3D-Visualization of the Ear Morphology of Penguins (Spheniscidae): Implications for Hearing Abilities in Air and Underwater
Proceedings of Meetings on Acoustics (POMA) https://doi.org/10.1121/2.0001291
Authors: Sylke Frahnert, Martin Lindner, Eva-Maria Bendel, Klara Henrike Frahnert, Natascha Westphal, and Michael Dähne
In this episode, we interview Michael Dähne of the German Museum of the Seas about the morphology of penguin's ears.
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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, The Journal of the Acoustical Society of America, also known as JASA, JASA Express Letters, Proceedings of Meetings on Acoustics, also known as POMA, and Acoustics Today. I'm your host, Kat Setzer, editorial associate for the ASA.
Joining me today is Michael Dahne of the German Museum of the Seas. Dr. Dahne is a coauthor on the article, “3D-Visualization of the Ear Morphology of Penguins (Spheniscidae): Implications for Hearing Abilities in Air and Underwater,” which appeared in the 37th volume of Proceedings of Meetings on Acoustics, and is based on a talk given at the 5th International Conference on the Effects of Noise on Aquatic Life. Thank you for taking the time to speak with us today, Michael. How are you doing?
First, can you just give us a little bit of background about yourself and your research?
Yes, I do have a mixed education. So I started out being an engineer for agricultural and also environmental protection. And then I graduated as a PhD in Zoology, about the use of acoustic methods for the protection of harbour poises in German waters. And I'm sort of a bio-acoustician. But now I'm curator of marine mammals. So I'm doing a lot of collection based research, especially on bones on how to issues of harbor porpoises and seals, for instance, but I'm also taking care of dissection. So we dissect marine mammals in two weeks terms and see what they died of.
That's really interesting, really cool. So what was known about penguins hearing before the study?
So actually there was not a lot known about penguin hearing before the study. There was actually one study that looked at the vocalizations of penguins looked at the frequency ranges, where the penguins vocalized in and sort of concluded from that where their hearing range might be best. And then there was one study on cortical evoked potential. So that was actually done in the 70s and it was sort of only done on one species, and only in a year. So it was not the underwater hearing abilities that were analyzed, but only the in ear hearing of the so called Jackass penguins or African penguins. And then there was some papers about the middle medium morphology of penguins. But otherwise, it was uncharted territory.
Oh okay. So why did you decide to study penguins ear morphology?
So first of all, it's the basis of hearing research to look at the morphology of the inner ear, the middle ear and the outer ear. But then there are some specialties about penguins. One is, of course, that they are semi-aquatic species, so they do dive in the seas quite to some depths of about 500 meters.
But they also spent a lot of their time in air breeding and taking care of the chicks. And also rearing the youngs. So it's a quite an interesting species to look at and there is another specialty at the Deutsches Mary's Museum, the German Museum of the Seas. We do have penguins on the roof of the Orsinium, which is one of our exhibition sites. And those Humboldt penguins are one basis for our hearing research as at the moment because they are trained for India hearing tests.
Oh that’s so fun! So, what methods did you use for the study of the morphology of penguins ears, can you detail how iodine staining works?
So, yes, what we did was to look at micro CTs of penguin ears and Penguin ears are quite small, but they do sit in quite large skulls. So actually having a micro CT and the size of a Penguin's color was not easy to find. So we work together with the Museum of Natural History Museum in Berlin. And they have a very fancy machine that can hold dinosaur bones and can micro CT those dinosaur bones and we use that machine for doing the micro CTs scans we and CTs is of course, a methodology where you use x-ray images and you stack them and then you look at sort of portions of different tissues. So, you can outline a bone which you call a segmentation, and then you go through the stack of pictures, and therefore, you then have 3d representation of the individual bones or the tissues. So, for the tissues, it's actually quite difficult to get those tissues to be very well visible. And therefore, you use the so called iodized staining potassium iodine staining technique, and within this method, you actually try to get as much iodine into the tissue and the more iodine is in in the tissue it then changes the density or the radio densities of the different tissues. And therefore you will have with more iodine in the tissue, you will have better reflectivity or a better visibility of the tissue and therefore then you have different contrasts between the different tissues. So some tissues absorb the iodine very quickly, others do not. And that enhances the contrast of each of the pictures and then the segmentations in the end.
