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Every animal has its own sensory world, its own thin slice of the fullness of reality that it can detect. Evolution has shaped the senses of animals according to their needs, but no animal can sense everything. No animal is perfect at everything. There is so much information out there that to be able to detect it all would be an overwhelming experience and also unnecessary. There's also a cost to the senses. Senses don't come for free. 

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To build a sense organ and to maintain all the neurons that feed into that sense organ takes up a lot of energy, which is why their senses are so refined and so constrained by their evolutionary needs. So the word umwelt was popularized and defined by a German zoologist named Jakob von Uexküll in the early 20th century. It comes from the German word for environment, but he meant the animals' sensory environment. And that's the specific set of sights, smells, textures, and sounds that that animal has access to and that another animal might not. When you really think about the senses, you do start to understand the very different kinds of information that those senses offer their owners. 

So we obviously taste with our tongues, but a catfish is essentially a swimming tongue. It has taste buds all over its skin. If you put little pieces of food near the flank of a catfish, it will be able to taste it and turn around and snap it up. For most animals, taste is about food. It's about trying to work out whether something is worth eating or not. And for humans, food is something that we put in our mouths. But if you are a very small animal, food can be something you land on. 

And which is why for many insects taste buds are some things that are found in their feet as well as in their mouths. A fly landing on the apple that you are trying to eat can taste it just by walking on it before you put it in your mouth. I, like most of you, have two eyes. They sit in the front of my face and they point forwards, which means that my visual world is always in front of me and I walk into it. But most birds have eyes on the sides of their heads, which means their visual world is around them. They often have close to wraparound vision, seeing to the sides and also a little bit to the back. And that kind of wraparound vision is really hard to wrap your head around. And then of course there are changes that can occur over an animal's lifetime. So the umwelts of an adult might be different to the umwelts of a juvenile. 

Jumping spiders are very driven by vision. They have excellent eyes. But those eyes also become more sensitive as they get older, more sensitive to light, which means that I think the world of a jumping spider will get brighter as it ages. One scientist describe this to me as a jumping spider watching the sun rise as it gets older. So a sea otter has very sensitive paws. They don't look very sensitive. They look like these weird sort of cauliflowery mittens. They have a sensitive touch that is equal to our exquisitely sensitive fingertips. One of the key differences is that they are also extremely skilled at using that sense of touch. They're very fast about making touch-based decisions. 

A sea otter will dive down into the ocean and very quickly root around with its paws. It will grab that sea urchin, yank that clam, and then rise to the surface before eating its food. A sea otter doesn't have the benefit of blubber that a whale or a walrus might have. It has very thick fur, but it can find enough food to eat because it has not only very sensitive hands, but very fast hands too. Even in a completely dark room where the very large eyes of an owl might not be of much use, they can still hear and they hear really well. The dish of feathers around an owl's face that gives it that distinctive owl-y look acts as a radar dish funneling sound towards its ears. 

Those ears are incredibly sensitive, but they also have a unique trick that allows the owl to work out exactly where sound is coming from. Based on when sound arrives in my ears, where those first arrives at the left or the right, I can tell where a sound is coming from in the horizontal plane. I can't do that trick in the vertical very well because my ears are level with each other so sound arrives at both of them from above or below at the same time. An owl solves this problem because its ears are offset. So they're asymmetrical. So one ear is slightly higher than the other. And when sound arrives at that ear first, the owl knows where in the vertical plane it's target is. And that's why an owl in the dark can land exactly on a mouse. It's why owls in the wild can bust through snow to pick up scurrying rodents that they couldn't even see. 

One of the primary uses of scent in the animal kingdom is for navigation, for finding your way around a landscape. You know, my dog, Typo, absolutely can do this. He knows where we are by cross-referencing his memories of the smells of the neighborhood against what he's smelling at any given moment. But there are other animals that use scent for navigation in even more extraordinary ways. A lot of sea birds, the group known as tubenoses, use the odorscapes of the ocean to find food. The ocean looks featureless to us, right? We can glide over it and just see this endless expanse of uniform blue, but it's not featureless to an albatross. 

Underwater features like mountains and valleys leads to concentrations of nutrients, which then concentrate food, plankton, and then krill, the kinds of things that a seabird might eat. And so the ocean has this undulating odorscape: odors that reveal the concentration of possible food and then areas of no scent that reveal scarcity in the deep. Elephants can do this too. Elephants can navigate over long distances. Obviously, they have that trunk. 

They have constantly scanning about with this extremely elongated nose. You know, they'll react to the imminent arrival of rain. People have suggested that they can find buried sources of water by smelling it. It's quite difficult to understand exactly how elephants smell because they are large, intelligent animals that are difficult to work with. Part of this relies on us using our imaginations like watching their incredible behavior, looking at their trunk, and trying to just make educated guesses about what their olfactory world might be like. 

There's a wonderful quote by Marcel Proust that I think captures what's magical about the umwelt concept. He said that the only true voyage would be not to visit strange lands, but to possess other eyes to see the hundreds of universes that each of them sees. That's how I think about the sensory worlds of other animals. I think that when I get to empathize with the smell world of a dog or the touch world of a sea otter, I feel like I'm traveling, like I'm leaving the confines of my own body and my own lived experience and going on this fantastical voyage into the world of another creature.

Ed Yong is a Pulitzer Prize–winning science writer on the staff of The Atlantic, where he also won the George Polk Award for science reporting, among other honors. His first book, I Contain Multitudes, was a New York Times bestseller and won numerous awards. His work has appeared in The New Yorker, National Geographic, Wired, The New York Times, Scientific American, and more. He lives in Washington, D.C., with his wife, Liz Neeley, and their corgi, Typo.

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