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Ed Yong on How Animals Sense the World

Sean Carroll’s Mindscape Podcast


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0:00:00.0 Sean Carroll: Hello everyone, and welcome to the Mindscape podcast. I’m your host, Sean Carroll. If you spend any time on the Internet, or maybe even if you don’t, you probably have heard about the amazing visual abilities of the mantis shrimp. The mantis shrimp is a tiny little guy, it’s a shrimp that has probably the most complex eyes in the entire animal kingdom. We human beings, we have color vision, our color sensitivity is pretty good. Human beings have pretty good eyes and we have three different kinds of cones in our eyes, three different kinds of color sensitive photoreceptors that are… The three different kinds, sensitive to slightly different wavelengths. If you’re an astronomer, like I used to be, it’s like three different filters that pass different wave bands through to the eyes. So we can combine these three different kinds of photoreceptors, trichromatism, it is called, to reproduce what we see as different colors of light, what we perceive as different colors of light.

0:01:00.4 SC: Now, the mantis shrimp has 12 different kinds of photoreceptors with different sensitivities across the spectrum. So it has been in popular imagination imagined that the mantis shrimp sees a whole bunch of kinds of colors that we human beings can’t actually see. Sadly, that’s not true, and on today’s podcast, you will learn that that is not true. The mantis shrimp doesn’t actually take advantage of its dodecachromatism or whatever you want to call it… It’s actually, according to physiological tests, not able to discern even as many colors as human beings are. Sorry to undo that little legend for you. But the point remains the same, which is that we use our senses to make pictures of the world, to build a model of what is really out there. And of course, there’s a trivial sense in which different kinds of animals sense the world differently, if they’re sensitive to different wavelengths of light or different frequencies of sound.

0:02:00.5 SC: But there’s also a more profound sense in which animals of different species perceive the world in wildly different ways because they either have more sensory apparatuses or entirely different kinds of sensory apparatuses. There are animals that live off of sensing vibrations and can basically touch things far away via signals sent by air or water. Not to mention animals that have actual receptors to magnetic fields or electric fields in ways that human beings just don’t have. So all of these animals really construct in their minds a very different kind of picture of the world that we human beings do. That’s what we’re gonna talk about today with Ed Yong, who has a new book out on exactly this topic, “An Immense World: How Animal Senses Reveal the Hidden Realms Around Us.”

0:02:49.0 SC: I’ve known Ed for a long time, we were both briefly bloggers at Discover Magazine, I don’t know, 15 years ago or whatever it was. Since then, Ed has gone on to great things, he’s a staff writer for the Atlantic, he did a lot of coverage of the coronavirus COVID-19 pandemic, and won the Pulitzer Prize for his coverage of that. But the book that he just came out with is actually… Or I guess it will come out tomorrow, is much more along his traditional beat of looking at ways in which animals are both similar to us and different to us. And of course, it helps us understand ourselves to think about the fact that we are constructing images of the world predicated on our sensory access to that world. But more importantly, it just helps us understand that animals are different, that what it is like to be a bat or a mantis shrimp etcetera is very different than what it is like to be a human being.

0:03:45.8 SC: Okay, so occasional reminder that we have a webpage here on preposterousuniverse.com/podcast, so you can find all the old episodes of Mindscape, complete with transcripts, show notes links etcetera. Links to the books like today, a link to Ed’s book, “An Immense World.” And also, we have Patreon, if you would like to support on Patreon, you can go to patreon.com/seanmcarroll. You get to ask questions for the “Ask Me Anything” episodes, which will now be regular Monday episodes going forward. And also, of course, you get ad-free versions of the podcast. So many ways to experience the Mindscape podcast experience. So with that, let’s go.


0:04:39.9 SC: Ed Yong, welcome to the Mindscape podcast.

0:04:42.0 Ed Yong: Hello, thanks for having me.

0:04:44.0 SC: I wanted to start by asking you if you’ve ever done Dining in the Dark? Do you know what that is?

0:04:51.3 EY: I have heard of it but I have not done it. It’s when you are having dinner… I feel like it’s… The clue’s in the name, right? Isn’t it…


0:05:00.0 EY: Just where you have dinner in a completely dark room and there’s waiters with night vision goggles?

0:05:04.7 SC: Well, there’s waiters. The way that we did it, the waiters were actually blind, so they were just used to…

0:05:10.9 EY: Oh, okay.

0:05:11.2 SC: Getting around without any vision. But they turn off all the lights… It’s a great scam if you have a restaurant ’cause you don’t need to decorate your restaurant. And also…


0:05:20.9 SC: The gimmick is that the rest of your sensorium becomes more sensitive so the food doesn’t need to be fancy, it just needs to be kind of vivid. And you taste all the differences between the different kinds of lettuce in your salad and so forth, and you learn to get around without seeing things.

0:05:38.9 EY: That’s interesting. Was it good? Did you enjoy it?

0:05:41.8 SC: It was good, I did enjoy it. They also come out with little games to play to see if your sense of touch has improved because you can’t see anything for a couple of hours, but…

0:05:51.0 EY: Interesting.

0:05:52.0 SC: It made me think of it, ’cause it’s sort of like sensorium tourism, right? Your book is all about how different…

0:05:58.0 EY: Right.

0:05:58.1 SC: Animals experience the world in different ways through different senses. And it just made me think of how we can sort of fake that by depriving ourselves of senses and how the world appears differently when we do that.

0:06:12.3 EY: Yeah, absolutely. I think that I… So the book is about this concept of the umwelt, the idea that every animal has its own sensory bubble and it has its own coterie of smells and sights and sounds that it can tap into and others can’t. And the man who pioneered this term, Jakob von Uexküll, he thought of the study of Umwelten as an act of travel, as an act of adventure. And that’s what I think of when you talked about sensorium tourism, that it is sort of a wonderful mini adventure to push against the constraints of your own senses.

0:07:04.2 SC: Yeah. And I think that when you mention to the person on the street that different animals sense the world differently, there’s sort of easy examples that come to mind and hard ones, right? Like different wavelengths of light. Okay. I mean, it’s pretty easy to imagine animals sensing that so the example you use, I guess, is sunflowers look like they have a bull’s eye in the center to certain birds and bees.

