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

What do cats and dogs hear that we don't? Plus, what a “smell map” of the receptors in a mouse nose tells us about this sense.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:03Flora Lichtman?

  Hey, it's Flora Lichtman and you're listening to Science Friday.

• 0:07Speaker 01

  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?

• 0:22Flora 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 Shifely. He's a neuro-audiologist 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.

• 0:51Speaker 02

  Thank you. Thanks for having me.

• 0:53Flora Lichtman?

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

• 0:58Speaker 02

  Oh, well, I and my medical and audiology students think so. It's way out of the norm. I mean, audiologists, you know, work with humans, but very there's, we are the first group to actually work with audiology of animals.

• 1:17Flora Lichtman?

  It's a one-of-a-kind lab.

• 1:19Speaker 02

  Yes, absolutely.

• 1:21Flora Lichtman?

  Wow. Okay, 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?

• 1:29Speaker 02

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

• 2:03Flora Lichtman?

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

• 2:09Speaker 02

  It's a little bit of both. Over the millennia, the ear has changed from the most primitive animals. When the ears change, they change 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, you know, 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, the bat 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.

• 2:52Speaker 02

  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.

• 3:15Flora Lichtman?

  Hair cells, that's the key, different hair cells.

• 3:17Speaker 02

  Yep.

• 3:19Flora 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.

• 3:28Speaker 01

  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? You know, and is it bothering their pets? And maybe they can do something to make their pets' lives more comfortable.

• 3:51Flora Lichtman?

  Pete, is public radio torturing pets across the nation?

• 3:57Speaker 02

  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, you know, dogs that are in the house and kids got a rock band going in the garage. And, you know, yes, it's good. They will suffer eventually a hearing loss, noise-induced hearing loss.

• 4:25Flora Lichtman?

  Do people bring in their pets with hearing loss to the fetch lab?

• 4:30Speaker 02

  We don't see many pet dogs that are, 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, you know, lose its hearing. But basically, every Friday, we see puppies for testing for congenital deafness, special operations, military dogs and Homeland Security dogs.

• 5:04Flora Lichtman?

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

• 5:11Speaker 02

  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.

• 5:37Flora Lichtman?

  Wow. If we zoom out for a second, is there an animal that, you know, an audiologist like you considers to have the best hearing? And I know it's a complicated question because I know, you 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 you, when you think about, wow, that is the majesty of the animal kingdom, I think of this animal.

• 6:06Speaker 02

  Okay, well, I, I, 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, they make a rumble. So an elephant, you know, 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, and the other elephant will know what elephant is making the sound.

• 6:43Flora Lichtman?

  A mile away? That's amazing.

• 6:46Speaker 02

  Yeah. 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. If we, if you jumped in a swimming pool, you know, when we're kids and try to talk to somebody under the water, it's, our ears just don't want to do that.

• 7:27Flora Lichtman?

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

• 7:31Speaker 02

  Yeah. But they, but they can hear, just like you're hearing me talk. Because the brain has changed so much.

• 7:39Flora Lichtman?

  Dr. Pete Scheifli is a neuro-audiologist 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.

• 7:50Speaker 02

  Thanks for having me. I appreciate it.

• 7:53Flora Lichtman?

  This was fun.

• 7:54Speaker 02

  It is.

• 7:55Flora 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.

• 8:13Speaker 03

  And from the Alfred P. Sloan Foundation, working to enhance public understanding of science, technology, and economics in the modern world.

• 8:27Flora Lichtman?

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

• 8:43Speaker 04

  Thanks for having me.

• 8:45Flora 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?

• 8:54Speaker 04

  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.

• 9:23Flora Lichtman?

  Right, and it's progressive, right? So like, one part of the ear does low frequencies, and then you move along and you get to high frequencies.

• 9:30Speaker 04

  Exactly. It's this beautiful, continuous map. That's exactly right. And similarly, your eye in 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.

• 10:01Flora Lichtman?

  How many do we have?

• 10:02Speaker 04

  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. The neurons, for the most part, in your nose that detect smell, choose which receptor to express at random.