Okay, so what did you end up finding out about penguins outer ears?
So actually the outer ear of penguins is quite similar to other birds, it's round to elliptic this pathway is airfields. The interesting thing about it was that there were no muscles surf was circular. But penguins must have some mechanism to open or close this otters externa, the outer ear, and we don't know yet exactly how that is working.
Okay, and then what did you find out about the penguins middle ears?
So the middle ears, they start out with the tympanic membrane and that attaches to the cranium. And most prominently also due to the current rate, which is a small bone that is attached to the lower jaw. And on the tympanic membrane sits the columella, which is actually the single article, the single ear bone and as most of you know, there are three articles in human ears or in mammal ears in general, which is the models the hammer, the Incas, the anvil, the stars, the stirrup, but those three bones are completely replaced in birds, and also in reptiles by the kolomela, which is a relatively long elongated bone and it has then a low footplate. And this attaches to the oval window to the inner ear. You can calculate ratios between the tympanic footplate area, towards the within the footplate area towards the tympanic membrane area. And actually, you see that the penguins are on the lower end of the range for birds. So, not too, obviously, an adaptation, but we'll come to that probably later on a little bit more, the tympanic cavity is air filled. And therefore penguins need to have some sort of adaptation towards diving. So when they dive down deep, they have to protect the tympanic membrane from rupturing. And therefore there needs to be some mechanism for pressuring or adapting the pressure was in the middle ear cavities to the outside pressure. And then we found that the yields are not connected via functional into our our pathway.
Okay, so then I guess that leads us into our next question, which is, so what did you find out about penguins’ inner ears?
So for the enemy is the cochlea of birds looks a little bit different from the one in mammals. So for mammals, we always see this very, very round circular shape. But bird ears look a little bit elongated. They don't have this sort of curvature to it and actually the cochlea and the lebaran looked very similar in the species and we couldn't look into the soft tissues of the inner ears, because during the fixation of the material, often there is a degradation of the tissues. And therefore, then you cannot use the staining too much to get the contrast and then you don't see the tissues in to the degree that you need to actually differentiate which tissue is doing what. And since these structures are also very filigree, it means that we cannot say much about the ear in general.
Okay, so, how do the structural features of penguins ears affect their hearing in and out of water?
So, as I said before the tympanic rain brain and the columella must be protected during dives. And since the columella is a very thin and long bone, it must be protected from breaking, which should not happen. But then the tympanic membrane must adapt towards pressure. And this means that the pressurization of the middle ear needs to be somewhat realized there also needs to be the protection from seawater that is coming into the outer ear because that may affect the hearing abilities. And then what we also found was that the columella in the area of the eardrum, actually they are thought to be a mechanism of amplification of the signal. So, if it is on the lower end of this range means that the amplification is probably very large compared to other birds. But we have to look further into it whether how actually this protection of the middle years realized.
Okay, so how did the iodine staining you used for part of the study impacts the results? Can you break down the differences and the pre staining and post staining scans?
Yeah, so actually, there were quite a lot of scans in between and the scan, the staining took from some days to weeks, to nearly half a year. So it's rather a long procedure, so that potassium iodine staining is penetrating all the different tissues. And if you do a scan at the beginning, which we did, then you actually see that all those tissues are similar gray tones. And then with increasing staining, you see that the contrast is much more easy to differentiate the different tissues. And then you can actually see in the end very thin membranes even. But unfortunately, the staining was not working all that well, for all of the tissues. So for instance for the extra kolomela, which is cartilage. It didn't work that well. And for the membranes of the inner ear. It also didn't work that well as I said before and this may be two issues arising from the conservation status of those birds that we have used within the study were collected quite long ago and they were fixated in different different solutions. So for instance the poor formalin fixated animals, they had to be the formalin had to be replaced by alcohol first, and then you could use the staining on the material. So it's rather long procedure. And so that the fresher the material is when it was collected, the better the fixation worked at the beginning and the less formalin was used, the better actually the results became what the study.