0:07:26.3 EY: Right, that’s right if you can see ultraviolet light then then yes flowers look different. The plumage of many birds look different and some birds that look identical across the sexes to us actually look quite distinct to the birds themselves.

0:07:44.8 SC: But then there’s also these wild examples of completely different kinds of senses. Like the mosquitoes sensing CO2. I don’t know if that counts as smell or how would we characterize the fact that mosquitoes know to find people by sensing their carbon dioxide emissions?

0:08:01.7 EY: So I think that is a very specific kind of smell but you know, again, it is a form of smell that we don’t readily have access to and does stretch the imagination, maybe to the same extent as imagining an ultraviolet that bulls eye on a flower might do, you know, you could imagine the smell of someone’s breath being incredibly attractive rather than like, whatever we think of as being as our reaction to breath. But you are right that there are other kinds of sensing that do seem exotic and that really push the limits of our imagination. So being able to sense the magnetic field of the earth as many songbirds and turtles can do, being able to sense electric fields as many electric fish can do, being able to sense body heat as rattlesnakes or vampire bats can, these all feel much more alien it’s not just like the sunflower example where you are, you are essentially like swapping pallets.

0:09:08.8 EY: You’re just translating from you know, bee vision to human vision but do you think even the, one of the extraordinary thing about the sensors I think is that even when you are thinking about more familiar senses, so not like electro reception or magnetic reception, the ones that we just don’t have, even something like vision that we have it doesn’t take much to start getting into territory that challenges our imaginations in the same way. And it’s one really easy example a duck that’s sitting on a pond because of the placement of its eyes can see the entirety of the sky without having to turn its head. I cannot imagine that, I can try, but it’s, it feels hard and effortful in my… I’m so used to having my visual world be right in front of me and be like, you know, 180 degree swath of space directly in front of my head that it’s really, really hard to imagine seeing behind me to imagine walking in a straight line and have part of your visual world recede away from you, as well as while another part goes towards you that feels very challenging.

0:10:33.4 SC: Do you think, and maybe this is something we just don’t know, but do you think that the brain of a duck is literally different kind of hardware to deal with that? Or is it maybe just a software problem?

0:10:48.2 EY: I think that it’s a bit of both and I think the hardware piece of it becomes more obvious when you think about something that is like much more different to us than even a duck. So the example I give in the book is, is an octopus, right and an octopus has a large nervous system, but most of that nervous system exists in its arms. Its arms have a large number of neurons more collectively than its actual that the brain in its head does. And those neurons allow the arms to work semi autonomously, like the arms have a bit of their own agency and they can move and do things independently of what the main animal is doing. And there’s some connection between those two things but then when you think about the sensors, it becomes even weirder because the arms have taste and touch receptors on the suckers.

0:11:45.5 EY: And then the head does vision obviously with the two eyes, so you have this creature that has this distributed nervous system and that’s doing different kinds of sensing with different parts of that nervous system. So, you know, how do you put all of that together? Again, it’s very hard to imagine. And, you know, in the book I write about how throughout fantasy and science fiction literature, we have many examples of people who you know, project their consciousness into the minds of other animals and this is like a very standard like mythological trope. But like, my argument is because the hardware is different, it just wouldn’t work like the… You know, the mind and brain of a human have evolved to process information that comes in from the body of a human. So you can’t just like, shunt that into the body of an octopus and expect that to work in this sort of weird, like, you know, dualist way. I think it, that, you know, the whole thing exists as a package and must be understood as a package.

0:12:56.1 SC: No, that makes perfect sense to me and I think what I’d like to do, you know, given your book that is coming out, by the way, I’ll tell people the name of it.

0:13:03.3 EY: Oh thank you.

0:13:06.3 SC: It’s called Immense World, how animal senses reveal the hidden realms around us. And we go through in the book, all these senses, both the familiar ones and the less familiar ones and I wanna do that, I wanna hit some of the highlights, but first I like what you’ve just said, because it drives home the fundamental fact that it’s not just a difference of degree, right? It’s not just a couple wavelengths here or there. It’s a very different kind of sensing and you have this wonderful sentence in your book early on, where even if it’s with the same senses that we know about, they can be deployed in different ways. So you say there are animals with eyes on their genitals, ears on their knees, noses on their limbs and tongues all over their skin. And you can just imagine following Thomas Nagel and what it is to be a bat that the…

0:13:52.0 SC: The world of such a creature is just a different kind of world. Even if it’s the same physical world, even if you’re eating the same food and dining in the dark your, your reconstruction of it inside your brain is an utterly alien phenomenon.

0:14:06.6 EY: Absolutely and I think Nagel was very prescient and exactly right on this, there is a sort of fundamental unknowability to that phenomenon for exactly those reasons. You know, the sensors are different, the animal’s body is different. It puts those sensors together in a different way. And, yeah, even part of the, I think part of the reason for writing this book and, and part of why I’m interested in this topic is it turns like even the most familiar of ideas into things that feel like unfamiliar and weird. You know, so, so you just listed a bunch of examples of sense organs appearing in different, in different body parts. So for, for humans, taste is something we do with our tongues.

0:14:53.3 SC: Yeah.

0:14:54.8 EY: That exists in our heads. And partly that’s because of our size, we’re a medium bodied animal. And so the foods that we eat is of… When we eat, we put food inside our heads. If you are a very small animal, like an insect food, can instead be something that you land on and walk upon. And for that reason, many insects from butterflies to flies have taste receptors on their feet. So a fly that’s landing on the apple that you are about to put in your mouth is tasting that apple as it’s walking around it. You know, and again, like, I think that truly challenges your imagination. You can just about imagine, like, you know, if I’m, I’m touching like the, the mattress that I’m sitting on now, you know, if I, I don’t know what it would be like to have, you know, to be able to smell or taste or have a chemical sense of that in the way that a fly and an octopus might.

0:15:49.0 SC: Well, and it makes perfect sense because as I was reading about taste in your book, in some sense, the sense of taste comes too late to be a good warning system, right? The food is already in our mouth, but for a creature where taste is external, that can actually be useful information before you start ingesting the stuff.

