Marcel Proust once said, "The only true voyage ... would be not to visit strange lands but to possess other eyes ... to see the hundred universes that each of them sees." An Immense World by Pulitzer Prize-winning science journalist Ed Yong is a travelog of "the true voyage" that Proust describes. The book contemplates the lives of our often neglected fellow companions on Earth: animals. Yong gathers anecdotes and knowledge from experts in the field to reveal how other animals encounter the world through their own senses. The journey is full of surprises that invite us to think differently about animals, step into their experiences, and allow their perceptions of the world to fill us with wonder. By treading the edge of human perception, Yong helps us discover newfound solidarity with other creatures—a solidarity not founded on superiority, but on a profound appreciation and advocacy for animal diversity.

In June 1998, Mike Ryan and his former student Rex Cocroft wandered through the thick Panamanian rainforest searching for treehoppers (a sap-sucking insect that creates vibrations by contracting its abdomen). After flipping through some leaves, they spotted a treehopper mother encircled by her babies. Cocroft placed a simple microphone on the plant’s stem and handed Ryan the headphones. Then he gently jerked the leaf. As the insects began to flee, Ryan expected their abdomen vibrations to sound like a "scurrying noise." Instead, he heard mooing cows. When the baby treehoppers regrouped with their mother, their mooing became a synchronized chorus. Ryan removed his headphones and was transported to an entirely different universe. He could hear birds, monkeys, and insects chirping. He looked down at the treehoppers, and they were quietly sitting on the leaf. "It was the coolest experience," he said. "It was sensory travel." 

Ryan experienced what the Baltic-German zoologist Jakob von Uexküll labeled Umwelt: the limited parts of the environment that an animal perceives through its senses. Reality is made up of numerous sights, smells, textures, sounds, electric fields, and magnetic fields. If an animal could sense all of them, it would be overwhelmed by stimuli and unable to function. Umwelt limits an animal's perception of the world and only reveals those parts that help it perceive food, shelter, threats, and mates. A tick, for instance, cannot see the color of a tree, of snow, or of the sky. Its Umwelt can only sense body heat, the touch of hair, and the smell of butyric acid that emanates from skin, since that is all it needs to find mammalian blood. 

Hamlet famously tells Horatio, "there are more things in Heaven and Earth . . . than are dreamt of in your philosophy." Though his statement refers to supernatural realities, this is true of the natural world too. As humans, we easily forget that our experience of reality is not all-encompassing. We look at animals as mere machines to make use of and categorize, but they are agents with peculiar and extraordinary inner worlds.

There is no need to pity the goldfish in its bowl. We, humans, are also trapped in our own perceptual bowls. We spend a lot of time wondering about alien life without realizing there are multiple interpretations and experiences of the world happening all around us. All species share the illusion that all they sense is all reality has to offer. What happens outside our senses, in a sense, doesn't exist to us.

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When we think of animals skilled at smelling, most people think of dogs or elephants. However, the sense of smell is so diverse and unquantifiable that rating one animal's ability to smell as superior to another’s is impossible. When thinking about other animal senses, asking the right question is important. Instead of thinking in terms of superiority we should ask: "How important is smell to that animal?" or "What does it use its sense of smell for?" 

Ants might be one the most dramatic examples of creatures that depend almost entirely on smell. Their antennae collect smelly substances from the environment called pheromones, enabling them to perform a wide range of individual and collective activities. 

Ants also emit a variety of pheromones, all with different weights. The lighter the pheromone, the faster it travels in the air. If you crush the head of an ant, light-weight pheromones will quickly send an alarm to the whole colony, and they will all immediately charge into battle. The heavier pheromones sit in the ants' bodies and can only be picked up by other ants' antennae. Ants will tap other ants to identify whether they are fellow citizens or threats. Queens use these heavier substances to stop workers from breeding or to mark unruly members of the colony as punishment for challenging authority. 

Many experiments have demonstrated ants' dependence on smell. One experiment involved smearing the pheromones of a dead ant on the body of a living ant. When the scientists reintroduced the marked ant to its colony, the other ants carried its body to the colony’s disposal area. It did not matter that the ant was jerking or moving. It smelled dead, so it was treated as dead. In another  experiment, researchers created a trail of pheromone that looped back on itself, forming a circle. The result: ants followed the trail in a spiral despite exhaustion until they died. 

