51 pages • 1 hour read
Ed YongAn Immense World: How Animal Senses Reveal the Hidden Realms Around Us
Nonfiction | Book | Adult | Published in 2022A modern alternative to SparkNotes and CliffsNotes, SuperSummary offers high-quality Study Guides with detailed chapter summaries and analysis of major themes, characters, and more.
Chapters 10-11Chapter Summaries & Analyses
Chapter 10 Summary: “Living Batteries: Electric Fields”
Many fish produce their own electricity, and the first batteries were inspired by electric fish. Electric eels produce electricity and have large electric organs. They send energy out into the world, which causes the muscles of other animals to twitch, revealing their location. The eel then sends stronger pulses out that paralyze its prey.
Most fish, however, are “weakly electric” and do not use electricity for hunting or for defense; these fish perplexed Charles Darwin. It is now widely believed that weakly electric fish use their electric fields as other animals use their senses, to navigate and to communicate, electrolocating as bats echolocate. Their electroreceptors can detect the difference between conductors, such as flesh, and insulators, such as rocks. Black ghost knifefish have electroreceptors all over their bodies; just as humans create images as a result of light falling on their retinas, electric fish “see” by perceiving the varying voltages that flash onto their skin, with conductors shining and insulators casting shadows.
Active electrolocation, like echolocation, takes a lot of effort. The stimulus must be created, not simply received. Unlike echolocation, however, electric fields do not move. This means that when electric organs are fired, electric fields immediately occur. There is no need to wait, for example, for a returning echo. Electrolocation works only at small distances, but it is panoramic, like the human sense of touch: Humans can sense touch all over the body, and electric fish can sense electricity all over their own bodies. This electric sense evolved from the lateral line but differs from it in that electric fields aren’t overwhelmed by turbulence and do not require any movement to perceive the world.
Electric fields are also used for communication among electric fish. Since electric fields don’t travel, they simply exist in the space between electric fish. In fact, electric fish “shine” for other electric fish in the perceptual field, well beyond the short distances in which the electric field yields information about other creatures.
Sharks’ and rays’ electric sense is a passive one called passive electroreception. They do not actively create electric fields, but they are very good at detecting the electric fields of others. Yong notes that all animals produce some amount of electricity. While hunting, sharks use their sense of smell at very long distances. Once they get closer, they rely on their vision, then their lateral line, and then their electric sense.
Air is an insulator and does not conduct electricity nearly so easily as water does, so it would be reasonable to assume that the electric sense lies only in marine animals. However, the air has a voltage of 100 volts for every meter up from the ground, and bees have electroreceptors in their hairs. When the positive charge of a bee meets the negative charge of a plant, pollen quickly transfers from plant to bee, even before the bee lands. This raises questions about electroreception in the air and whether flowers evolved to produce electric patterns.
Yong concludes his observations on animals’ electricity by noting that spiders travel long distances by “ballooning,” which was once thought to depend solely on the wind. Recently, scientists found that electricity launches the spiders into the air.
Chapter 11 Summary: “They Know the Way: Magnetic Fields”
Earth has a magnetic field that is always present and is constant. The core of the planet is a huge magnet that is made of solid iron and surrounded by molten iron and nickel. Humans use compasses to navigate it, but other animals sense the magnetic field without such accessories. Unlike the other senses, humans do not know how the electromagnetic field is sensed. Yong notes that this is the sense about which the least is known.
Many birds demonstrate zugunruhe, German for “migration anxiety,” when they are caged and not allowed to migrate. In captivity, they hop toward the direction in which they would ordinarily migrate, regardless of whether they are exposed to light. This appears to be a case of magnetoreception across many species. This sense is not used for communication, and the only magnetic field to which animals respond is the earth’s.
Turtles have two magnetic senses. They have an internal compass, and they also have a second magnetic sense that “reads” the geomagnetic field through inclination—the angle formed when geomagnetic lines meet on the ground—and intensity—differences in geomagnetic strength. Inclination and intensity work similarly to longitude and latitude.
It is not usually difficult to locate sense organs. However, magnetic fields can pass through any biological matter, so magnetoreceptors could be anywhere on the body. This is the only sense for which researchers have been unable to locate the sensors.
Chapters 10-11 Analysis
Electroreception and magnetoreception are both senses that humans lack, and they are some of the least understood senses. Structurally, the book proceeds from human senses and the senses of those animals with which people are most familiar to animals with which they may be unfamiliar. In Chapters 10 and 11, it continues this trajectory of highlighting difference as it explores sense organs that cannot be located and senses that humans do not possess.
The movement out from the human self to animal selves that are more distant from human experience or dissimilar to humans concludes Yong’s exploration of different stimuli and senses. Through this progression, the author also urges his audience to imagine the umwelten of species who perceive the world in ways that differ most dramatically from human experience. The discussion of electricity and geomagnetic fields places the most challenging scientific material late in the text, which supports its purpose as a science text written for a broad audience. Throughout the book, Yong consistently increases the level of scientific understanding necessary to understand his material. This allows readers to become accustomed to his writing style and to his approach to sensory material by processing the content that is likely to be less challenging first.