0:16:08.0 EY: Yeah. Absolutely you know, for, for us, like, well, you know, even for us, like taste is, is a quick warning system. You know, if, if we, if something tastes bitter, we can, we can spit it out. But you know, if you, if you are very small, if you’re a… If you are a fly, you can just take off, this incidentally is part of the reason why DEET works you know, DEET tastes repulsive to mosquitoes. And if a mosquito lands on an arm that’s covered with DEET, it tastes something foul and takes off.

0:16:40.3 SC: Good. So it’s not, I guess I would’ve thought it was smell, but you’re saying it’s really taste.

0:16:45.3 EY: Well. Okay. So [laughter], so I talk about this in the book, right? Yeah. Like the distinction between taste and smell is actually quite counterintuitive. And, and hard to explain to the extent that I even asked, like some biologists, like, what is the difference between them? And some of them struggled for an answer, you know, I think you can go through some of the obvious guesses, right? Like, some people would say, okay, taste is something that you do with your tongue and smell is something you do with your nose, but how does that work when you are a fly and you’re tasting with your feet? Or how does that work when you’re a snake, which is tasting with its tongue? So it’s not just about the organ. Some people say it’s about the distance of the stimuli. So taste is about close contact, like sensing chemicals that are, that you have to make physical contact with that exist in liquid or solid form.

0:17:36.0 EY: Whereas a smell is about detecting chemicals floating over long distances through the air. But again, there are problems with that, right? If you are an aquatic animal, the distinction between those two things become like collapses into almost nothingness. And even for smell, those molecules are dissolving in a liquid layer in your nose. So there’s always a liquid phase. I think that the actual distinction between them as one person who studies taste explained to me is in their use, taste is a largely innate and inflexible sense that is used most, almost entirely to work out if something is good to eat or not.

0:18:19.8 SC: Okay.

0:18:20.3 EY: It’s very simple and it’s quite binary. It’s like, yes, no you know, spit or swallow. And smell is more complex, more dependent on experience and more, more tailorable you know, smell depends on your positive and negative experiences with chemicals over the course of your life. And animals put it to a very wide range of uses from navigation to finding food, to social interactions. So you know, that thing that the mosquito is doing when it lands on a DEET covered arm and reflexively takes off, falls very much under the rubric with taste, it’s the kind of thing that taste allows an animal to do.

0:19:09.0 SC: Guess I would have intuitively thought that my personal sense of taste is more subtle and vary than my sense of smell. Maybe I’m just not very good at smelling things.

0:19:21.0 EY: Well. Okay. So I think that’s partly because most of what we think of as taste is actually smell like when we, when we eat food that like sense of like having a refined palette to that sense of connoisseurship of being able to identify flavor, almost all of that is smell, all your taste buds are really giving you is, is the five basics, right? It’s sweet, sour, bitter, salt, and umami. The, you know, the, the richness that we associate with meals that like a lot of the pleasure that we derive from food is really about smell it’s about those, those you know, things hitting your nose as you’re putting food into your mouth, or like going up the back of the pallet. Yeah, it’s interesting that culturally taste is the sense that we associate with like connoisseurship, right? We talk about people having like fine tastes. Whereas it’s actually the cruder and less sophisticated sense than smell.

0:20:22.5 SC: I guess I should have known that. Are you familiar with Ann-Sophie Barwich who is a philosopher?

0:20:28.5 EY: Right. Yeah, I know the name. Yeah those.

0:20:31.0 SC: She’s a philosopher of smell and she was a previous guest on Mindscape and, you know, she’s a champion of taking smell more seriously. And I should have remembered that lesson from, from talking to her. There’s a lot going on that, that but I mean, actually let’s just get into it because we don’t need to go in the order that I was imagining things, smell and taste seem different fundamentally to me than many of the other senses we’re gonna talk about, which are kind of vibration based. Right. You know, electrical, electromagnetic waves, sound waves, literal vibrations. But is tell me if I’m wrong here, but I get the feeling that there’s a molecular kind of fit and lock and key mechanism going on when we talk about the sense of taste and smell.

0:21:15.9 EY: Yes. And, and I caveat this by saying that actually a lot of how lot of the details here are still unknown. And so we certainly with smell, there is very much a lock and key thing. There are receptors that are specially shaped that to recognize different molecules around us. And those receptors are the sort of fundamental Lego bricks that our sense of smell operates on. But the universe of possible smells is huge. And certainly more vast than you know, than our like genetic repertoire of odor receptors. And so there has to be this a quite complicated, like combinatorial code that goes from like the basic hardware to our nose, to like our ability to distinguish between this just huge universe of possible odors. Both individually and in combination smell is super complicated. And a lot of that intermediate step between like detect like snagging a molecule from the air and like shoving it into another molecule that recognizes its shape. And then, you know, what exists in our heads like the aroma of baking bread and all that, that is still largely mysterious. I think and that’s, that’s for us, let alone for other creatures.

0:22:52.7 SC: Do you have an opinion especially after writing this book about the claims of wine connoisseurs, to be able to pinpoint, you know pencil shavings and apple rhymes and leather smells and tastes in a good bottle of wine?

0:23:09.5 EY: So, I do think that I’m sure that there certainly are people who are very good at identifying particular odors and, you know, and, and recognizing them. And I think this partly is a thing that humans are just not very practiced at. You know, it’s the… As you’ve said, as others have noted humans kind of deprioritize our sense of smell, like just culturally, it’s not a thing we have a rich vocabulary for, most of our words for describing smell are borrowed from other senses, or they’re just like nouns, you know, like lemon and we’ve sort of underappreciated it and thus underdeveloped it. Now, I think a lot of people have a much keenness in smell. I think a lot of people are better at putting words to their experiences.

0:24:05.3 EY: There are even entire cultures of people who have rich smell vocabularies. But you know, I think that the wine question is interesting because while I’m sure that it is possible to identify specific smell components of it, like the, this is very heavily tied into the question of like, you know, is an expensive bottle of wine actually better than, than a cheap bottle of wine. And I think for that, like there have been a lot of trials showing that actually it’s not the case, right. Like if you sort of blind if, if you do like a blinded experiment you know, often, like people can’t tell the difference between them in terms of like purported quality. But I also feel like this speaks to some of the malleability of smell and of our senses, right.