Ants may be unable to smell tumors, bombs, missing people, diseases, low blood sugar, or oil pipeline leaks like dogs can. But their antennae allow different ant species to achieve equally fascinating feats. Antennae allow army ants to rise above individuality and join together to create a predator superorganism. Argentine ants form mega colonies connected through mile-long trails, and leaf-cutter ants garden their own fungi. All animals, whether deliberately or inadvertently, use smell to send messages and communicate with the world in vastly different ways.

As humans, we have evolved to see more detail than almost every animal. However, because we rely so heavily on vision, we often fall into perceptual bias when thinking about other animals' Umwelt. 

For example, one of the most prominent hypotheses about zebras is that they evolved black and white patterns to camouflage in a herd. The stripes were thought to allow a zebra to lose its individual outline and blend with the rest, messing with the vision of predators like lions and hyenas. In 2012, however, Amanda Melin, a scientist who studies vision, quickly shot down the hypothesis. Most carnivores do not have the same visual acuity as humans. If they are hunting at dawn or dusk, which most are, they would definitely not be able to see the stripes. Predators would only see the stripes at close range, but by the time they are that close they can already smell the zebra. 

Time after time, we have examined other animals through the wrong eyes—ours. 

But there are no norms or standards when it comes to eyes. There are as many variations of eyes as there are animals. Creatures could have one eye or hundreds of them. Their eyes could be as small as an amoeba's nucleus—like the fairy wasp’s—or as big as a soccer ball—like the giant squid’s. Animals can have eyes attached to their mouths, arms, and armor. Some can see in the dark or in two directions at once. Flies register events in slow motion, whereas turtles see the world in time-lapse. Jumping spiders actually become more sensitive to light as they age. For them, getting older is "like watching the sun rising." Mallard ducks can see in front, behind, and above their heads at the same time. Unlike humans who move forward into the world, a mallard simultaneously moves toward and away from the world. 

We like to use the phrase "a birds-eye-view." What we mean when we say that is what our eyes would see when placed on a bird's head. But animal vision is much more than just a variation on human vision.

Humans have long admired the melodies of birds. Some birds can sing in completely synchronized duets. Others, such as zebra finches, learn their songs by listening to each other. Whip-poor-wills sing so fast that a human ear can only pick 3 of the 5 notes their melodies comprise. On the other hand, a mockingbird can mimic all five notes exactly, as well as a few hundred other songs. 

That's just the beginning. A zebra finch's song is composed of syllables that it always sings in the same sequence. For simplicity, let's say the syllables are organized as A-B-C-D. Zebra finches learn this song in their youth. When scientists have shuffled the sequence, however, it does not seem to matter to them. Whether you play A-B-C-D or C-D-C-B-A, as long as the composition contains all the syllables, the zebra finches will recognize both as their own song. For humans, this behavior would be like paying attention to (and making sense of) the vowels of a conversation rather than the order of the words. Zebra finches only care about the details.

Furthermore, zebra finches' songs are so complex that they may represent an entire language. One experiment swapped syllable B from one recording to syllable B from another recording. The birds identified that something had changed. Their ears can hear extremely fine acoustic structures that sound the same to humans. Their responses to subtle changes could have infinite variations and meanings that we could never grasp. After all, zebra finches sing to meet lifelong partners, find each other when they are apart, keep each other on track when traveling, and organize parenting duties. Conceivably they could be sharing information about sex, health, and identity, which could all be encrypted in the tiny details of their songs. 

Without a bird's brain and ear, we will never fully know the meaning of a birdsong. To bypass the communication barrier we would have to bypass the barrier of our senses, our Umwelt. Animals do not just communicate in ways we cannot understand—they communicate in ways we cannot perceive.

Every year on September 11, the New York City sky is lit by two columns of 88 xenon bulbs with 7,000-watt intensities. The annual installation is called Tribute in Light, commemorating the terrorist attacks of 2001. These bright blue lights divert around 1.1 million migrating birds. Because migrations require debilitating amounts of energy, a slight detour can kill many birds. But the Tribute in Light happens just one night of the year. Many more birds die annually in the United States due to similar man-made diversions. Communication towers kill 7 million birds each year. 

