An Immense World (young Readers Edition)

Ed Yong explores how animals perceive the world through senses radically different from human experience, building a case that every creature inhabits its own unique sensory reality.

Yong opens by asking readers to imagine a school gym shared by a human, an elephant, a mouse, a robin, a rattlesnake, an owl, a bat, a spider, and a mosquito. Each animal perceives the same space differently: the mosquito smells skin chemicals, the bat hears ultrasonic squeaks, the elephant produces rumbles too low for human ears, and the robin sees ultraviolet colors people cannot detect. To frame these differences, Yong introduces the Umwelt, a term used by German zoologist Jakob von Uexküll in 1909 to describe the particular slice of the environment an animal can sense and experience. Human senses are also limited, Yong argues, but our Umwelt feels complete because it is all we know.

The first section covers smell and taste. Yong uses his corgi, Typo, to illustrate how dogs experience the world through their noses: a dog's airstream splits so that scented molecules enter a dedicated smell chamber, and the nostrils create swirls that pull in fresh odors continuously. Dogs can distinguish identical twins by smell and detect bombs, tumors, and many other targets. Ants coordinate colonies through pheromones, smelly chemicals detected by antennae; when scientist Daniel Kronauer removed a smell gene from ants, they abandoned trails, ignored grubs, and wandered from their colony. A snake's forked tongue collects odor molecules from two spatial points and delivers them to the vomeronasal organ, a paired smell-sensing structure connected to the brain, allowing the snake to steer toward prey.

Yong turns to vision, which varies enormously across species. Measured in cycles per degree, a unit of visual sharpness, humans score 60 to 70, wedge-tailed eagles hold the record at 138, and lions score only 13, near the threshold of human legal blindness. Scientists Amanda Melin and Tim Caro showed that lions can see zebra stripes only from about 90 yards away, proving stripes serve not as camouflage but as a deterrent to bloodsucking flies. Giant squids evolved soccer-ball-sized eyes likely used to spot the bioluminescent shimmer of charging sperm whales, their primary predator.

The color chapter explains that color is created by the brain when neurons compare signals from cone cells. Most mammals are dichromats with two cone types, but tens of millions of years ago some African monkeys evolved a third cone, becoming trichromats, an inheritance humans share. Ultraviolet (UV) vision is the norm among color-seeing animals: Flowers display dramatic UV bullseye patterns visible to bees, and most birds are tetrachromats with four cone types, perceiving nonspectral color combinations—colors produced by mixing non-adjacent wavelengths, which do not appear in the standard rainbow—invisible to humans. The mantis shrimp has 12 photoreceptor types yet proved poor at distinguishing similar colors in experiments, likely collapsing the spectrum into roughly 12 categories. Yong closes the chapter with the principle that eyes define nature's palette: Flower colors evolved to match pre-existing bee vision, not the reverse.

The chapter on pain distinguishes nociception, the reflexive detection of harmful stimuli, from pain, the conscious suffering that follows. Fish pain research sparked debate when 2003 experiments showed trout exhibiting prolonged distress after acid injections, and later studies found zebrafish choosing a boring tank dissolved with painkillers over an enriched one. Neuroscientist Robyn Crook revealed that a squid's entire body becomes sore after injury, while an octopus localizes pain and cradles injured arms, showing that even closely related animals experience pain differently. Yong argues the productive question is not simply whether animals feel pain but how and when pain provides an advantage.

The temperature chapter describes how animals sense heat through TRP channel proteins that open at species-specific thresholds. The thirteen-lined ground squirrel tolerates temperatures from 39°F during hibernation to 130°F in summer, and the Saharan silver ant forages on sand reaching 127°F. Fire-chaser beetles detect distant forest fires via infrared-sensitive organs, and rattlesnakes have pit organs that function like simple infrared eyes, allowing them to strike prey in complete darkness.