0:24:57.0 EY: That we can, that it’s also a bit top down rather than bottom up, that it depends like on our experience also depends on like the cultural value that we impart on these things. And so, you know, one way of looking at the wine thing is “Ha ha look at all these rubes thinking that this wine that they’ve paid a lot of money for is special.” But another, perhaps more generous way is isn’t there something like, kind of cool about the way our brains and senses are constructed, that you can take something that is basically… Like… Actually indistinguishable from something that’s like from another type of wine and imbue it with what really, really feels to people like something special.

0:25:45.1 SC: My own feeling about those studies is that they can’t possibly be studying people who are practiced and trained to taste wine. ‘Cause it seems to be, it’s a pretty big difference between a fine bottle of wine and a cheap one. But I’d have to look that up to be, to be sure. But certainly, you know, our training matters and our context matters, you know, as you say in the book precisely the same smell can be, and Ann-Sophie Barwich said the same thing in our interview can be either repulsive or attractive depending on its context. Right. It can, it can be like cheese or awful depending on how you encounter that smell.

0:26:24.1 EY: Yeah. Absolutely. And you know, that’s true within humans, for both reasons, both cultural and genetic, you know, there are some people with genetic variants that mean that body odor smells like vanilla. Very, very lucky people, I feel rather envious of them. But then those differences become even more extraordinary across the animal kingdom. Is a very basic example, like my dog Typer, is not repulsed by the smell of poop in the same way that I might be. I think this is a good example of what the umwelt means, different creatures, vary in what smells they have access to and then what those smells mean to them, and how those smells are used in their life.

0:27:17.2 SC: Well, and you do point out the very important thing in the book that we humans when taking our dogs for walks, deny them their umwelt sometimes because we don’t understand that they need to smell things, let them smell things, that’s their lives. [chuckle]

0:27:34.4 EY: Right, right. It’s such a fundamental part of being a dog. And I think a lot of dog owners, not knowing that pull dogs away from smell experiences hurrying them along on a walk and stopping them from sniffing other dogs in the genitals because they think it’s indecorous for some reason. I do think that it’s a problem. There are studies by people like Alexandra Horowitz, who has many great books in dog cognition, about how dogs that are allowed to sniff and encouraged to sniff end up being basically happier, less anxious, more optimistic, because they just get to be dogs. And I encourage this, like when Typer and I go out for walks, at least once a day, we have a walk that is his to control. He gets to decide how he wants to spend that time, we don’t have a destination in mind, we might make it one side of a block we might go for… We might go further afield, but it’s his to control and what he does with that time is he sniffs really intensely. He explores even bits of street that we’ve walked along hundreds of times over. I think like that, that alone is indication of how important that smell… The sense of smell is to dogs. If you give them full agency, the way they… A lot of them choose to exert that agency is by sniffing everything.

0:29:07.5 SC: Yeah, and I don’t claim to understand that at all. I have two cats, one of which will not want you to scratch her until she has sniffed you, your fingers thoroughly. The other one couldn’t care less. He’s like, “Yes. Scratches I’m with them now.”


0:29:20.2 EY: Yeah, right, right. Right, right. There’s species-level differences. And there’s absolutely individual-level differences, too. One thing that I’ve said a few times in talking about this book, and which I plan on saying as my disclaimer in front of any Q&A in any in-person event is I cannot tell you why your pet does that weird thing that it does.


0:29:51.9 EY: It is as much a mystery to me as I’m sure it is to you. But I think that we can start getting somewhere. We can start thinking about the kinds of hypotheses that make sense or the kinds of things that we… The kinds of questions we need to ask. I’ll give you an example there are some times when Typer looks out the main window of our living room and freaks out where he just starts barking at the window. And it’s not when… Sometimes it’s obvious. Sometimes someone’s rang the doorbell there’s some really loud… There’s a loud van driving past, but sometimes we don’t know why he does it. And watching that, firstly, gives me a lot more sympathy for people who believe in things like ghosts, for example, like paranormal stuff, because if you’re not used to thinking about animals as existing in a completely different sensory world, then if they are reacting to something you can’t sense that’s can’t natural, it’s gonna be supernatural.

0:31:00.8 EY: But I think if you’re used to thinking about umwelt then you can start asking questions like, Okay, is there a vibration out there that I’m not feeling? Is there a high-pitched noise that exists beyond the level of my hearing but that Typer can hear? Are there smells drifting in through the cracks in the window that are freaking him out? Is it a weird shape that my much sharper eyes can identify as something innocuous but that he has somehow mistaken for a threat? I might never know the answers but I think those are the right kinds of questions to be asking and they flow from this understanding of the different sensory worlds of other animals?

0:31:41.7 SC: Well, and we have a lot of senses to cover here. So you’ve already mentioned that human beings are pretty good at vision. We’re pretty vision centered in our lives I think. We sort of, as you already said, the vocabulary that we use, even to describe other senses is often borrowed from vision. So let’s talk about that a little bit. You did discuss this theory… Maybe it’s a theory, maybe it’s pretty obviously true of how eyes develop over evolutionary time through different stages, first, just sensing the existence of light and then moving on maybe I’ll let you tell the story.

0:32:16.6 EY: Right. Yeah, and this is… I think this is certainly pretty well accepted among vision scientists. The ideas from a very, very reputable and well-respected team, but it sort of goes along four stages, that first, you have simple light-sensitive cells that do nothing more than detect the presence of light. And that’s very useful for animals that want to, for example, find… For animals that want to sense, like the time of day, you could see, imagine like an aquatic simple animal that’s reacting to the present or absence of light. Then the next step up is also pretty simple. All you do is add some kind of shade to your light sensitive cells. So maybe it’s a dark membrane or a spot of pigment. And what that does is it blocks light coming in from a certain direction, which means that now you not only can sense light but you can tell which direction it’s coming from. And that allows you to do another more like an expanded class of things, including crawling towards shade, finding protection and shelter. Then the next step up, if you take a lot of those shaded cells and you put them together, then suddenly you have the ability to sense an image. You know, a low resolution image at first, but then that gives you the ability to do more stuff in the world around you.

