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 7-9Chapter Summaries & Analyses
Chapter 7 Summary: “The Rippling Ground: Surface Vibrations”
Some might categorize surface vibrations as a form of sound, but airborne sound waves move in the direction of the force, whereas the movement of surface waves is perpendicular to the direction of the force. Throwing a stone in a lake illustrates this point: The stone sinks, but the waves move out across the water, perpendicular to the direction in which the stone fell. Surface waves move across both ground and water. The most important distinction that Yong makes between sound and surface waves is that scientists have ignored surface waves.
Plants easily carry surface waves, and around 200,000 insects communicate through vibrations that create surface waves on plants in “vibrational songs.” The vibrations can be converted to sound that is audible for humans with a vibrometer. Because surface waves travel only along flat planes, they do not lose their energy as quickly as sound waves do. Thus, they travel far, and these vibrational songs are not correlated to body size: Very small insects often sound very large when their songs are listened to with a vibrometer.
Listening to surface waves, for some animals, is as important as making surface waves. Scorpions, for example, have slits in their feet, called slit sensilla, that sense surface vibrations and allow them to find food. Scientists realized only recently that sand carries surface waves well.
Some worms can sense a digging mole underground through vibrations; thus, they will head to the top of the ground, where the mole does not try to catch them. Other predators of these worms, however, will shake the ground so that these moles rise to the surface and are then caught. Animals, then, tap into the umwelten of other animals.
Being able to sense seismic vibrations predates animals’ sense of sound. Three of the jawbones through which seismic vibrations are processed became the middle ear bones: the hammer, anvil, and stirrup. Many animals pay more attention to surface waves than to sound waves. Humans, like many animals, hear via the sensory organ of the ear as well as via bone conduction. This is why most people do not recognize their own voices when they hear them recorded, instead of filtered through their own bone conduction.
Many spiders create the flat, circular webs that are called orb webs. The orb web is used to trap insects. However, Yong insists that the web is not entirely separate from the spiders themselves. Like scorpions, spiders have slit sensilla that enable them to detect their prey, and they create webs through which they can sense their prey. For some spiders, their slit sensilla works in direct coordination with the webs they construct, making the web part of the spider’s sensory system. Most spiders with orb webs sit in the middle, with their legs on the radial spokes, through which they feel vibrations that they can differentiate. They can also pluck the silk thread to obtain more vibrational information when needed. The spider can adjust the silk by creating threads of different thicknesses, pulling the thread out of the body at different rates, and adjusting the web’s tension. Yong’s argument moves beyond the sensory, however. He asserts that the web “is not just an extension of a spider’s senses but an extension of its cognition. In a very real way, the spider thinks with its web. Tuning the silk is like tuning its own mind” (208).
Chapter 8 Summary: “All Ears: Sound”
Sound is produced by waves of pressure. As these waves travel, air molecules bunch up and then spread out in the same direction as the pressure waves, resulting in sound. The frequency of a sound is determined by how many times the molecules come together and disperse; this is pitch, which is measured in hertz. How far the molecules move determines loudness, or amplitude, which is measured in decibels. Hearing is a response to these molecules’ movements.
Both hearing and touch are mechanical senses: They both have receptors that function via mechanical stimulation. This is obvious with human touch. With hearing, sound waves reach the ear, moving the small hairs within it. Hearing, unlike touch, can happen across long distances.
An owl catches prey by hearing alone. The ear is composed of three parts: the outer, middle, and inner ear. The outer ear in an owl is essentially the owl’s entire face. Owls’ ears are asymmetric, which allows them to hear both horizontally and vertically. While humans generally hear something and then look to determine the source of the sound, owls simply hear to determine the source.
Hearing is useful, but it is not as essential as touch or nociception. Animals simply hear what they need to, and some animals do not need to hear at all. All mammals have ears, and they always have two located on the head. Most mammals have good hearing. Insects, however, have a wide variety of ears that appear just about anywhere on the body, but most insects probably do not hear.
