What cats and dogs hear + A ‘smell map’ of the nose

[MUSIC PLAYING] FLORA LICHTMAN: Hey, it’s Flora Lichtman, and you’re listening to Science Friday.

PAUL: Hi, I’m Paul. I’m calling from Davis, California. And my question is, what is it about dogs and cats that makes them able to hear a much wider range of sound frequencies than humans?

FLORA LICHTMAN: Good question, Paul. Why do our fave animal companions seem to be able to hear things we can’t? Is it their ears? Is it their brains? And what would life be like if you could hear the world through your dog’s ears? Here to fetch us some answers is Dr. Pete Scheifele. He’s a neuroaudiologist at the University of Cincinnati and the executive director of the Fetch Lab, an animal audiology clinic and research lab. Pete, welcome to Science Friday.

PETE SCHEIFELE: Thank you. Thanks for having me.

FLORA LICHTMAN: The Fetch Lab sounds like a pretty cool place to work.

PETE SCHEIFELE: Well, I and my medical and audiology students think so. [CHUCKLES] It’s way out of the norm. I mean, audiologists work with humans, but we are the first group to actually work with audiology of animals.

FLORA LICHTMAN: It’s a “one of a kind” lab.

PETE SCHEIFELE: Yes, absolutely.

FLORA LICHTMAN: Wow. OK, let’s get to Paul’s question. First of all, do cats and dogs hear a wider range of frequencies than we do? Is that true?

PETE SCHEIFELE: Yes, absolutely. Humans can only hear frequencies from 20 hertz to 20 kilohertz. But dogs, and, to an extent, cats, can actually hear starting at 30 hertz, but they can hear all the way up to 57 kilohertz. So they’re very on the high end of things. They have it covered, and we can’t hear those frequencies.

FLORA LICHTMAN: And what allows them to do this? Is it their ears or how their brains process the sound?

PETE SCHEIFELE: It’s a little bit of both. Over the millennia, the ear has changed from the most primitive animals. When the ears changed, they changed because of some reason that that animal needs to be able to hear those frequencies. So, for instance, a bat can hear way up into almost 500 kilohertz. But it’s also, when it’s looking for food, it’s looking for primarily mosquitoes and June bugs, which are very small. So a low frequency that that wouldn’t even know that the mosquito was there. It has to be high frequency, so that the wavelength is so small that the bat can hear it.

But when it comes down to dogs and cats, basically, a dog’s ear has more hair cells in it, outer hair cells, which allow it to hear the frequencies that we cannot. So the ear had to change for that particular animal, and over time, it did.

FLORA LICHTMAN: Hair cells– that’s the key, different hair cells.

PETE SCHEIFELE: Yep.

FLORA LICHTMAN: Paul, our listener who kicked off this quest, was curious about what this all means for the experience of pets. And he gave this example.

PAUL: My mom is practically deaf. And so when she does anything around the house, the volume is at 10. And I’m just wondering if there are other people out there that have pets, if they’re like, are they listening to the TV too loud? Are they listening to NPR too loud? And is it bothering their pets? And maybe they can do something to make their pets’ lives more comfortable.

FLORA LICHTMAN: Pete, is public radio torturing pets across the nation?

PETE SCHEIFELE: [LAUGHS] It can. So dogs and cats and a myriad of other mammals are subject to noise-induced hearing loss, just like a person is. And so, yes, if things are playing loud, dogs that are in a house and kids got a rock band going in the garage, then, yes, they will suffer, eventually, a hearing loss, noise-induced hearing loss.

FLORA LICHTMAN: Do people bring in their pets with hearing loss to the Fetch Lab?

PETE SCHEIFELE: We don’t see many pet dogs that have a noise-induced hearing loss, unless sometimes you get the hunting dogs. And so the dog is being shot over. And so eventually, if they don’t use some kind of hearing protection for the dog, the dog will lose its hearing. But basically, every Friday, we see puppies for testing for congenital deafness, Special Operations military dogs, and Homeland Security dogs.

FLORA LICHTMAN: Are there dog breeds that suffer from deafness, congenital deafness more than other breeds?

PETE SCHEIFELE: Yes, absolutely. So we think that when 101 Dalmatians came out, a lot of people wanted a Dalmatian puppy. Breeding went rampant. People weren’t watching what they were doing from a genetic point of view. And so now, one out of every five Dalmatians in a litter will be born either bilaterally or unilaterally deaf.

FLORA LICHTMAN: Wow. If we zoom out for a second, is there an animal that an audiologist like you considers to have the best hearing? And I know it’s a complicated question because I know best is subjective and scientists don’t like that, but do you have a top pick, an animal that can do something amazing with their ears, that when you think about, wow, that is the majesty of the animal kingdom, I think of this animal?

PETE SCHEIFELE: [LAUGHS] OK, well, I think as far as that goes, we’ve been studying elephants and rhinos in South Africa. And the elephants are particularly of interest to me because they make a rumble. So an elephant, where I’m standing, if I’m standing next to it, will send out rumbles which vibrate through the ground. They’re seismic frequencies. And that can be picked up over a mile away by another elephant. And the other elephant will know what elephant is making the sound.