0:33:47.9 EY: Beyond just like head towards dark area. And then the fourth stage is when you add like focusing elements to that, when you add things like lenses, like basically deluxe features that turn what might be a very blurry image into a sharper one. And that’s where, how you end up with the kind of eyes that we have. And that allow us to do with vision, all the things that we do reading you know, looking at each other’s other facial expressions, things that involve high resolution. And yeah, I think that this, there something, a few things that are notable about this pathway. Like we have examples of animals that exist at every point along that spectrum. And so Darwin lamented this idea that our eyes seemed perfect and that he, it was hard to imagine how they, the gradual steps by which they evolved, but we now have those steps.

0:34:43.6 EY: We have those steps in theory and in practice. But also Darwin was wrong, I think in saying that in thinking of our eyes as the sort of perfect example, right? Like there’s no destination eye that animals were sort of evolving towards, all of those eyes and those intermediate services exist because they are very well suited to their owner’s needs. So starfish has pretty simple eyes on the tips of its arms, which it can use to find the shelter of a reef. But that’s what, that’s all it needs to do. A starfish doesn’t need an Eagle’s eye doesn’t need able to spot prey from miles away, an Eagle does. So in many ways, I think the eye is a great example of evolution tuning and animal senses to the needs of its owners.

0:35:34.1 SC: Well, and a point you also make in the book is that having good senses requires resources. There’s reasons not to be perfect at all the senses because we have to give and take what we can given the amount of energy and space in our bodies and brains we wanna devote to these senses.

0:35:52.0 EY: Right. Totally. And I think that may be, I think that can be a counterintuitive idea in part because a lot of sense organs are basically holes, right? So it’s very easy to think of them as passive receptacles, like the eyes absorb the light, like sound goes in through the ears. The sense organs become like these sort of vacuums that soak up stimuli in the world around us, but for any of them to work it takes a surprising amount of energy, like even getting the neurons in the retina, ready to fire at the moment when light tend to cells detect light they need to be perpetually set in the state of excitement. The analogy I give is it’s like you’re drawing a bow and knocking an arrow and holding it there, always.

0:36:55.8 EY: So that at the moment when you could fire the arrow, it’s ready. That’s what, like the nervous system that operates our senses is doing all the time. And so to have that in place takes a considerable amount of energy. So even when I’m closing my eyes, even if I’m sitting in the dark, having good eyes drains my resources, and it means that there’s always going to be trade offs. It means that no animal is going to be able to sense everything because it has energetic limits. But also no animal needs to sense everything and that’s partly why umwelt exists at all. It’s because evolution has tailored the animal sense and animal senses to the needs of its owners according to the limits of its energy budget.

0:37:48.0 SC: Well, and there’s another layer of sophistication, right? Because the story you told about the development of eyes up to the focusing and so forth is kind of parallel to how we would imagine a really good camera being developed. But eyes have this extra thing where the different photo receptors are used for different purposes like rods and cones in our eyes at a very basic level. But then am I, you’re gonna correct me if I get this wrong, but jumping spiders have different eyes for motion versus sharp vision. This is just a whole another level of elaborateness I think.

0:38:25.0 EY: Right, right. We, so we have different parts of our eye that are devoted to different tasks. So we have the center of our retina just start off the center, the fovea is where our vision is sharpest. It’s why when I’m looking at you right now I can see your face in detail whereas if I look off to the side even a little bit, now your face is blurry in my field of vision, even though I can still see it. A jumping spider has something similar, except it has fully like it has done this full division of labor thing between its sets of eyes. So that the center pair of four pairs does sharp detail or the vision and the pair just on either sides of that, the lateral central, the lateral pair does movement detection. And that’s just… Crazy to me. [laughter], you know, you can, I’ve watched experiments being done where the lateral eyes have been blocked and you move like the shape of a cricket in front of the spider and the spider cannot track it. You unblock those eyes. And suddenly the lateral eyes are telling the middle eyes where to go and the spider is following the cricket. That’s…


0:39:53.9 EY: How do you even begin imagining what that might be like? I mean, I assume that for the spider, this all fuses into like a single stable, unified visual experience. Like it does for my eyes with those sort of do different jobs existing in different parts of the same eye, but like, is it, you know, that the fact that they are separate does absolutely blow my mind, you know, I’ll give, you another example, right? Like, so, colour vision exists throughout my retina it’s sort of best in the fovea, but you know, I’m looking around me and everything I see is in colour. But you have like larval fish where, the part of the retina that sort of that’s pointing upwards is monochrome. So it’s really only seeing in black and white, a part that is facing, forward, and a little bit down, is sensing mainly ultraviolet to pick out its prey against the sort of ultraviolet fog of the water. And there’s another part that is doing Tetra chromatic colour vision. So that’s seeing like more, another dimension of colour beyond what we can see. So you have the same eye, that’s doing three different kinds of colour vision in the eye of like a baby fish.


0:41:13.5 EY: Again, I cannot imagine what that would be like.

0:41:18.4 SC: Well, clearly the brain knitting together, all of these signals is a very, very important part of this whole story. I remember being very struck, stricken, struck by, David Eagleman telling me that if you watch someone dribble a basketball walking away from you eventually, you know, it takes longer for the sound to get to you than the light. So they will become out of sync. But for a long time, they remain in sync because the brain puts them into sync, even though it’s taking the sound longer to get to you. So there’s a certain point at which they suddenly go out of sync, ’cause the brain says, all right, I give up [laughter]

0:41:53.7 EY: Yeah, absolutely. And, that piece of it is very, very hard to get at.

0:42:00.3 SC: Yeah.