Despite the fact that many animals do not have the sense of hearing, hearing can be useful in detecting predators. Animals that listen for predators often also communicate via hearing. In fact, the ears—and the sounds to which they are receptive—determine the sounds of communication. This is called “sensory exploitation.”
Birds’ hearing happens on a faster scale than human hearing. Yong discusses zebra finches, which have songs of several syllables that always follow the same sequence. When these syllables are switched around by humans and played back to the birds, the finches seem to be unable to detect the changes that are so obvious to human researchers. However, researchers found that the sequence of the syllables does not carry meaning for the finches; instead, they care about the individual notes. This is counterintuitive for humans, who identify birds through the sequences of syllables they repeat; thus, people have long assumed that the choreography of their songs is important to birds. For some birds, sequence of syllables does matter, but for other birds it does not. The songs probably sound entirely different to the birds themselves than they do to humans, and it remains impossible to fully know what meaning these sounds carry for them.
In the case of vision, as previously discussed, eyes can be exceptional either at resolution or at sensitivity, but they cannot excel at both. Similarly, ears can be exceptional either at temporal resolution or at pitch sensitivity, but they cannot excel at both. Some birds, such as chickadees, however, go back and forth between the two, depending on the season. Their calls and hearing change based on environmental and social conditions, and their ears change with these conditions. They may also hear their own songs differently, depending on the season and on their sex. In the case of house sparrows, both sexes hear the same in the fall, but in the spring the males and females hear differently. Thus, their “umwelten converge and diverge through the year” (230). Umwelten constantly change and are never static.
Sound below frequencies of 20 hertz is infrasound, which is generally inaudible to humans. Infrasound can travel very long distances, especially in water. Many whales call out in infrasound, with their calls traveling thousands of miles, potentially even across entire oceans. Whales may also use these infrasounds to map the ocean. This mapping may occur in the moment as a navigational tool, but whales may also revise maps based on sound memories they possess. Yong observes that this infrasound navigation is extremely complicated for humans to consider; it is hard to imagine the scale of a whale’s hearing, which involves coordinating its navigation with the time that an echo takes to return to it across very long distances. At the same time, whales’ heart rates can be as low as 2 beats per minute, and they probably operate on a very different time scale than that of humans. Elephants, too, produce sounds below human ranges of hearing.
Other animals produce sounds that are above human ranges of hearing. For example, many rodents communicate in ultrasound. They communicate during play and when stressed, and they seem to laugh when tickled. In general, the smaller the animal’s head, the higher the frequency it can hear; the larger the head, the lower the frequency.
High-frequency sounds, as opposed to low-frequency ones, do not travel far. This allows animals to target a specific audience and provides for “secret” communication that is almost akin to whispering. Thus, a baby mouse’s alert call does not attract predators but is heard by its mother.
Chapter 9 Summary: “A Silent World Shouts Back: Echoes”
Echolocation, a form of determining location via sound waves, puts energy into the world, rather than picking up on energy that is already there. This is different from vision, hearing, and touch. Echolocation requires that a stimulus—a call—be put out into the world and returned to the caller in the form of an echo that “maps” an object, revealing information. There are many challenges to echolocation, the most obvious of which is distance, as these waves lose energy quickly in air. Bats’ sonar calls are generally at high decibels that are beyond the capacity of human hearing. They can contract their middle ear muscles as they call and then restore full hearing a split second later for the echo. This prevents the bat from going deaf.
Bats must navigate many specific challenges. From an anthropocentric viewpoint, every echo provides a picture. As bats get closer to their prey, they increase their number of calls, calling out as frequently as their muscles allow. This is called the terminal buzz. Bats’ echolocation is much more sensitive than human vision; they can detect delays of one millionth of a second, which translates physically to distances of less than one millimeter. A broad “picture” via echolocation is useful until the terminal buzz phase, when more specific information is needed. At that point, a bat analyzes echoes at their various frequencies to build a “picture” of the entire insect. Sometimes bats ignore their echolocation and act on what they already know. Echolocation requires a lot of energy, and bats often make mistakes while attempting to save it. This is why they often fly right into new doors at the entrance of caves: They are “cruising,” rather than echolocating.