Because the planet is artificially brighter every year by 2 percent, we can hardly see the Milky Way anymore. Scientist Sonke Johnsen reasonably encapsulated the depth of this loss when he said, "The thought of light traveling billions of years from distant galaxies only to be washed out in the last billionth of a second by glow from the nearest strip mall depresses me to no end."

Humans have not only extinguished darkness with lights, but we have also drowned silence with noise. Nature depends not only on the voices and songs of animals but on the unintended sounds of their movements and interactions. Aircraft, roads, and the mining and oil industries have polluted these soundscapes. The impact on nature is detrimental. Great tit birds have been forced to sing at higher frequencies to bypass the noise of the cities. Nightingales in Berlin are now belting out their tunes more loudly. The noise has made it hard for birds to find mates and for owls to hear their prey. Prairie dogs are compelled to stay underground, parasitic Ormia flies struggle to find their hosts, and sage grouse abandon their breeding sites. 

Consider the shoes you are wearing. They were likely shipped from overseas on a cargo vessel reverberating a noise that spread for miles. The noise overpowers the songs of whales, the hum of toads, the grunts of cod, and the whistles of seals—which are all sounds that sustain the harmony of the animal world. Since WWII, global shipping has tripled and raised the levels of low-frequency noise by 32 times. 

Through centuries of work, humans have learned about the sensory experiences of other species. Despite this knowledge, we have continued to pollute their sensorial landscapes. Sensory pollution is one of the ecological sins that often gets ignored and contributes to this era of “biological annihilation,” as scientists have called it. Instead of stepping into other animals' Umwelt, we are forcing them into ours.

Valuing nature in so far as it represents something remote and otherworldly gives people the illusion that nature is separate from humanity. It makes us believe that nature is not all around us, that we do not exist within it. Consequently, we easily neglect it. Since the 18th century, thinkers romanticized only those landscapes that seemed to highlight their own mortality and connection with the divine. As environmental historian William Cronon noted, "God was on the mountaintop, in the chasm, in the waterfall, in the underworld, in the rainbow, in the sunset." In other words, the concept of wilderness became unjustly associated with grandeur. 

This reductionary bias is evident in the United States national parks, which exclusively protect environments that match at least one of Cronon's descriptions. Since very early on, Yellowstone, Yosemite, Grand Canyon, Rainier, and Zion have been revered as landscapes exemplary of nature’s best. But it wasn't until the 1940s that the park service recognized a swamp by establishing Everglades National Park. 

Understanding other animals' senses cultivates an understanding of nature’s value that goes beyond grandeur. By relearning our concept of natural beauty as something that is not distant, we will begin to see how we are corrupting nature and how we can preserve it. We will begin to see that the beauty of the natural world is more immense than the biases of our own Umwelt. It dwells in the unexpected universes of all other animals. 

Most pollutants are very hard to fix. Radioactive waste, pesticides, and plastics will keep hurting the environment for decades, even if we stop using them tomorrow. But we can stop sensory pollution immediately. We are deeply acquainted with how other animals perceive the world, and we also have the technology available to help them. We only lack awareness of many other animal worlds, how they work, and why they are essential. 

Changing sensory pollution is possible. We can restrict vehicles from entering important wilderness areas. We can dim lights, replace white lights with the more naturally hued red lights, and soften the traffic noise on land and water using sound-absorbing technologies. In a 2007 experiment, commercial ships in the Mediterranean slowed down by 12 percent and produced half of their regular noise. In another experiment, people placed signs at the Muir Woods National Monument in California, establishing quiet zones. Visitors were encouraged to turn off their phones and speak quietly. The results reduced overall noise in the park by the equivalent of 1,200 fewer visitors. 

Sensory pollution disconnects other animals and us from our surroundings and the cosmos, but there is hope. We can enter into the world of other animals and shelter their sensory experiences. Humans' most valuable sensory skill is our ability to investigate and care for other animals' sensory worlds. Nature has given us this gift—we must cherish it and use it productively.

These insights are just an introduction. If you're ready to dive deeper, pick up a copy of An Immense World here. And since we get a commission on every sale, your purchase will help keep this newsletter free.