0:42:00.6 EY: You know we can just about get at it with humans. Right. It’s really hard. And that that’s the sort of, that’s the Nagle thing that like, even, if you, you know, you can do all the, the sort of fancy experimentation you want, but that final piece, like how the brain knits that together into something subjective and cohesive is a little ineffable. You know, so one example, right? Like, so bats echolocate, bats send out high-frequency calls and they hear the rebounding echos and through that they can sense the world around them. They can navigate through obstacles, capture insects, yada yada, because of the nature of that sense, echolocation should be stroboscopic. You know, the bat is putting out a call.

0:42:46.4 EY: It is hearing the echo. It is putting another call. It is hearing the echo, every set of call and echo creates a snapshot of the world around it, so it should be like the equivalent of watching a movie, right. Where you have different frames, each depicting something static. Now, when we watch a movie, we obviously knit together those frames into something that makes sense to us right. Into a sense of movement and into like this cohesive moving world. I assume that that is what bats do you know? I don’t imagine that a bat’s sense of the world as it flies and echo locates is like, you know, discreet this strobe thing. But I don’t know that, it’s just an educated guess and, partly like it’s an educated guess I can maybe in part, because like a bat is a mammal, it has a brain like, you know, smaller, but not entirely dissimilar to what I have. So it’s not hard to imagine that it has like the right, like, processing power to create that like cohesive movie from all those snapshots, that might be very different. If you were thinking about something that had a much smaller or simpler brain.

0:44:03.2 SC: Well, and, even in the case of vision, our brain is doing a lot when it comes to colour, right. We haven’t talked about colour yet, except you briefly mentioned the Tetrachromatic aspect of things, for some animals, but to a physicist you know, we think that we understand what colour is, it’s the wavelength of light, but of course, almost everything we’re looking at has multiple wavelengths coming at us and our eyes filter it just down to three, I guess, three sort of filter windows, astronomers are very familiar with this, but then our brain have to reproduce or reconstruct what it actually seems like as a colour to us. And it’s probably very different to different animals.

0:44:42.1 EY: Yeah, totally. You know, a, wavelength of, what is it like 700 nanometers, feels like red to us. But it’s not necessarily red to another creature. You know, to Typer my dog, it’s going to be closer to like a kind of dark muddy yellow, because he has a different set of hardware in his eyes. Yeah. Like, you know, this colour I think is interesting because it is, it really is fundamentally inherently subjective. There’s nothing specific about 700 nanometers that makes it red. It’s red because that’s what we, that’s what our sense organs and our brains…

0:45:28.6 EY: That’s the sensation that our nervous system creates, and that’s going to be very different for other animals. So for a dog the visual spectrum goes from a dark yellow to a dark blue and in the middle where we have green, they just have like whites and grays, and for a bird it’s going to be a lot more complicated. It’s going to go from red to ultraviolet, but again, like I’m talking about the visual spectrum as if it was a linear thing and it isn’t, it’s not like birds with an extra type of color sensitive cone in their eyes, like push out the spectrum and its margins it’s that they have this whole other dimension of color that we don’t have access to.

0:46:21.4 EY: So you can imagine that an animal that only has one or zero cone types of cone in size can see like a hundred gradations from black to white and all the shades of grain between, if you add another class, so you get, what Typer has you add another hundred gradations, between blue and yellow, that sort of multiply onto those. So now you’re talking about, 10,000, different possible discriminable colors. If you add another type of console we have, then you add 100 gradations from red to green on top of that too. And then if you add what a bird has you add another, it’s all multiplicative, and that’s why that visual world of a bird is so difficult to imagine. If you have another animal, like say a bee, which is also trichromatic, the bee just sees like a different part of the spectrum.

0:47:14.1 EY: It just goes from like green to ultraviolet, you can recolour what we see into what a bee might see, but you absolutely can’t do that with a bird because just four into three, what doesn’t go. And so it’s really, really hard to imagine what a bird might see. You can say like, here is a particular type of color that a bird sees, and I don’t see, I can recolour like parts of the world that I have to match that to show where a bird might see it, but you can’t do that with all of the bits or all of the colors that the bird has access to.

0:47:52.7 SC: But all that, taking into consideration, you’re still a little deflationary about the mantis shrimp, which is very famous on the internet for seeing all sorts of colors.


0:48:01.7 SC: And you’re saying it’s been exaggerated a little bit.

0:48:05.7 EY: Yes. Right. So mantis shrimps have, 12 or more types of color sensing cells in their eyes, right? So people are like, “Oh do they have like… Are they, what is it, dodecachromats?” Do they have this like 12 dimensional colour vision? And the answer seems to be no, in fact, in terms of discriminating between different colors, they seem to be substantially worse at it than humans, or, fish, or basically anything else that’s been tested. And it seems that what they’re doing, people’s best guess of what they’re doing is that… Okay. So in our eyes, we are taking the outputs from three kinds of cone cells, and doing quite basic arithmetic between them, adding and subtracting those signals in a process called opponency.

0:48:58.1 EY: And that is how we go from three to like the millions of possible colors that we can see. A mantis shrimp probably isn’t doing that. It seems to be taking the raw signals from its 12 types of color cells, and just sending them straight to the brain and then comparing them to a lookup table. This is apparently what satellites do. And it’s very different to what, basically all other animals do with color, rather than like having this 12 dimensional spectacular rainbow in its eyes. It basically seems to be collapsing the entirety of the visual spectrum into what tantamounts to like a children’s coloring book, right? It’s 12 different types of area that each get a different color and then you are sort of passing the world in that way. And which I think is the opposite of the reputation that they have, but also like very cool, still.

0:49:53.4 SC: Well, look, it’s a shrimp, we shouldn’t judge it too harshly. It doesn’t have a lot of processing power in the brain.


0:50:00.8 EY: Right. Right. And this takes us back to the point where the size of the brain matters, right? Like you can have as much… An animal with a brain, the size of a mantis shrimp really shouldn’t have the processing capacity necessary to deal with like 12 dimension colour vision. And, it turns out it doesn’t right? So it all goes hand in hand. The sensors are not just about what exists in the sense organ, vision isn’t just about the retina. It’s also about what the brain does with the signals from that retina.

0:50:37.4 SC: Okay. Again, lots of senses to cover here. So let’s move on to touch. I’m gonna count vibrations in with touch, right? In some sense, touch is a very direct thing. We’re touching things, but then again, one of the points you make in the book is that other kinds of animals have, in some sense, extended touch, they use the medium they’re in whether it’s air or water or whatever, for all intents and purposes touch things that they’re not touching.


