Researchers know that bats will alter the calls they use for echolocation depending on their task or environment. In this episode, we talk to Léna de Framond and Holger R. Goerlitz of the Max Planck Institute for Ornithology about their research into whether bats from temperate regions, which experiences large variations in temperature and humidity which could affect call attenuation, vary their calls based on weather.
Associated paper: Léna de Framond, Verena Reininger, and Holger R. Goerlitz. "Temperate bats may alter calls to partially compensate for weather-induced changes in detection distance." J Acoust Soc Am 153, 2867 (2023). https://doi.org/10.1121/10.0019359
Read more from The Journal of the Acoustical Society of America (JASA).
Learn more about Acoustical Society of America Publications.
Music Credit: Min 2019 by minwbu from Pixabay. https://pixabay.com/?utm_source=link-attribution&utm_medium=referral&utm_campaign=music&utm_content=1022
Researchers know that bats will alter the calls they use for echolocation depending on their task or environment. In this episode, we talk to Léna de Framond and Holger R. Goerlitz of the Max Planck Institute for Ornithology about their research into whether bats from temperate regions, which experiences large variations in temperature and humidity which could affect call attenuation, vary their calls based on weather.
Associated paper: Léna de Framond, Verena Reininger, and Holger R. Goerlitz. "Temperate bats may alter calls to partially compensate for weather-induced changes in detection distance." J Acoust Soc Am 153, 2867 (2023). https://doi.org/10.1121/10.0019359
Read more from The Journal of the Acoustical Society of America (JASA).
Learn more about Acoustical Society of America Publications.
Music Credit: Min 2019 by minwbu from Pixabay. https://pixabay.com/?utm_source=link-attribution&utm_medium=referral&utm_campaign=music&utm_content=1022
Kat Setzer 00:06
Welcome to Across Acoustics, the official podcast of the Acoustical Society of America's publications office. On this podcast, we will highlight research from our four publications. I'm your host, Kat Setzer, editorial associate for the ASA. Today we'll be talking with Lena de Framond and Holger Goerlitz about their article, "Temperate bats may alter calls to partially compensate for weather-induced changes in detection distance," which recently published in JASA Express Letters and has been featured in an AIP Publishing Scilight. Thanks for taking the time to speak with me today, Lena, and Holger! How are you?
Lena de Framond 00:43
Good, thank you.
Holger Goerlitz 00:44
Thanks. Very good. Thanks for having us.
Kat Setzer 00:46
Good. Yeah. Thanks for being here. So first, just tell us a bit about yourselves and your research background.
Lena de Framond 00:53
I'm a bioacoustician. So I'm studying the vocalizations of animals, mostly birds and bats, in relation to their environment. So temperature, humidity, weather, and especially noise.
Holger Goerlitz 01:09
I'm also a biologist, focusing on sensory ecology. That is, how do animals perceive their environment? What kind of information do they use? And as Lena said, we're focusing on sounds and there, focusing on bats and insects: how they interact, how the bats perceive the world, how do insects perceive the world, and how do they interact as predators and prey?
Kat Setzer 01:30
So in this, your research has to do with bat echolocation, specifically the effect of weather on bat echolocation. Can you give us a bit of an overview about how the environment affects echolocation?
Holger Goerlitz 01:42
There are multiple aspects that are affected. So the environment, that can be, for example, the habitats: Where do the bats fly? Or where do they live? Where do they go foraging? Do they, for example, fly out of a cave or commute through a forest? Do they forage higher up in open space or just close to the ground? It can be whether there are other animals around, for example, other bats of the same species or other species, and also whether there's prey or not. This all affects how they echolocate. And also external factors such as the weather, the air temperature, the air humidity can have effects on bat echolocation.
Kat Setzer 02:19
Okay, got it. So what's currently known about how weather affects bat calls?
