Smell is a primary sense for dogs. It's the way they socialize, it's the way they navigate. It's the way they explore.

That's Ed Yong. He's an author and Pulitzer Prize winning journalist at The Atlantic. He also just published a book called An Immense World: How Animal Senses Reveal the Hidden Realms Around Us. And as he was writing the book, Ed became a dog dad.

So his name is Typo. He's a corgi. We we got Typo after I'd written about dog and smell in this book, and it has profoundly influenced the way we've raised him.

Dogs have an incredibly sensitive sense of smell. Some have been trained to smell bombs and drugs, even cancer, even COVID. And for dogs, going for a walk isn't just a chance to do their business and get in their doggy steps. It's also an exciting smell adventure, one that Ed and his dog Typo go on every day.

We go on what we call a sniff walk, like we go at his pace, and we do what he wants. And what he wants to do is sniff. Like he will spend 10 minutes exploring a bush. You know, he will take half an hour to just go around the block because he's exploring the world with his nose. And I find it kind of wonderful, like watching him take the utmost care, like sniffing every leaf on a plant.

For dogs, it's also a way of connecting with other dogs.

I had this moment when I was like looking at Instagram on my phone and watching Typo sniff a patch of pee that some other dog had left a few hours earlier and thinking, "This is the same activity. This is a deeply social act and it's an act of like keeping in touch with what friends of ours have been up to, even though they're not directly in our presence."

We all know about the five human senses - sight, smell, hearing, touch and taste. They literally help us make sense of the world. We need them to move around to find food and shelter and to communicate and connect with each other. Animals, of course, have senses, too, including a bunch that we don't have. So to kick off the new season of Chasing Life, I sat down with Ed to give you a bird's eye view, pun intended, of the realm of the senses in the animal kingdom. Did you know that scallops can have as many as 200 eyes? Until I read Ed's book. I didn't even know that they could see. I also didn't know that sea turtles use the Earth's magnetic field to allow them to swim around the world and then still find the exact beach where they were hatched. Learning about these animal senses made me feel like a kid again, full of wide eyed wonder at nature's beauty and creativity. It also gave me a richer sense of the world and a better understanding of our place in it. I really hope it does the same for you. I'm Dr. Sanjay Gupta, CNN's Chief Medical Correspondent. It's time to start chasing life.

So the senses are about taking the coursing chaos of the world and making meaning out of them. You know, they take in all the fairly abstract stimuli around us, like waves of pressure that we call sound, like electromagnetic radiation that we call light. And through some, like chemical wizardry, turns that into neural signals that give us the experience of a sunset or the smell of a cup of coffee or, you know, the sound of birds singing.

It's really incredible to realize that our bodies can even pick up on these external stimuli and then convert those into experiences. But what's even been more amazing to me is that each of us, humans or otherwise, does this differently.

You know, I start the book with this thought experiment to imagine that you're sharing a room with an elephant and a bee and rattle snake, a spider, a bat. You could all be in the same physical space, but you would have radically different experiences of that space. Rattlesnake will be able to sense the body heat of the animals around it. The elephant could make low infrasonic rumbles that the other creatures couldn't hear. A dog in that space would be able to get so much scent that its fellow animals couldn't get. So each of us is trapped in our own sensory bubble and perceiving just this thin sliver of the fullness of reality. But this this feeling of getting everything is is such an illusion, and it's an illusion that every animal shares. I think there's something wonderfully uniting about that idea, but also very humbling. It tells us that even the most familiar parts of our world are full of unknowns and extraordinary things.

You know, it's interesting when you think about what the purpose of senses are, and if you just dial up the aperture a bit and senses seemingly exists to to make us survive, to to help us keep away from danger. What purpose do they serve beyond that?

Well, think about everything an animal needs to do, you know, find its way around the world, interact with other creatures, detect danger. Yes, but also interact with other members of a community. You know, find mates, find companions. And what's important, I think, is that nothing needs to sense everything. Through the course of evolution, our senses have evolved to give us exactly the kinds of information that we most need. And that's why these perceptual bubbles there's this concept called the umwelt, that's why that exists.

I'm curious. Can you just explain what is umwelt? How do you define the term?

The word just means environment in German, but it's not used to mean the physical environment. So the umwelt, my umwelt is not like the desk that I'm touching or the chair I'm sitting on. The umwelt means the perceptual environment. So the parts of the world that we can sense and perceive and the other creatures might not be able to. So my umwelt, for example, includes a spectrum ranging from red to violet. A bee's umwelt goes from green to ultraviolet. Birds have a whole dimension of colors that they can perceive and that we can't. My umwelt does not include the Earth's magnetic field that a turtle might be able to perceive, doesn't include the electric fields a shark or a platypus can sense. So each creature has its own set of sights and smells and textures and sounds that it can perceive and that are unique to it. And that is what the umwelt is. It is the thin sliver of reality that we have access to.