0:51:06.4 EY: Yep, yep. Right, right. Yes. And I think that is a bit counterintuitive, right? We can do that to an extent, I’ve a ceiling fan blowing now, I can feel the current from that fan, but that touch at a distance is very much power for the course where a lot of other creatures. A shore bird probing into the sand can detect objects buried in the sand that are beyond the reach of its bill, by sensing the ways the pressure waves created by the probing bill are deflected by objects in the sand. Fish and manatees can sense the currents created by other things, swimming in the water around them.

0:51:55.7 SC: And they can use that to sort of navigate their environments to sense each other, even a blind fish can school successfully without bumping into its neighbors, because of the scent. A seal has whiskers that allow it to detect the trails left behind by a swimming fish that still exist in the water after that fish is gone. And that’s wild to me, right. [chuckle] I don’t think… Before writing this book, I would never have thought of a fish as leaving a track. But it does, it leaves behind, turbulent water that continues to royal long after the fish is gone and that an animal with the right equipment, like a seal with its whiskers can follow. Even in the air there are examples of this too. There are, you know, you have things like crickets and spiders that can detect the ludicrously faint air currents created by other, critters around them. There are spiders that can detect the wind created by a fly precisely enough to then leap into the air and catch that fly. Then there are crickets that can sense the breeze created by a charging spider enough to run away from it. And it really does seem like a lot of these creatures are pushing against the limits of physics. Their sense organs are about as sensitive as they could possibly be.

0:53:29.6 EY: And I certainly cannot imagine what it is like to be a cricket for exactly these reasons. I mean that the world is a different place, seems like a different place. I guess, the right way to say it is the world is the world, but you’re sensitive to such a different part of the world that it might as well be a different place.

0:53:46.9 SC: Yes, yes, exactly. If you were in the same room as a cricket you would be sharing the same physical environment, but you would both have a radically different experience of that environment. And that is very much like the core of what an immense world is about.

0:54:06.3 EY: And this is again, skipping ahead a little bit, but it absolutely reminds me of the electrical fish, which you talk about. And I actually had Malcolm McIver on the podcast, talking about such things. And I love you have a little picture in the book of this fish, which has its electric field around it, and an insulating object, versus a conducting object, versus a high capacitance object, all appear different to the fish that it just able to extend its electric field a little bit around it.

0:54:39.3 SC: Yeah. And again, oh, like how wild, right. So the electric sense, so more than any other sense, I think touch is the closest analog for the electric sense. The electric sense occurs across, over an electric fish’s entire body. It’s again, in like an extended form of touch it goes for about an inch beyond the surface of the fish’s skin. And it is sensitive to different qualities of the environment that we don’t care about. So you talked about like conductivity, capacitance, these are I guess what to an electric fish, what things like brightness, or color might be a like to us, you know, what things like volume and pitch would be to our ears. I think that the thing that really blows my mind about the electric sense is that, so these fish are producing their own electric fields and they’re sensing the way those fields are distorted by objects around them. That’s how they can make their way through water, which is often too murky to into to see. But those exact same pulses are also messages that they use to communicate to other fish. So for them, the line between perception and communication is very blurry. When electric fish fight sometimes the loser will convey submission by ceasing its electric field [chuckle] basically like someone just sort of shutting up and backing away from a fight, right.

0:56:22.0 EY: And closing their eyes.

0:56:23.0 SC: Because that is… Right, right. Exactly right. But because that is the same field it uses to navigate, it’s also as if someone is losing a fight, you know, keeping quiet, but also like covering its eyes and ears and backing away, right. The fish becomes oblivious to its surroundings in the act of ceasing its communication. And that’s really weird. [chuckle] That’s, it’s hard to think about. It’s hard to think about as a writer and reader of this book, but it’s also hard to think about as a scientist studying these fish, right. If you watch the animal doing a thing, what is it doing? Like, is it communicating? Is it doing something like perceptual? Is it doing something communicative a bit of both or neither, right. It’s hard to know.

0:57:15.5 EY: Or is this distinction even relevant if you’re a fish?

0:57:18.8 SC: Yes, exactly. Exactly.

0:57:22.2 EY: And even though, I mean, as you said, and I also sort of latched onto there’s absolute similarities between this electrical sense the fish has and this extended sense of touch that we have from vibrations in water or air or whatever, it is more alien to us human beings, we don’t have any electrical sense roughly speaking, right?

0:57:44.0 SC: Yes. I think that’s true. Well, I mean, I can sense an electric feel kind of right. If I like lick a battery or if I like stick my finger in the socket, I will feel something, but it’s gonna suck. And it requires like a lot of stimulus for that to happen. These animals put electric fields to and everyday use in a way that, yeah, absolutely we cannot do. I think that’s challenging to our imagination for a lot of ways. Like, partly we just don’t have this sort of analog of it, but we’re also limited in our vocabulary, right? Like when we’re talking about what an electric fish feels, we’re talking about things like capacitance and potential like things that…

0:58:38.0 EY: That where, that feels just a bit like cold and abstract, that isn’t like the rich lexicons that we have to describe things that we see or smell or hear. And you know, that, it I think it’s a good example where the limits of our own senses and the limits thus of our like language and our style of thought. It acts as a barrier to really appreciating what these other creatures are experiencing.

0:59:06.6 SC: Do you know of any science fiction writers who have imagined aliens that have electrical senses and have invented a whole new vocabulary for them to describe the, oh, that’s a capacitance object over there. It must be alive. Maybe it’s tasty. [laughter]

0:59:22.3 EY: Right, right. I bet there are, I’m sure there are, I just don’t know any of them off the top of my head. So if anyone’s listening to this and can think of…

0:59:29.4 SC: A good opportunity.

0:59:31.0 EY: Sci-fi, electro receptive, sci-fi let me know.

