What this covers
Ed Yong and Sean Carroll discuss how animals construct radically different sensory worlds from one another, drawing on examples from across the animal kingdom to show that perception is not a simple matter of better or worse versions of human sight, hearing, and smell. The conversation centers on the idea that each creature inhabits its own umwelt—a distinct sensory bubble shaped by evolutionary pressures, body plan, and metabolic constraints—and that these alien perceptual universes are largely inaccessible to human introspection. Yong walks through the machinery behind sensation: how senses are energetically costly systems that force evolutionary trade-offs, how different sensory hardware cannot simply be transplanted into a human mind, and how recognizing this gap should reshape both our empathy and our environmental responsibility.
The discussion ranges across multiple sensory modalities and anatomies. It covers how cones multiply to give birds a fourth color dimension humans cannot access, how electric fish blur the line between sensing and communicating with the same organ, how an octopus distributes neurons throughout its arms for semi-autonomous sensation, and how many animals use the medium around them—air currents, water pressure, vibrations—as extensions of touch. Yong addresses why common beliefs often mislead: that more sensors always means better perception (the mantis shrimp's dodecachromatism does not grant superior color vision), that taste is our refined sense (it is mostly smell and cultural meaning), and that consciousness could transfer between species (the mind, brain, sensors, and body form an integrated package). The conversation also turns to concrete harms: how humans flood environments with light and noise pollution that breaks sensory landscapes built over billions of years, disrupting animal communication and navigation in ways that are, unlike persistent toxins, immediately reversible.
Yong argues that every animal inhabits its own sensory bubble (umwelt) shaped by its specific hardware and needs, so the perceptual world of any creature is fundamentally different and largely unknowable from a human perspective — and recognizing this should make us both more imaginative about other minds and more responsible about sensory pollution.
- Senses are not passive receptacles but energetically costly systems evolution tunes to an animal's needs, forcing trade-offs
- Different hardware (eyes, neurons, body plans) cannot simply be 'piped' into a human mind, so other umwelten are inherently alien
- Because we don't share other animals' senses, we pollute their environments with light and noise without realizing the harm
Sensory perception is top-down and malleable, shaped by context, experience, and cultural value rather than stimulus alone.
- Smell is partly top-down rather than bottom-up: the same odor can be repulsive or attractive depending on context (e.g., cheese versus something awful), and perception depends on experience and the cultural value imparted on things, both within humans and dramatically across species where the same smell carries different meanings.
“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”
- The true distinction between taste and smell is not the organ or the distance of the stimulus but their use: taste is a largely innate, inflexible, near-binary sense used almost entirely to decide if something is good to eat (spit or swallow), while smell is complex, experience-dependent, tailorable, and used for a wide range of purposes from navigation to social interaction.
“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”
- Most of what people experience as taste — the refined palate and connoisseurship — is actually smell; taste buds only deliver the five basics (sweet, sour, bitter, salt, umami), while the richness and pleasure of food comes from odor molecules reaching the nose, making taste the cruder, less sophisticated sense despite its cultural association with fine discrimination.
“most of what we think of as taste is actually smell... all your taste buds are really giving you is, is the five basics, right? It's sweet, sour, bitter, salt, and umami”
- Blinded experiments have repeatedly shown that people often cannot tell expensive wine from cheap wine in terms of purported quality, which speaks to the malleability and top-down nature of smell and the senses — the cultural value imparted to something can make an essentially indistinguishable substance feel special.
“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”
- Color is fundamentally subjective — there is nothing about 700 nanometers that makes it 'red'; it appears red only because that is the sensation our sense organs and nervous system create, and the same wavelength would appear as a dark muddy yellow to a dog or something entirely different to a bird, because color is a construction of the brain not a property of light.
“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... That's the sensation that our nervous system creates”
Each animal inhabits a unique sensory bubble determined by its evolved hardware and neural architecture.
- Because an animal's sensory hardware and body are fundamentally different and the brain evolved to process information from its own body, the science-fiction trope of projecting human consciousness into another animal couldn't work; the mind, brain, sensors, and body must be understood as an integrated package, not separable in a dualist way.
“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”
- Sense organs are not passive receptacles that soak up stimuli; even when idle they consume substantial energy because the nervous system must keep neurons perpetually primed to fire (like a permanently drawn bow), which means no animal can sense everything and every animal faces trade-offs — and this energetic limit is partly why distinct umwelten exist.
“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”
- Every animal inhabits its own sensory bubble (umwelt) — a unique set of smells, sights, and sounds it can access while others cannot — meaning creatures sharing the same physical environment can have radically different experiences of it.
“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”
- Darwin was wrong to treat the human eye as a perfect example animals evolve toward; there is no destination eye, and all eyes including intermediate forms exist because they are well suited to their owner's needs — a starfish's simple arm-tip eyes suffice to find reef shelter, while an eagle needs eyes that spot prey from miles away, making the eye an example of evolution tuning senses to needs.
“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”
Animals deploy sensory organs and subsystems distributed across their bodies in ways driven by body size and ecological role.
- Jumping spiders have taken the division of visual labor to an extreme: the central pair of their four eye pairs handles sharp detail vision while the lateral pairs handle motion detection, and experiments blocking the lateral eyes show the spider cannot track a moving cricket until those eyes are unblocked, at which point the lateral eyes direct the central eyes where to look.