How do we really know what their umwelt is? I mean, we see their behavior and are we sort of retro engineering what we think their umwelt must be based on their behavior? Is that it?

Yeah. You can look inside the cells in animals' eye and you can say like what wavelengths of light so therefore what colors might it be sensitive to? You can do behavioral experiments, you can play specific sounds or show specific kinds of images and see what an animal responds to. American philosopher Thomas Nagel wrote about this in his essay. What is it like to be a bat? I can imagine, like having leathery membranes across my fingers and flapping through the air. But understanding the subjective, conscious experience of being a bat or an elephant or a songbird or a sea turtle is, I think, actually fundamentally impossible. Science can take us very close to the edge of that, but there's always going to be this chasm of understanding between where we are and where they are. And I think that's kind of wonderful, right? Like you need feats of imagination to cross that gap. And it's a task that I don't think we will ever fully achieve. But there's something glorious about nonetheless attempting it. I think it makes us flex our mental muscles, like our empathy muscles, in a way that we don't often do.

If you ask anybody to name the senses, they're going to name vision, hearing, smell, taste and touch. Do we have more than these five senses?

Oh, yeah, definitely. So the reason we think that there are five is Aristotle. That's his classification scheme and he missed some. So there are a lot of internal senses that tell us about the state of our own body. Equilibrioception. That's the sense of balance, right? So it helps me keep steady. Proprioception is the sense of where your body is. So I close my eyes. I know that I'm holding up my arm right now. That's proprioception. But then you can lump or split them in lots of different ways. You know, some people argue that what we call touch is a bunch of different senses. We have different receptors and neurons for detecting cold and hot. It's not a single like temperature sense, they're actually two distinct ones. Is is our sense of pressure or vibration different? Do they count as separate senses or are they all part of touch? And things get more complicated when you look at other animal senses that they have that we don't, like magnetic sensors and electric senses, but there are also a lot of creatures combine the senses in strange and unfamiliar ways. So a lot of insects with their antennae are sort of tasting and touching at the same time. Do those feel separate to them or do they have a single sense of taste-touch? You know, this question of how many senses are there? Turns out to be surprisingly difficult, even for humans and definitely across the rest of the animal kingdom.

The traditional human senses all basically work in the same way. There is an outward facing sensory organ that collects input from the environment. For example, special receptors in the eyes pick up light waves, and then the sensory organ communicates that information via the nervous system to the brain. Those signals are then interpreted by the brain into, for example, the image of a face, the smell of coffee, the sound of a baby crying. But for some of the animal senses, it's not always clear what's going on.

So, for example, songbirds and sea turtles can detect the magnetic fields of the earth. Sea turtle swimming in the Atlantic knows exactly where it is and which way to go. A songbird in a completely dark room with no landmarks when it comes time to migrate, knows exactly which direction to point into. How does that work? We genuinely don't know. And that's partly because magnetic fields permeate through tissue. They penetrate through living matter. So the sense organs that detect magnetic fields do not need to be on the surface. They don't need to be an ear or an eye or a nose. And that, combined with the fact the magnetic fields are so counterintuitive, makes it a very difficult sense to study. And I kind of love that. People have been thinking about the senses and the way animals perceive the world for centuries, if not millennia. And yet, here is a sense that was only discovered in the 1950s and whose fundamentals we still don't understand.

Is it a question of I mean, do we all have these capabilities of having these senses but don't use it?

I think it's unlikely. You know, I said that the senses come at a cost. So even if I close my eyes, having eyes at all and having all the neural machinery that makes vision work, drains energy. The way I think about it is, imagine you're drawing a bow and you're just holding that string taut so that when the time comes, you can fire the arrow. That's what it's like to have sense organs. So to even get your neurons ready to fire at the point when you see something drains a ton of energy. So organisms typically don't have senses that they don't use just lying around. There are some exceptions which are that we might be able to use the senses that we do have in amazing ways that we don't really think about. So in the book I talk about humans who can echolocate, you know, much like bats and dolphins. They produce noise and they perceive the world by being attuned to the echoes coming back from the objects around them. This is an ability that a lot of blind people have had for a long time. You know, so here's a case where humans actually can have this quite extraordinary sense. Even for our own species, different individuals have their own unique perceptual bubbles.

What was the most mindblowing example, if you will, of a sense in an animal that you came across something that you're sharing at have cocktail parties or whatever?