0:59:34.6 SC: Well, and the other thing is because we don’t have it, it is more mysterious to us. I mean, you do a very good job in the book, for every sense, you discuss literally the sort of biochemical way in which it is activated, you know, the proteins that help us vision and so forth. But how does that work for the electrical sense? And I’m not gonna give away the punchline, but you point out it all stems from hair cells at the end of the day. [laughter]

1:00:03.2 EY: Right. Yeah. Right, right. Like it’s very strange that… So we talked about this distributed sense of fish being able to touch at the distance that’s their lateral line, and it comes down to small hair cells that they have in their bodies. Fundamentally like most forms of touch come down to this, right. It’s like a small structure that gets deflected. And some neural setup that can detect that deflection, that is how most touch operates. You put that the thing in a capsule, you’ve got the pressure sensor, you put it on skin, you have something that detect like water and air currents. You put that in an ear and it’s something that can now detect sound waves channeled into the air. That’s basically how hearing works. And you can turn the… So the electoral receptors that electric fish use to detect to their own electric fields, evolve from those same types of cells. So in many ways, electric, the electric sense really is a very specialized form of touch. So, you know, you talked about like wanting to talk about touch and vibrations and all the rest in the same category. Like if you were go, if you were to be like super lumper about all of this, like all of these senses are basically the same thing. Yeah. You know, they like touch, vibrations, hearing, echolocation and advanced form of hearing electro reception. They all feel like just extensions of the same kind of mechanical sense.

1:01:48.7 SC: You also have all these other wonderful senses. I mean, I’m lumping a couple of senses together like we did with touch and electricity, but heat and pain are listed as senses In your book. I don’t think of pain as a sense usually. Right. But I guess it is. And you do a pretty good job of distinguishing between nociception, which I guess is the actual sense versus pain, which is an experience something that the conscious brain sort of feels after the fact.

1:02:23.2 EY: Yes. And, right. So how to deal with this was actually a challenge in the book because, it’s kind of a mishmash sense, right? The type, most of the book is organized by stimuli. Like there’s a chapter on… The chapter and hearing is really a chapter about sound. The chapters on light and color are really, on vision and color are really chapters about light. But pain is about detecting all kinds of things that are United by their capacity to cause us harm. So, you know, some of that might be chemical. It might be detecting noxious temperature, so extreme heat or extreme cold. So how you deal with that is an interesting question. And eventually I decided like it was just worth putting it in a separate chapter of its own because it does have this interesting distinction.

1:03:12.3 EY: A lot of scientists talk about nociception, which is the detection of the harmful stimulus, and pain, which is the emotional response to that detection. So I touch a hot pan and my finger recoils, before I realize what has happened. That’s nociception that reflects the detection of the hot stimulus. Pain is the suffering, pain is the fact that that incident sucks. And that I continue being in anguish afterwards. There… This distinction is interesting because a lot of people use it as a way of denying the idea that animals can experience pain, that they have this sort of emotional reaction to harm that they only do the nociception bit and not the pain bit. And this discussion, this question of whether animals can do pain, they have the emotional experience that accompanies nociception is a huge part of that chapter.

1:04:15.8 EY: You know, I talk about live debates about fish and insects and crustaceans and cephalopods. But I think that it’s important to get the… I think it was important me to get that quite early on in the book because some people have argued that. And so I actually that, things like fish and certainly fish and cephalopods, have can experience pain. They, it might be exactly the same as what we experience, but it is certainly like of kind. But some people have argued that this distinction is meaningless even in the pain literature, because for example, vision scientists, aren’t making distinction between photo reception, the detection of light and vision, the subjective thing, right? It’s only when we are talking about an area that affects like the welfare of animals and like our own morals and like what we can eat, then we draw the distinction. I actually think that line of argument is wrong. Because as we’ve said, like vision scientists absolutely do make a distinction between those two things. You know, there is basic photo reception. They do draw a line when like something actually starts counting as an eye or such counting as true vision. Um.

1:05:34.7 EY: Why? So the distinction exists. The reason why it matters for pain is that it feeds into all of these moral and ethical and even economic debates. So trying to get readers to appreciate this distinction between objective and subjective. The chapter on pain gives us an insight into the fundamental unknowability in this, that we’ve already talked about and that Nagel pinpointed.

1:06:04.2 SC: Yeah, and there’s some incommensurability, the fish or the shrimp or whatever feel something, and whether or not its pain might not be an answerable question in some philosophic sense, but bringing up these moral, ethical, other questions, I will… I promise this is the last question, but I do wanna give you an opportunity to talk about the issues raised in the last chapter of the book, that we human beings, because we don’t sense things in the same way that other animals do, have been willy-nilly polluting their sensoria with light and sound and all sorts of other things, and we are really making the earth a worse place for other animals in various ways that aren’t very necessary, we could do a better job.

1:06:51.8 EY: We could. We flood the environment with light, especially at night, and breaking this several billion-year hot streak that the world had of cycling between light and dark. [chuckle] We flood the world with noise, we drown out alarm calls and mating songs. And all of this does harm to the animals around us, I think it harms us too. It disconnects us from the nature that we’re surrounded by, disconnects us from the cosmos, it’s really hard to see the stars at night, and most people who live where we live have never seen the Milky Way before. I think that’s profoundly sad and it comes from our umwelt, the fact that we don’t think of these things as pollutants, but they very much are to other creatures, and I think that they are pollutants that we can remedy relatively easily. It’s not like plastics in the environment that are going to persist for centuries, it’s not like DDT and other pesticides that are similarly gonna wend their way through the world long after all of us listening to this have gone. You wanna fix light and noise pollution, often you can just flip a switch.

1:08:07.2 SC: Yeah.

1:08:07.4 EY: And it disappears. It’s a easy ecological win and one that I think we should try and fix.

1:08:18.1 SC: And even if you don’t know what it’s like to be a bat, we can at least try to be nicer to the bats and let them enjoy [chuckle] their own umwelt.

1:08:24.0 EY: Absolutely, we can give the bats more space to be bats, that’s a worthy cause.

1:08:30.1 SC: That’s a perfect closing thought, so Ed Yong, thanks very much for being on the Mindscape podcast.

1:08:35.0 EY: Thanks Sean, good to speak to you. Take care…


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