Holger Goerlitz 02:26
Well, first of all, it's known that the weather affects the attenuation. This is how strongly the sound becomes fainter while it's traveling through the air. And this is particularly pronounced for ultrasound. And as the temperature changes, as the relative humidity changes, the attenuation also goes up or goes down in a very complex manner, and this affects how far the echolocation reaches, that is, over what distances bats can detect their prey. What is mostly unknown is then, in turn, whether the bats do react to these changes in attenuation and in some way, alter their echolocation calls. And we know that bats are extremely flexible. They alter their echolocation calls all the time in response to all kinds of external situations, habitats, and so on. And we were wondering whether they also adjust echolocation in response to changing weather conditions.
Kat Setzer 03:17
Yeah, it totally sounds like it would make sense that they would change their calls based on weather, if they can change it based on a variety of other factors. In this you looked at bats in temperate areas. Why did you choose to study these bats in particular?
Lena de Framond 03:31
So we know that bats in tropical areas do change their call according to the changes in temperature and humidity. And we also know that there's species-specific differences in the calls in relation with the climate, with the average annual temperature and humidity. But what we don't know is what bats do in temperate areas, because in temperate areas, the bats have to face a really, really broad range of temperatures and humidities. For example, in the cold nights in March, it's 6 degrees; in the warm nights in July, it's going to be 26 degrees. So that's a 20 degree difference between the cold and warm nights. And that also translates into a 20% change in the atmospheric attenuation condition.
Kat Setzer 04:23
Oh, wow. Yeah. So that's a lot that they have to contend with. Okay, so what was your hypothesis regarding how these bats handle variations in weather?
Holger Goerlitz 04:32
We had two key hypotheses. The first is that the bats could simply increase the call energy. So if attenuation goes up, and the calls are getting fainter more quickly, we hypothesized that they would simply call louder or longer to put out more energy. And the other hypothesis is that they would lower the call frequency because lower frequencies are less attenuated, so this could be another mechanism to counteract the effect of attenuation, by calling at lower frequencies.
Kat Setzer 05:02
Okay, that makes sense. So then how did you set up this study?
Lena de Framond 05:06
To study the changes in echolocation calls relative to the environmental weather condition, we wanted to know what the bat said. And as said before, when sound propagates through the air, especially ultrasound, it suffers from a lot of propagation loss. First because there's the spreading of energy through space, and because the atmosphere absorbs a lot of energy. So what we had to do is to reconstruct the call as emitted by the bat. And to do this, we had to know the distance between the bat and the microphone. So we used a microphone array, so that's four microphones that are arranged in sort of star-shaped thing with one microphone in the center and three arranged in a triangle around. And we used the time difference between the arrival of the call at each of the microphone to triangulate the bat position. So since the bats are calling really often, they're calling about 10 times a second, we ended up with nice trajectories with one position every 50 centimeters, more or less.
Kat Setzer 06:16
Okay, that makes sense. It's kind of like bat GPS. So in your article, you state that to deal with atmospheric attenuation, bats can lower call frequency or they can increase call energy. So you ended up analyzing the frequency and energy of the bat calls. Can you explain how you did that?
Lena de Framond 06:33
Yes, so as said, we had the nice trajectories, the position of the bat relative to the microphone. So from the recordings, we could correct the recording for the distance-dependent geometric spreading and atmospheric attenuation to reconstruct what the bats was saying. It's a bit like artificially putting the microphone 10 centimeters in front of the bat's mouth. And then we took some frequency measurements, did standard frequency analysis, and we calculated the energy as a combination of the call duration and call amplitude. And then we'd make correlations with the acoustic attenuation condition.
Kat Setzer 07:13
Okay, got it. And then you also estimated detection distance. Why was that important, and how did you do it?
Lena de Framond 07:20
So the detection distance is actually the one biologically relevant parameter for the bat. When the bat is echolocating, it wants to know what's around it and to detect the insect prey. And how far the echolocation can go, how far the bat can see or detect prey, this is what is going to be affected by the change in atmospheric attenuation. So to calculate the detection distance, we used a model. So we know how sound propagates through the air. So we can artificially propagate the echolocation call from the bat into the space, then make the sound reach an insect target, which will have some reflective properties that we can set, then have the echo propagate back and check whether the echo is loud enough to be detected by the bat or not. And we can do this for different distances between the bat and the target and check when the bat cannot detect the target anymore.