FLORA LICHTMAN: A mile away.

PETE SCHEIFELE: Yeah.

FLORA LICHTMAN: That’s amazing.

PETE SCHEIFELE: So when I first started working with animals, it was with bottlenose dolphins and beluga whales. And they’re in a whole different situation because they’re in the water. Water is more dense than air, and consequently, the speed of sound in water is five times faster than it is on land. And so their brain and their ears had to change to accommodate being able to hear under the water. So if you jumped in a swimming pool when we were kids and tried to talk to somebody under the water, our ears just don’t want to do that.

FLORA LICHTMAN: Yeah, I can imagine it. We all know what that sounds like, the wah-wah-wah-wah sound.

PAUL: Yeah, but they can hear just like you’re hearing me talk because the brain has changed so much.

FLORA LICHTMAN: Dr. Pete Scheifele is a neuroaudiologist at the University of Cincinnati and the executive director of the Fetch Lab, an animal audiology clinic and research lab. Thanks for being here, Pete.

PETE SCHEIFELE: Thanks for having me. I appreciate it.

FLORA LICHTMAN: This was fun.

PETE SCHEIFELE: [LAUGHS] It is.

FLORA LICHTMAN: Don’t go away. Our sensory journey continues after the break. Scientists are trying to map the nose to understand our sense of smell. Stay with us.

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FLORA LICHTMAN: From ears to noses, for the first time, scientists have created a smell map of the nose using a mouse model. Dr. Bob Datta, neurobiologist at Harvard Medical School and lead author on the study, is here to tell us more. Bob, thanks for being here.

BOB DATTA: Well, thanks for having me.

FLORA LICHTMAN: When I hear “smell map,” I’m thinking of an analog to the tongue map. Like, you taste salty here or sweet here. Is that what you are working towards with smell?

BOB DATTA: Yeah, we want to understand how smell is organized in space within your nose. And that’s because smell has long been thought not to be organized in space. Most of your other senses take advantage of space to organize what’s important to them. So if you think about hearing, your cochlea has actually a map of frequency within it, where neurons in your cochlea that are near to each other respond to similar frequencies. And those neurons that are far away from each other tend to respond to different frequencies.

FLORA LICHTMAN: Right, and it’s progressive, right? So one part of the ear does low frequencies. And then you move along, and you get to high frequencies.

BOB DATTA: Exactly. It’s this beautiful, continuous map. That’s exactly right. And similarly, your eye and the retina contains a continuous map of space. And so people have long wondered whether or not there’s a similar sort of spatially organized map for smells in your nose. And the way you’d look for that is by looking at the pattern of expression of smell receptors. So these are receptors that are dedicated to interacting with particular smells in the world. Mice have about 1,000 of these.

FLORA LICHTMAN: How many do we have? Yeah.

BOB DATTA: Humans have more like 400. And these receptors were first identified, believe it or not, 35 years ago. And so for many, many years now, people have wondered whether or not the receptors that detect smells in your nose are organized in space in much the same way that information about frequency is organized in the cochlea. And for a long time, people thought there was no real spatial organization to smell, that neurons, for the most part, in your nose that detect smell choose which receptor to express at random.

FLORA LICHTMAN: Why did people think there was no spatial organization? If we have spatial organization for every other sense, why would smell be an exception?

BOB DATTA: I mean, have you ever seen the inside of a nose?

FLORA LICHTMAN: [LAUGHS] I mean, I– no. [LAUGHS] That’s an excellent question.

BOB DATTA: [CHUCKLES] If you look inside a nose, it’s organized into these structures called turbinates, which look kind of like curlicues or scrolls. And so the anatomy on the inside of your nose is incredibly complicated if you just take a peek inside of it.

FLORA LICHTMAN: It’s like the topography is complex?

BOB DATTA: Exactly. It has this really confusing and complicated topography. And interestingly, animals that smell better often have even more complex topography. So our noses are less complicated than a mouse nose. It’s less complicated than a dog’s nose, which is super complicated. And the fact that the topography is so complicated means that it’s really difficult for us as scientists to know what direction is up and down and what direction is left and right. So that was one main limitation is that it’s just, unlike many of the other structures in our body, our nose has this really complicated and confusing shape.

The other limit was, for many years, we were kind of limited technologically. So there’s an old technique that allows you to look at where a given gene is expressed in a tissue. And that technique, called in-situ, typically allows you to look at one or two genes at a time. And so people would say, pick one of the 1,000 receptors in the mouse, and ask, where is this receptor expressed in the nose? And as they did that multiple times, they couldn’t really, looking one by one in this really complicated tissue, discern a pattern. And yeah, that led them to think that it was random.

FLORA LICHTMAN: And the genes tell you the receptor, right? They’re the way that you identify what the receptor is?

BOB DATTA: Yeah, absolutely. So here, they’re doing it in situ for receptor genes, trying to figure out where individual receptor genes are being expressed. And of course, those genes encode the receptor itself.

FLORA LICHTMAN: Because you’re not looking for the receptor. You’re looking for evidence of the receptor through the gene.