Most bats are FM (frequency modulated), but there are many species with calls that last much longer and occur at only one frequency. They are listening for a particular kind of echo, and these are called CF (constant frequency) bats. Their pulses are much longer than those of FM bats, so they can capture “acoustic glints,” such as when a wing is perpendicular to the incoming call and makes a particularly sharp echo. CF bats can use these glints—or “flashes,” to place this in a visual context—to find insects against backgrounds that would be formidable for FM bats.
Dolphins also echolocate. They can echolocate on an object that is visually concealed and then recognize that same object visually on a TV screen. They may see through sound; this characterization of their echolocation as visual is not a result of anthropocentrism. Rather, it means that the dolphin is translating one sense to another: taking sound and translating it to vision. Although other senses can be used to explore the world, rather than to communicate, echolocation is “inherently exploratory” (264). Dolphins send out calls through their noses and listen through their jaws.
While bats and dolphins use echolocation in similar ways, echolocation works differently in water than in air. Calls echo off solid surfaces in air, but they penetrate them in water. Bats can make out the outlines of objects, but dolphins hear and see inside objects. Yong states that dolphins can see inside other animals, detecting anatomical features that would require an MRI for humans to see. This also changes the way these dolphins perceive one another. Many male beaked whales, for example, have unusual skull formations that resemble deer antlers. Since they can echolocate on one another, both inside and out, beaked whales can carry antlers without disrupting the sleekness of their exterior shapes.
In addition, underwater sonar has a very long range. A bat moves incredibly quickly, constantly responding in split seconds. However, Yong asserts that a whale has the opportunity to “plan” due to the distance that underwater sonar can cover.
Chapters 7-9 Analysis
These three chapters are organized around surface, sound, and sonar waves. The most important distinction that Yong makes, however, between sound and surface waves is that scientists have ignored surface waves.
As in previous chapters, this section dispels many common human assumptions. For example, birds’ hearing happens on a faster scale than that of humans, and their songs may sound entirely different to birds. In his discussion of birds’ hearing, Yong introduces the ways that sense organs, sensory perception, and umwelten are constantly changing. The umwelt is not, in fact, a solid and unchanging sensory world for species. It is constantly shifting and is never static. An individual, then, may have a broad umwelt in which variations occur.
Yong presents whales’ scale of existence as challenging for readers to comprehend. Their size helps to create their sensory systems, perceptions, and experiences, and they experience space and time very differently than humans do. The author explores this different perception through a discussion of echolocation. This approach connects to his prior indicators about other animals’ unique senses of time, such as flies’ extremely fast vision. Animals have different definitions of time, space, and even, as seen earlier, heat. Dolphins, in fact, “see” one another, through echolocation, from the inside out. These examples demonstrate that intuiting animals’ specific perceptions and experiences is impossible.
Yong presents spiders that create orb webs as further examples of the ways that animals’ senses challenge human conceptions of the senses. He shows that spiders’ cognition extends beyond their brains, and their webs work in coordination with their brains. Thus, their thinking occurs via the web.
Yong also wants to make clear to the reader that infrasound and ultrasound are terms that emerge from a discourse that is anthropocentric, or centered on human experiences. These sounds are “low” and “high” only according to human hearing. Constantly drawing the reader’s attention to human biases, he also remarks that rodents “have been fixtures of laboratories since the seventeenth century. All that time, they’ve been spiritedly talking to each other without any human realizing” (239). This points, as Yong repeatedly demonstrates, to scientists’ dangerous biases. They are deaf to both the literal talk of rodents and to their umwelt, even when they are supposedly observing them. The passage offers a soft criticism of scientists’ anthropomorphism and prejudice.