Kat Setzer 08:16
So let's talk results. Did you see any changes in bats call parameters with changes in weather?
Lena de Framond 08:22
Yes, so we saw things, but not what we expected to see. We, as Holger explained, we expected the bats to lower the call frequency to put their calls in a position where they suffer from lower atmospheric attenuation. And we studied three species. One of the species lowered the call frequency, but the two others didn't. And the other call parameter that we studied was the call energy. We expected the bats to call with higher energy when the atmospheric attenuation increases. And we found that two of the species we studied didn't change the call energy and the third species, instead of increasing the energy, it actually decreased the energy and not just a little bit, but actually quite a lot. And finally, when we looked at the detection distance, the fact that some species actually didn't change their call, of course, it didn't help them to maintain the detection distance. And this one species that decreased both the frequency and the energy, instead of maintaining the detection distance, it ended up in the lower detection distance as if it hadn't changed anything.
Kat Setzer 09:30
Okay, that's a little confusing. So you did say that one of the three species you studied did have some changes to their calls in response to the weather changes. Can you tell us a little bit more about those changes? What effect do they have on echolocation? Why do you suppose they happen?
Lena de Framond 09:45
Why they happen we don't really know. It's difficult to say what they do. So if this device is calling at lower frequency, one of the consequences is also that the wavelength is then larger, and when the wavelength is larger, it doesn't reflect as well on smaller targets. So not only the bat will change the propagation property of the calls, but it also changes what insects it would be able to detect. So lowering the call frequency also means that it will detect only larger prey and not the smaller prey anymore. Whether it really has an impact on what the bat can catch, we don't really know.
Kat Setzer 10:22
Okay, and so then two of the three types of bats you studied didn't actually end up changing their calls based on changes in weather. Why do you suppose that was?
Lena de Framond 10:31
So our measurement of detection distance, this is only a model. So we don't really know what the bats in real life will see or detect, or whether it's really relevant for them to detect the prey 3 meters away or 3.5. We don't really know also, if this will affect their foraging success, if they will benefit from detecting so many other prey further away or not. And we don't really know how they could actually react to the changes in atmospheric attenuation. So one hypothesis is that they use the temperature as a proxy for atmospheric attenuation. But it's not that easy, because the relation between the temperature and atmospheric attenuation is not linear. So sometimes, if we increase temperature, it will increase atmospheric attenuation, sometimes it will decrease atmospheric attenuation. So it's not that easy. And we don't really know whether these changes in atmospheric attenuation are very relevant for the species we studied. We studied species that are very generalist, they're eating pretty much everything they can find. So the change in detection distance of 20% that we measure, maybe it's relevant, or maybe not.
Kat Setzer 10:41
Yeah, I guess they're not like flying around with smartphones with weather apps and just be like, Oh, it's, you know, it's 20 degrees out right now need to go a little quieter, louder, whatever. So what are your overall takeaways with regards to behavioral plasticity?
Holger Goerlitz 12:01
That's a good one, because probably it doesn't, we don't have any takeaways directly for that, I would say. Well, we do know that bats are extremely plastic. This is what I find so fascinating about them. Like within less than a second, when they catch insects, they can go from emitting 10 calls a second to more than 200 calls a second. They change frequency. They change call duration to a 10th of the original length and just within a half a second. So they are extremely plastic, which is why we did this study. But apparently, as Lena just explained, these changes in weather conditions are apparently not a factor that requires them to show the plasticity that they do actually have.
Kat Setzer 12:44
Well, thank you again, for taking the time to speak with me today about your research, even if it was a bit of a negative result, I guess, but I never realized how much bats can vary their call echolocation. So this was a good learning moment. It is really interesting to think about the ways animals can modify their behaviors to adapt to their environment, and when environmental changes are significant enough to merit that kind of adaptation. I'm sure our listeners will enjoy hearing about your research. And like I said, thank you again for speaking with me today.
Holger Goerlitz 13:13
Thank you very much.
Kat Setzer 13:18
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