not lost in old world fruit bats so maybe there was some kind of increased hearing capability in the ancestor of bats. And then later on in different lineages that was further developed to create the more advanced echolocation that we see in some extant species.
Georgia: Oh, amazing. So bats managed to do this twice we think?
Kalina: Well, It's very much debated. So some people don't believe that at all and think that it was definitely lost in old world fruit bats and that it was only evolved once. From my point of view, I think that it's more likely that it has evolved multiple times because I just think it is, as we said earlier, echolocation is kind of like a superpower of the bats. So if it had evolved in the ancestor and then all of them were capable of it, is it likely that the old world fruit bats would have lost this amazing ability? We don't really see anything that might constitute if a trait is a loss. So if you think of when a traits lost, for example, one of the senses that's most commonly lost is vision in different animal species, for example in blind cave fish, and there you see this degeneration of the eye structures and loss of gene function,. So you see pseudogenes and things evolving. So in terms of old world fruit bats, there's no evidence of any kind of relaxation in terms of the morphology of the inner ear. So you don't see any kind of increased variability or anything when you look at the different shape variation across species. And there isn't a single gene that we know that looks like it's lost its function or showing kind of relaxation or something.
Kat: Kalina Davies from Queen Mary University of London.