“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”
- Because insects are so small that food is something they land on and walk upon rather than ingest into a head, many insects from butterflies to flies have taste receptors on their feet, so a fly landing on an apple tastes that apple as it walks around it — a placement of sense organs driven by body size.
“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”
- Many animals use the medium they live in (air or water) to effectively touch things they are not physically touching: shorebirds detect buried objects beyond their bill's reach via deflected pressure waves, seals follow the turbulent trail a swimming fish leaves behind in water with their whiskers, blind fish school without collisions by sensing currents, and spiders detect the faint air currents of a fly precisely enough to leap and catch it.
“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”
- An octopus has most of its neurons in its arms rather than its head, giving the arms semi-autonomous agency to move and act independently, and the arms do taste and touch via suckers while the head does vision via the eyes, producing a creature that performs different kinds of sensing with different parts of a distributed nervous system.
“an octopus has a large nervous system, but most of that nervous system exists in its arms... those neurons allow the arms to work semi autonomously... the arms have taste and touch receptors on the suckers. And then the head does vision obviously with the two eyes”
- Most forms of touch, and by extension hearing, echolocation, the lateral line, and even the electric sense, fundamentally come down to small hair cells or structures that get deflected and a neural setup that detects the deflection; electroreceptors evolved from these same cell types, so touch, vibration, hearing, and electroreception can all be seen as extensions of the same basic mechanical sense.
“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”
Eyes and color vision evolved through stages matching organism needs, and visual sophistication varies dramatically across species.
- Eyes evolved through four well-accepted stages: (1) simple light-sensitive cells detecting only presence of light; (2) adding a shade or pigment spot that blocks light from one direction, enabling sensing of direction; (3) clustering many shaded cells to sense a low-resolution image; and (4) adding focusing elements like lenses to sharpen the image, and living animals exist at every point along this spectrum.
“first, you have simple light-sensitive cells that do nothing more than detect the presence of light... the next step up is also pretty simple. All you do is add some kind of shade... if you take a lot of those shaded cells and you put them together, then suddenly you have the ability to sense an image... the fourth stage is when you add like focusing elements”
- Despite having 12+ types of color-sensing cells, the mantis shrimp does not have superior color vision; physiological tests show it discriminates fewer colors than humans because it sends raw signals from its 12 color cells straight to the brain to compare against a lookup table rather than performing the opponent processing (adding and subtracting cone signals) that lets humans see millions of colors.
“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”
- Adding cone types multiplies discriminable colors multiplicatively, so a bird with a fourth color-sensitive cone (sensitive into ultraviolet) does not simply extend the spectrum at its margins but gains a whole additional dimension of color we lack, which is why we can recolor parts of our world to show where a bird sees a color but cannot fully represent a bird's color experience — four into three won't go.
“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”
Human-caused light and noise pollution breaks evolutionary cycles and harms wildlife, yet remains easily reversible.
- Smell is so fundamental to being a dog that dogs allowed and encouraged to sniff end up happier, less anxious, and more optimistic; humans who hurry dogs along on walks and prevent sniffing deny them their umwelt, and giving a dog full agency on a walk reveals it chooses to sniff everything intensely.
“There are studies by people like Alexandra Horowitz... 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”
- Scientists distinguish nociception — the detection of a harmful stimulus, as when a finger recoils from a hot pan before you realize what happened — from pain, the emotional suffering and continued anguish that accompanies it; some people use this distinction to deny animals can experience pain (claiming they only do nociception), but evidence suggests fish and cephalopods can experience pain, even if of a different kind than ours.
“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... Pain is the suffering”
- Because echolocation works by sending out a call and hearing each rebounding echo, it should in principle produce a stroboscopic series of static snapshots like film frames; humans presumably knit movie frames into smooth motion, and a bat — being a mammal with a not-dissimilar brain — likely has the processing power to fuse its echo snapshots into a cohesive moving world too, though this remains an educated guess.
“echolocation should be stroboscopic. You know, the bat is putting out a 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”
- For humans, taste comes too late as a warning because food is already in the mouth, but for animals with external chemical senses (like a fly that can simply take off, or a mosquito that lands on a DEET-covered arm, tastes something foul, and departs) taste provides useful protective information before ingestion.
“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”
- When an animal reacts to something a human cannot sense (a vibration, a high-pitched noise, a smell drifting through cracks), people unfamiliar with the idea that animals occupy different sensory worlds are inclined to attribute it to the supernatural; understanding umwelt reframes such episodes as answerable empirical questions about what the animal might be sensing.
“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”
- Humans flood the environment with light and noise, breaking a several-billion-year cycle of light and dark and drowning out animals' alarm calls and mating songs, harming wildlife and disconnecting us from nature and the cosmos; unlike plastics or DDT which persist for centuries, light and noise pollution can often be fixed by simply flipping a switch, making them an easy ecological win.
“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... We flood the world with noise, we drown out alarm calls and mating songs”
Most mechanically distinct senses reduce to hair cells detecting deflection, making touch the fundamental sense underlying hearing and electroreception.
- For electric fish, the same electric pulses serve both to perceive the environment (sensing how the self-generated field is distorted by objects) and to communicate with other fish, so the line between perception and communication is blurry — when a losing fish ceases its electric field to signal submission, it simultaneously becomes oblivious to its surroundings, as if backing out of a fight while also covering its own eyes and ears.
“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”