Oh, they're all my babies. It's hard to choose between them, and I try and give a different answer each time. So. There are lots of animals that can sense electric fields. All animals, especially in the water, produce their own very faint electric fields. Things like sharks and platypuses can detect that. So even if a fish is buried under sand, a shark can use its head like a living metal detector and finds that buried fish. There are electric fish that can produce their own electric fields. By then sensing how those fields are distorted by objects around them, they can find their way around incredibly dark and murky water, and it's really hard to think about how that electric sense might feel. One researcher I asked, imagined like an electric fish swimming alongside a rock and maybe like a cold sensation travels down its flank. But it's such an alien sense, it's very difficult to conceive of how it might work. And then the last example, I'll say there are some spiders. Baby spiders often do a thing called ballooning, where they will release small strands of silk and take off in the air and travel for miles to reach new places. It was long thought that this work through wind, like the wind, is just blowing the silk along. And it turns out that it works through electric fields. There is an electric field that goes from ground to sky, a kind of planetary electric field caused by lightning storms existing around the world. And it's that that the spider effectively rides. So, you know, you have baby spiders flying through the air by riding the Earth's electric fields. And I think that's just incredible.

When we come back, we're going to explore a sense that isn't traditionally thought of as a sense at all.

It's a bit of touch. It's a bit of smell. It's hard to categorize, but does perform a very specific and important role in our lives. And it varies it varies quite considerably around the animal kingdom.

We'll be back in a moment.

And now back to Chasing Life and my conversation with award winning journalist Ed Yong. Every individual animal, from humans to spiders to shrimp, is limited by their own umwelt, or their sensory bubble. This makes it really hard to put ourselves in the shoes, so to speak, of another living organism, to really imagine the world that they experience.

You know, I write about what it might be like to be a bat or a mantis shrimp or an elephant or a star nosed mole. But those are feats of imagination as well as science. It's sort of fusion of those things. And I do think that this is a type of question that people have asked for, probably the entirety of our history as a species. So many of our myths and stories feature characters who can transfer their consciousness into the bodies of other animals. So a lot of the ancient gods would do this on the regular. You know, you have characters from Game of Thrones who can do this. The idea that even in a fantasy world you might be able to do this stems from this idea of dualism, the idea that the mind and body are separate and that you can take a mind and put it into another body. And it would work. And it really wouldn't. You know my mind, and my senses are a product of my body and my own evolutionary history and my needs. I can't imagine putting my consciousness into the body of an octopus and expecting it to work because it just wouldn't work. So I think what I'm saying here is that to really understand what it's like to be another animal and why it's such a difficult and almost impossible task is, it's not just as simple as even studying the senses. You have to understand the entire the animal's entire body, its nervous system, its behavior, its history, its ecology. You need to know all of that to really think about what that creature's experience is like. And that's fiendishly hard.

Including previous experiences that that particular animal had.

Absolutely. Yeah, absolutely.

That brings up this this idea of something that I've been very interested in lately, which is which is pain. You have a chapter devoted to this in the book about pain being a sense. Why did you decide to include that?

Right. Because we've talked about how we have these senses outside the traditional five. And I really wanted to talk about pain because it touches on some of that. Right. So it's a bit of touch. It's a bit of smell. It's it's hard to categorize, but does perform a very specific and important role in our lives. And it varies it varies quite considerably around the animal kingdom. When it comes to animals feeling pain, a lot of people go through one or two extremes. They either think that no animals feel pain like we do, or all animals feel pain like we do. And in either case, it's a dumb question to ask. And it's not like an animal's experience of pain can be radically different, even between supposedly closely related species. So if you take squid and octopus, for example, if an octopus hurts one of its arms, it knows which arm has been hurt. Just like if I stub my toe, I know my toe hurts and it will cradle and groom that arm. It clearly has a sense of pain that is very similar to our sense of pain. A squid does not have that. Like if a squid injures its arm, its entire body becomes hypersensitive. So it's as if I stub my toe and suddenly my elbow's sore, or my ear hurts to the touch. And that's possibly because a squid doesn't have the long, dexterous arms of an octopus. It can't explore much of its body. So it doesn't really pay. It doesn't help the squid to understand, like I've been injured, like on my back near the fin. It's much better for the animal to have this sort of general sense of like, I have acquired an injury. I'm in a bit more danger than I normally am. I should be more cautious. So even creatures like squid and octopuses, which seem very similar to us, can have radically different experiences of pain.