Oh, okay. Yeah, that's very interesting. So how are penguins ears set up to prevent water from penetrating the external ear canal and interfering with hearing?
So what I said before was that we did not find any circular muscles and circular muscles would be used to actually close the ear canal directly, and then prevents the water from entering. So this is not feasible for the penguins. We didn't see any passive mechanism like curvature, which would start to bya pressure gradient to close the outer ear. So what we think might be feasible is that some muscles pull the auditory meters to a slit and then they're also fed discovering the outer ear. So it might be a very simple mechanism that prevents the seawater from entering. But it's not 100% clear from what we have found was in the study and must also say that micro CT scans are not the perfect methodology to look into different tissues of different types. Sometimes this commentary is helping much more. And therefore, there needs to be more studies on that I must say that histology is also intrusive, so you cannot use the material twice and those birds in those collections, they are something that needs to be kept in a very good state. So actually, micro CT is has some very good advantages and conserving the material in the same state as it is. So you, you must find a good way in between getting the best results for your study and preserving the material to the best degree.
Okay, I see. So how are penguins’ ears adapted for diving rapidly into deep water?
So this is still a very hard question to answer the there have been studies about it, especially on King penguins. And for the King penguins. It was found in histology that they have a so called corpus covering ozone in the middle ear, which is a thin, very venise tissue, and it's thought to be flooded with blood during the time when they die, so by getting this dense tissue, it will inflate the material and therefore decrease the room available to any air filled cavities was in the middle ear, and therefore this is a pressure equalization mechanism but the interesting thing was that we did found it in the emperor penguins. We did also have scans of Rockhopper penguins and Gentoo penguins. And within those twos, we didn't find those adaptations. So there must be something else that is working for other penguin species who also dive quite deep. So King penguins and emperor penguins, emperor penguins can dive down to about 500 meters. But some of the other species can even dive down also to 200 meters, there must be some other mechanism that is preventing the tympanic membrane from rupturing during those dives. And those are very rapid dives. So they really go down with one very fast motion.
And then we also found that the inter aura pathway was lost. And the inter aural pathway is an air filled cavity that is connecting the two middle ears and would lead to a press a pressure equitisation between the ears. But it is also thought to be a mechanism for directional hearing in the air. And it might be very
or might become clearer over time. But it seems that diving species in general have lost the inter aura pathway over time. And so it might be an adaptation to underwater sounds. And that actually, that directional hearing underwater works differently than in there.
Oh, that’s really cool! So do any of the morphological traits of penguins ears relate to those found in the ears of other vertebrates that can hear underwater?
So actually, those ears of the penguins that we found, or that we looked at are similar to other bird species in other words, for bird species, the earth also developed to have tympanic ears. And it seems like many marine animals like whales, for instance, they have lost this inter oral connection but for whales especially, it is also so that the ear is not connected, the middle ear and the inner ear to be specific is not connected anymore to the bones of the lower jaw. So there are more adaptations within the animals that are purely aquatic species and therefore have adapted to live purely in the water.
And in comparison to other species that are semi-aquatic and therefore have to keep the ear hearing intact.
Okay, so what is the quadrate? And how does its relationship to the hepatic membrane affect penguins hearing?
So the quadrate is a very small bone. And it's directly connecting to the tympanic membrane, therefore, it can actually put pressure onto the tympanic membrane or release pressure from the tympanic membrane. And it's directly connected to the lower jaw of the animal. So it is thought that actually by moving the lower jaw, the pressurization of the tympanic membrane can be released a bit or stressed a bit. And this may affect hearing capabilities in general.