The touch chapter reveals that the star-nosed mole navigates dark tunnels by tapping mechanoreceptor-packed rays on its nose against walls a dozen times per second. Harbor seals track the swirling water trails left by fish from nearly 200 yards away using their whiskers. Fish use lateral line sensors to detect water flow, enabling schools to part around a lunging predator. The tiger wandering spider detects the breeze ahead of a passing fly using hairs called trichobothria, striking once the fly comes within 1.5 inches.

A chapter on surface vibrations reveals a sensory world distinct from airborne sound. Scientist Rex Cocroft converted treehopper vibrations into audible sound, revealing deep, lion-like purrs from tiny insects. Elephant researcher Caitlin O'Connell confirmed that elephants sense seismic signals through their feet by playing alarm calls through ground-buried shakers and watching herds freeze defensively. Orb-weaving spiders extend their senses through their webs, tuning silk tension to detect different prey; a well-fed spider placed on a hungry spider's tense web hunts prey it would normally ignore, suggesting the web functions as an extension of cognition.

The hearing chapter traces scientist Roger Payne's barn owl experiments, which proved owls hunt by sound alone in complete darkness. Many insects evolved ears to detect predators, with ears appearing on butterfly wings, cricket knees, and the center of a praying mantis's chest. Infrasound, sound at frequencies below human hearing, plays a vital role for large animals: Fin whale calls can theoretically travel 13,000 miles underwater. Researcher Katy Payne, who studied elephant communication, discovered that elephants converse infrasonically through concrete walls.

Echolocation receives its own chapter. Bats produce calls reaching 138 decibels and detect echo delays of millionths of a second. Moths fight back: Body scales absorb sonar, tiger moths produce jamming clicks, and luna moths trail long tails that generate distracting echoes. Dolphins echolocate over greater ranges because sound travels faster in water, and their sonar penetrates flesh to reveal internal structures like bones and swim bladders. Daniel Kish, who lost both eyes to cancer before age two, demonstrates human echolocation by clicking his tongue and navigating by echo. Kish founded the nonprofit World Access for the Blind in 2000 to teach thousands of people who are blind to echolocate.

The chapter on electric and magnetic fields describes how roughly 350 fish species generate electricity using specialized cells called electrocytes. Most use weak fields for active electrolocation, sensing how objects alter their self-generated field. Sharks detect the faint electric fields all living things produce, and bumblebees sense the electric halos around flowers using their body hairs. The chapter then covers magnetoreception: European robins placed in cages oriented toward their migratory destination even with all landmarks hidden, and researcher Ken Lohmann proved that baby loggerhead sea turtles possess both a compass sense and a magnetic map sense. Magnetoreception remains the only known sense without a confirmed sensor organ, since magnetic fields pass through all body tissues.

The penultimate chapter argues that senses must be understood as integrated wholes. Mosquitoes combine carbon dioxide detection, heat sensing, odor, vision, and taste to find hosts, and these senses interact: Heat attracts only after carbon dioxide is detected. The octopus embodies this complexity, with two-thirds of its 500 million neurons residing in its eight semi-autonomous arms, which operate in a world of taste and touch while the head focuses on vision.

The final chapter warns that human-generated light and noise pollution degrade animals' sensory environments. Sea turtle hatchlings are lured from the ocean by lit roads, millions of migrating birds die from tower collisions, and ship noise drowns out sounds animals rely on for hunting and mating. Marine biologist Tim Lamont demonstrated that playing healthy-reef recordings over bleached coral doubled fish populations in 40 days. Yong emphasizes that these are rare problems with immediate solutions: The COVID-19 lockdowns of 2020 halved night-sky brightness near Berlin and ocean noise in Alaska's Glacier Bay.

The book returns to the imaginary school gym. Although humans lack ultraviolet vision, magnetoreception, echolocation, and infrared senses, Yong argues, we are the only creatures who know these senses exist, can imagine what they feel like, and can choose to protect the sensory worlds of other animals.