BOB DATTA: Exactly, exactly.

FLORA LICHTMAN: So we didn’t have the tools that actually allowed us to get the detail on which receptor was where.

BOB DATTA: Exactly. We lacked the technical ability to understand the precise pattern of expression for each one of these 1,000 or so receptors in the mouse, because we didn’t have that tool, and because the anatomy was so complicated. And people really didn’t what to make of what they were seeing when they asked where individual olfactory receptor genes are expressed in the nose. And that led them to hypothesize that maybe, for the most part, neurons are randomly picking which receptor to express.

FLORA LICHTMAN: And you found that was not true.

BOB DATTA: Right, exactly, exactly. So what we found was through the use of more modern techniques that let us look at many hundreds of genes at once, is that actually there’s a really precise pattern to the ways in which receptors are expressed in the nose, where the 1,000 receptors are actually organized into, essentially, 1,000 stripes, running from the top to the bottom of your nose. These stripes are overlapping, but their position is really, really precise. And it seems–

FLORA LICHTMAN: Wow.

BOB DATTA: –to be essentially invariant from one mouse to the next. So instead of each mouse having a different nose where neurons are just randomly picking what receptors to express, instead, it seems like the nervous system is capable of building this incredibly precise machine for detecting smells.

FLORA LICHTMAN: That’s a lot of variables, 1,000 different receptors to organize.

BOB DATTA: Yeah, so to be honest, I think a lot of the reason people assumed that receptors would be chosen randomly is because no one believed that the nervous system could solve the problem of placing 1,000 things into a precise order.

FLORA LICHTMAN: Yeah, it’s a hard problem, although the human body seems like a hard problem. So yeah.

BOB DATTA: Definitely.

FLORA LICHTMAN: So what is the equivalent for smell of sweet, salty, bitter, umami? Do we know?

BOB DATTA: Yeah, so now that we have this map, I think the main question we want to ask is, what does this map mean? Why are these 1,000 receptors in your nose in the order from top to bottom that they’re actually in? And the short answer is we don’t know, but having this map allows us to ask questions about how things are organized. And but I can tell you some possibilities.

So one possibility is that maybe odor qualities are organized in space in your nose. So you might imagine that citrus fruits are all at the top, and maybe meat is at the bottom. Or maybe these receptors are organized based upon what you like. Maybe smells that are really pleasurable to you are at the top, and maybe smells that you really hate are at the bottom. We don’t know, but a major program of research in the lab now is to try to figure out what this map we’ve discovered actually means.

FLORA LICHTMAN: What about the chemical composition of the smells? Could that be a way it’s organized?

BOB DATTA: Absolutely. So odors are actually small chemicals that float through the air, and each odor smells different because it has a different structure. And so one thing we did look at in our paper was whether or not different aspects of odor chemistry, like whether an odor contains a ketone functional group or an aldehyde functional group, whether different aspects of chemistry are actually organized in the nose. And what we found is that they’re organized a little bit, but not a lot. And so we wondered whether there’s something else going on. And that’s what we’re working on now.

FLORA LICHTMAN: Does the chemistry tell you whether it’s a smell you like?

BOB DATTA: Sometimes, but not always. So this is a major problem in olfactory science. If you look at a molecule, what makes it smell the way it does? And recent advances in artificial intelligence have improved our ability to predict what a molecule is going to smell like. And what we’ve learned from that is that sometimes, certain chemical features make things pleasant, but often, those very same chemical features can be found in an odor that’s unpleasant. So that’s a problem we’re still working on.

FLORA LICHTMAN: Mysterious. Does this smell map, could it help improve treatment for people who’ve lost their sense of smell?

BOB DATTA: We hope so. So we don’t know definitively whether this smell map exists in humans. We know that it exists in mice. Right now, with collaborators, we’re looking at human tissues to ask whether it actually exists in humans. To be honest, I really suspect it does. Everything we know tells us that actually the mouse and the human olfactory systems are really similar.

And so if this map exists in the nose, it means that if we try to build devices to artificially stimulate the nose, which is something people are doing in order to help people who’ve lost their sense of smell, or if we, say, put stem cells in the nose to help rebuild the olfactory epithelium after it’s been damaged by something like COVID, if we take any of those strategies, we have to respect this map of receptors that’s organized in space.

FLORA LICHTMAN: You have to know what goes where if you’re going to fix it.

BOB DATTA: Exactly. And so you’d want to rebuild the whole map and not just parts of it. And so it’s a really important constraint that I think we’ve discovered on future treatments, and probably an important part of developing treatments that work.

FLORA LICHTMAN: Dr. Bob Datta is a neurobiologist at Harvard Medical School. Bob, thanks for being here.

BOB DATTA: Thanks so much for having me.

FLORA LICHTMAN: This episode was produced by Shoshannah Buxbaum. If you have any questions you want us to sniff out, look, we’re all ears. Give us a call. Our listener line is always open. 8774-SCIFRI is our number– 8774-SCIFRI. And thank you, listener Paul, for the great question. I’m Flora Lichtman. We’ll catch you next time.

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