That's very cool. So then how does this research tie into your larger research projects currently?
So the research that we did here, was in the project hearing in penguins and hearing and penguins is a rather large project where we cooperating with institutions like the University of Southern Denmark, in audience, the zoo odunsi, the Marine Science Center in Rostock and the Natural History Museum of Germany in Berlin. And so it's rather an international collaboration within that project. We are training penguins for in a hearing tests at the OCR noon, which I said before. But we also do large part of public outreach, we train penguins into audience. Those are Gentoo penguins, and rockhoppers and King penguins, for ear hearing tests, but also for playback studies. We look into the ear morphology, which we are talking today about. And then we also look into the use of auditory evoked potentials for estimating hearing thresholds of the birds. And this is a methodology which is used, for instance, in newborn humans and work there, you get a little electrode onto the forehead. Usually, it's three electrodes, and then you are being exposed to a number of sounds which are repeated over time. And then some electronic equipment looks into what the auditory evoked potentials that you can get from the scalp actually reacting towards the sound that are repeatedly transmitted. And then if you find those reactions, then you think that the sound was heard. This is a very non intrusive methodology. And it can be used on a larger range of species. So our penguins, for instance, or to train to wear a hood. And then the electrodes are put on the scalp, they are not penetrating the surface. And we try to get hearing capabilities from those animals as well with these non invasive methodologies. So within the project, we do a lot of educational outreach activities, about noise at sea. Noise at sea is a problem that is very difficult to be transmitted to the general public. And it seems to be something that is very unrelated to us as humans, it seems to be something that is happening somewhere far away where we cannot hear what we cannot see, and where we don't see the effects of underwater noise. But there are a number of different sound sources that see this, for instance, explosions. In Germany, we are talking about the removal of world war two ammunition at sea, which will cause explosions at some times. But there's also explosions for clearing the wave for building activities at sea. This power driving for offshore wind farms, which is creating a lot of noise. There are seismic surveys, that can create a whole lot of noise or across ocean basins. And then there's multiple things that humans are not aware of that we do at sea, there's of course shipping, and everyone is aware of that this is increasing underwater noise. When so we made for this public outreach or educational activities. For instance, a video that is explaining about underwater sound. We created an animal otter Grimm database, where researchers could look at different hearing curves of animals. We have done a music hackaton this music hackaton Created by all sorts of underwater noises collected from different databases publicly available. And it was very fun being part of these experiences, then we have a website with which is in German and is educating about this issue, we had a yearly topic at the German oceanographic Museum, which was called No Noisy Sea, where we had a real sized Orca sculpture hanging within the museum, which was to dude was different noise symbols, like a movie is stitute was different things that impress him during his lifespan, and therefore make an impression on the being of the animal or the human. So there are lots of different activities that we have done. And we have also used the ear morphology of penguins, for doing outreach activities. So for instance, we have a video was in our exhibitions where the middle ear is the outer ear, the middle ear and the inner ear, are being seen and being seen as a three dimensional movie. It's explaining about the morphology, but it also puts it into context of the article. And rather pristine areas in an article, which we see that are probably flooded by human noise in the future to if we continue to increase, for instance, cruise trips towards Antarctica. And when the pressure on the Antarctic pristine ecology is increasing over time. So there's lots of things that we try to develop from the different activities within the project. And we think we have done that quite well. And I invite everyone to look at the different websites at the different activities that we have carried out.
Very cool. Thank you. Yeah. Well, if you'd like to give us those website links, we can include them in our show notes. Thank you again for taking the time to speak with us today. I know I learned more than I thought I could ever know about penguin ears today. I bet our listeners will find your insights as interesting as I did.
Thank you very much for inviting me and having the chance to speak.
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