An Immense World: How Animal Senses Reveal the Hidden Realms Around Us

by Ed Yong

A scientist’s explanations about other animals are dictated by the data she collects, which are influenced by the questions she asks, which are steered by her imagination, which is delimited by her senses. The boundaries of the human Umwelt often make the Umwelten of others opaque to us.

Leafcutter ants are so sensitive to their trail pheromone that a milligram is enough to lay a path around the planet three times over.

When African elephants reunite after a prolonged separation, they go through intense greeting rituals. Human observers can see their flapping ears and hear their throaty rumbles, but for the elephants themselves, the experience must also be olfactory pandemonium. They vigorously urinate and defecate, while aromatic liquid pours forth from glands behind their eyes, filling the air around them with scents.

In 2007, Lucy Bates found a clever way of testing this idea. She followed family groups of elephants and waited for one to urinate. Once the herd had left, she drove over, scooped up the urine-soaked soil with a trowel,

We use the phrase “bird’s-eye view” to mean any vista seen from on high. But a bird’s view is not just an elevated version of a human one. “The human visual world is in front and humans move into it,” Martin once wrote. But “the avian world is around and birds move through

For killer flies, speed is everything. “Their prey can come from anywhere, and the Mediterranean is so dry that it’s rare for them to have prey,” Gonzalez-Bellido says. They immediately take off after anything that could conceivably be a meal and, once airborne, catch their prey as quickly as possible so that they themselves aren’t cannibalized by others of their kind. Their chases are near impossible for even well-trained human eyes to follow. By filming these pursuits with high-speed cameras, Gonzalez-Bellido showed that they typically take a quarter of a second. They might even be over in half that time.

Humans are insensitive to nepetalactone, a chemical produced by the catnip plant that is intensely irritating to mosquitoes.

Its larvae abound in soil and water that are contaminated by feces. If an unlucky person stands or walks through such places, the worms swim toward them and penetrate their skin. Threadworms, along with hookworms and other skin-penetrating nematodes, infect around 800 million people around the world, from Vietnam to Alabama.

The emerald jewel wasp also has a long, probing organ with a touch-sensitive tip, but its goals and methods are far grislier than a red knot’s. The wasp—a beautiful inch-long creature with a metallic green body and orange thighs—is a parasite that raises its young on cockroaches. When a female finds a roach, she stings it twice—once in its midsection to temporarily paralyze its legs, and a second time in its brain. The second

cicadas, crickets, katydids, and more, Cocroft estimates that around 200,000 species of insects communicate through surface vibrations. Their songs aren’t normally audible, and so most people are completely unaware that they exist. Those who become aware often get hooked.

And everyone who can hear does so partly through bone conduction, which is why people often think they sound strange on recordings. Those recordings reproduce the airborne components of our voices, but not the vibrations traveling through our skulls.

Human hearing typically bottoms out at around 20 Hz. Below those frequencies, sounds are known as infrasound, and they’re mostly inaudible to us unless they’re very loud. Infrasounds can travel over incredibly long distances, especially in water.[*17] Knowing that fin whales also produce infrasound, Payne calculated, to his shock, that their calls could conceivably travel for 13,000 miles. No ocean is that wide.

Clark calculated that one individual was 1,500 miles from the sensor that recorded it. He could listen to whales singing in Ireland with a microphone situated off Bermuda.

They’d noticed that elephant families would often move in the same directions for weeks at a time, even though they were separated by several miles. In the early evenings, different groups would also converge on the same waterholes at the same time, but from different directions. Infrasound carries over long distances, even in air, and if elephants use it to communicate, that would explain how they can synchronize their movements across a savannah.

Some moths do make ultrasonic courtship calls. Males will follow a female’s pheromone trail, land next to her, and vibrate their wings to produce a volley of ultrasound. These calls are very quiet, almost like whispers. Like other ultrasonic communicators, these moths are probably making use of ultrasound’s limited range so that they’ll be heard by a prospective mate sitting nearby, but not by a hungry bat flying overhead. But unlike most songs, ultrasonic or otherwise, these calls aren’t meant to be attractive. They’re meant to sound dangerous. They mimic the calls of bats, prompting the females to freeze and allowing the males to mate more easily.

The third problem is one of speed. Every echo provides a snapshot. Bats fly so quickly that they must update those snapshots regularly to detect fast-approaching obstacles or fast-escaping prey. John Ratcliffe showed that they do so with vocal muscles that can contract up to 200 times a second—the fastest speeds of any mammalian muscle.[*5] Those muscles don’t always contract so quickly. But in the final moments of a hunt, when bats are bearing down upon their targets and need to sense every dodge and dive, they produce as many pulses as their superfast muscles will allow. This is the so-called terminal buzz. It is what Griffin first heard at his Ithaca pond. It is the sound of a bat sensing its prey as sharply as possible, and of an insect likely losing its life.

Around half of hawkmoths—another major group of around 1,500 species—can also jam bats. But unlike the tiger moths, hawkmoths produce their confounding clicks by rubbing their genitals together. They seem to have evolved this ability on three separate occasions, with each group repurposing a different section of their sex organs into bat-befuddling instruments. But bats, in turn, have evolved counters to moth defenses. At least two species—the barbastelle of Europe and the Townsend’s big-eared bat of North America—make very quiet calls that allow them to sneak up on moths unnoticed. With their stealthy whispers, they can get so close that their prey don’t have time to either dodge or jam.

None do this better than electric eels. Their electric organs take up most of their 7-foot-long bodies, and contain around 100 stacks of between 5,000 and 10,000 electrocytes. The most powerful of the three electric eel species can discharge 860 volts—enough to incapacitate a horse.[*2]

Over mile distances, a shark sniffs out its food. As it draws near, vision takes over. Nearer still, the lateral line chips in. Its electric sense only enters the fray at the close of the hunt, to pinpoint the exact position of its prey and guide its strike. That’s why the ampullae of Lorenzini are usually concentrated around the mouth.[*24]

When the time comes for birds to migrate, they become visibly restless. Even in captivity, they’ll hop, flit, and flutter. These frantic movements are known as Zugunruhe—a German word that means “migration anxiety.” The birds know it’s time. They long to get going.

A striking pattern emerged: On days with the most intense solar storms, gray whales were four times more likely to beach themselves.[*5]

When an animal decides to move, its nervous system issues a motor command—a set of neural signals that tell its muscles what to do. But on its way to the muscles, this command is duplicated. The copy heads to the sensory systems, which use it to simulate the consequences of the intended movement. When the movement actually occurs, the senses have already predicted the self-produced signals that they are about to experience. And by comparing that prediction against reality, they can work out which signals are actually coming from the outside world and react to them appropriately.[*5]

Whenever an animal moves, it unconsciously creates a mirror version of its own will, which it uses to predict the sensory consequences of its actions. With every action, the senses are forewarned about what to expect and can prepare themselves accordingly.

Over a third of humanity, and almost 80 percent of North Americans, can no longer see the Milky Way.

“If I said that I’m going to increase the noise level in your office by 30 decibels, OSHA would come in and say you’d need to wear earplugs,” Hildebrand tells me. “We’re conducting an experiment on marine animals by exposing them to these high levels of noise, and it’s not an experiment we’d allow to be conducted on ourselves.”