If you look at this scientifically, are we able to understand a particular animal's capacity for pain? Meaning, meaning, let's say, Typo your dog. If there's something that's painful to Typo, steps on something or whatever happens, by measuring, for example, the density of these pain receptors. Well, they're they're very densely populated by the paw, therefore Typo can experience a lot of pain. Is that how this is studied, or is it more based on the behavior of the animal? Typo, you know, wails and and sort of yelps if, you know, something happens.

Yeah, a lot of it is based on behavior. So, you know, looking at the physiology, like the number of receptors, all of that like that gets you a certain distance. So it tells you a bit, but it doesn't cross that subjective chasm we talked about. Right. It doesn't get me into Typo's mind. It doesn't it doesn't help me to imagine his experience. His behavior absolutely does. And, you know, in the case of a dog, an animal that's also a mammal like us, it's easier to interpret what he might be doing. It's much harder when you are looking at like a crab or a fish or a worm, you know, to to interpret what its movements might mean. Like, to what extent are they analogous to ours? And I think because of that and because the question of whether animals feel pain has so many moral and ethical and economic implications for us, we should go beyond just saying like, do animals have it or not? And to think about what the experience might be like, you know, how does the the sensation of pain vary from one species to another?

I mean, you said we may never really fully understand, but what is the importance of trying best as we can to understand another species' umwelt?

I think a few things. Firstly, if we don't, we risk harming them. In small ways, like yanking a dog along the walk, stopping it from engaging in this olfactory exploration that makes it happy. We are also polluting the world with too much light at night, with too much sound in quiet spaces. We don't think of light and noise typically as pollutants, but they are and they can be. And they are severely harming a lot of creatures around us in ways that will eventually, I think, bounce back towards us. You know light at night harms pollinating insects, which we rely upon. But I think that the promise of the book and the reason why I think people should think about this is is a bit less transactional than that. I think it's that I feel that I gained so much from thinking about the creatures around us. I understand them better and I understand my world better. I see even simple things like the streets around my house, the plants in the nearby garden, the surface of the sea, in new and amazing ways. I understand that even when things seem still and boring, they are, you know, replete with information that I can't perceive. And I think understanding that gives me a much greater and richer sense of the world. It helps me understand my place in it, and it makes me more appreciative of the other creatures I share the world with. And I think that that ability to think about the umwelt of another creature is probably unique to humans. And I think the ability to do that, which is so unique to us is, is a gift. And I think it's one that we should cherish and not take lightly.

It is a gift and it's one we should cherish. And I could tell you, at least for me, it's already had an impact on how I'm going to choose to experience and act in the world.

We got to get Typo and then my three dogs, Nuck, Hugh and Bruno, together sometime and just let them do their thing. Let them be dogs and go experience the world.

Oh, that would be great. I would love that.

I've typically thought of walking the dogs as a benefit for me, maybe to get some steps, to make sure the dogs do their business. I'm a "let's get it done" kind of guy. So when I was taking my dogs out for a walk, I was typically speed walking down the street. But it wasn't until I read Ed's book that I really thought about letting them stop, to smell the roses, to take in all that sensory stimuli. It's something that is necessary for the dogs. It's part of their umwelt. And even though this book isn't about the difference between humans as much as it is about animals, it has really made me think about other people's umwelt as well. It forces me to think about whether someone has had a bad day or a bad experience because that can affect how they are experiencing the world. Think about that. Their umwelt might change how they sense things around them. And when it comes to pain, it makes me realize that as much as we think about pain as a sensation, it really is more of an experience, an individual one perhaps to every creature on earth. The old saying is, you can't understand someone until you've walked a mile in their shoes. Well, it's really true here, and it's something I think we should all think about, especially during these very raw and challenging times.

I'd love to hear your thoughts on the umwelt of other creatures. Well, learning about it changed the way that you view the world? Will it change the way you behave in it? Record your thoughts as a voice memo and email them to asksanjay@cnn.com or give us a call 470-396-0832 and leave a message. We might even include them on an upcoming episode of the podcast.

Now, for the rest of the season, we're going to be looking at humans and our senses. We're going explore topics like neurogastronomy. That's how taste relates to emotion, memory and learning. Also synesthesia. That's what happens when our senses overlap and mix together. And face blindness. What it's like to not even recognize your own face in the mirror. We'll be back next Tuesday with an episode about losing our taste and our smell to Long-Covid and how some people are now trying to get those back. Thanks for listening.

Chasing Life is a production of CNN Audio. Megan Marcus is our executive producer. Our podcast is produced by Emily Liu, Andrea Kane, Xavier Lopez, Grace Walker and Rafa Farihah. Tommy Bazarian is our engineer and a special thanks to Ben Tinker, Amanda Sealey and Nadia Kounang of CNN Health, as well as Rafeena Ahmad from CNN Audio.