• 10:36Flora 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?

• 10:45Speaker 04

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

• 10:47Flora Lichtman?

  I mean, I've, no, I, that's an excellent question.

• 10:53Speaker 04

  If you look inside a nose, it's organized into these structures called turbinate, which look kind of like curly cues or scrolls. And so the anatomy on the inside of your nose is incredibly complicated if you just take a peek inside of it.

• 11:06Flora Lichtman?

  It's like the topography is complex?

• 11:08Speaker 04

  Exactly. It has this really confusing and complicated topography. And interestingly, animals that smell better often have even more complex topography. So, you know, our noses are less complicated than a mouse nose. It's less complicated than a dog's nose, which is like super complicated. And, 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, 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.

• 11:50Speaker 04

  And that technique called in C2 typically allows you to look at one or two genes at a time. And so people would say, pick one of the thousand 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 that, yeah, that led them to think that it was random.

• 12:12Flora Lichtman?

  And the genes tell you the receptor, right? Like they're, they're the way that you identify what the receptor is.

• 12:17Speaker 04

  Yeah, absolutely. So here they're doing in C2s for receptor genes, trying to figure out where individual receptor genes are being expressed. And of course, those genes encode the receptor itself.

• 12:26Flora Lichtman?

  Because you're not looking for the receptor, you're looking for evidence of the receptor through the gene.

• 12:30Speaker 04

  Exactly, exactly.

• 12:32Flora Lichtman?

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

• 12:38Speaker 04

  Exactly. We lacked the technical ability to understand the precise pattern of expression. For each one of these thousand or so receptors in the mouse. And because we didn't have that tool and because the anatomy was so complicated, and people really didn't know 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.

• 13:07Flora Lichtman?

  And you found that was not true.

• 13:10Speaker 04

  Right, exactly, exactly. So what we found was through the use of sort 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 thousand receptors are actually organized into essentially a thousand 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 to be essentially invariant from one mouse to the next. So instead of each mouse having a kind of 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.

• 13:55Flora Lichtman?

  That's a lot of variables, 1,000 different receptors to organize.

• 14:00Speaker 04

  Yeah, so to be honest, I think a lot of the reason people assumed that receptors would choose, 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.

• 14:15Flora Lichtman?

  Yeah, it's a hard problem, although we, you know, the human body seems like a hard problem, so.

• 14:20Speaker 04

  Definitely.

• 14:21Flora Lichtman?

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

• 14:28Speaker 04

  Yeah, so now that we have this map, I think the main question we want to ask is, what does this map mean? You know, why are these thousand 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 but having this map allows us to ask questions about how things are organized. And but I can tell you some possibilities. So, you know, one possibility is that maybe odor qualities are organized in space in your nose. So you might imagine that like citrus fruits are all at the top, and maybe meat is at the bottom. Or maybe, maybe, you know, these receptors are organized based upon what you like. Maybe smells that are 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

• 15:11Speaker 04

  program of research in the lab now is to try to figure out what this map we've discovered actually means.

• 15:16Flora Lichtman?

  What about the sort of chemical composition of the smells? Could that be a way it's organized?

• 15:21Speaker 04

  Absolutely. So odors are actually small chemicals that flow 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 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 wonder whether there's something else going on, and that's what we're working on now.

• 15:51Flora Lichtman?

  Does the chemistry tell you whether it's a smell you like?

• 15:55Speaker 04

  Sometimes, but not always. So this is like 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.

• 16:18Flora Lichtman?

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

• 16:24Speaker 04

  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.

• 17:06Flora Lichtman?

  You have to know what goes where if you're going to fix it.

• 17:09Speaker 04

  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.

• 17:21Flora Lichtman?

  Dr. Bob Data is a neurobiologist at Harvard Medical School. Bob, thanks for being here.

• 17:25Speaker 04

  Thanks so much for having me.

• 17:26Flora Lichtman?

  This episode was produced by Shoshana 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-CY-FRY is our number, 8774-CY-FRY. And thank you, listener Paul, for the great question. I'm Flora Lichtman. We'll catch you next time.