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 4-6Chapter Summaries & Analyses
Chapter 4 Summary: “The Unwanted Sense: Pain”
The experience of a sense is particularly important in the case of pain. This chapter opens with naked mole rats, which can survive extreme conditions, such as living up to 18 minutes without oxygen.
The experience of pain depends on nociceptors, which are neurons; almost all animals have them, including naked mole rats. However, these animals do not seem to experience acids as painful. Thus, the chapter begins with the reality that pain is not experienced the same way across species. Pain, too, is subjective.
Nociception is the sensing of harmful stimuli without the sensation of pain. Nociception is the detection of damage, while pain is the suffering that often accompanies this detection. Nociception triggers a response before what is happening is consciously perceived; pain sometimes, but not always, follows this response. Pain is produced in the brain, while nociception occurs at the sight of damage and in the spinal cord. People generally do not separate the two but lump them together.
Pain is the sense that people often wish they did not have, and the distinction between nociception and pain is important for moral reasons. It is relevant to the design and procedures of experiments conducted on animals as well as to considerations of how animals are trapped, killed, or prepared for human consumption.
Chapter 5 Summary: “So Cool: Heat”
The best understood temperature sensor is TRP channels, a variety of proteins that occur all over the body. For example, TRPV1 is the sensor that detects painful heat. It is activated at different temperatures for different animals, so each animal has its “own definition of hot” (139). Extremophiles—animals such as camels and penguins that live in what humans perceive as extreme conditions—have TRP channels that are adjusted to their environments, with very different definitions of hot and cold than those of humans. Temperatures that would trigger nociception and pain in humans and many other animals most likely do not do so in these animals.
Thermotaxis is the ability to sense heat through sensors, which allows animals to adjust to more favorable conditions. Some animals, such as the Melanophila beetle, or fire beetle, don’t have to feel heat. Rather, they can sense it from a distance and move toward it, if desired. Electromagnetic radiation increases when an object gets hotter, making its molecules move around faster and creating more radiation and at higher frequencies. Most of this radiation is in the infrared spectrum. For people to “detect infrared light from a distant source, […] [it] must be extremely intense" (143); Yong uses the sun as an example. However, fire beetles have adaptations that allow them to sense heat dozens of miles away and travel toward its source.
Warm-blooded animals can produce their own body heat. This condition is called endothermy. Evolutionarily, this enabled them to live in varied environments, but it also made them very trackable, especially for parasites, who often use their own sense of heat to track warm-blooded hosts. Uexkull begins his famous book A Foray Into the Worlds of Humans and Animals with the example of a tick, which he claims seeks hosts through the sense of smell. Scientists now believe that ticks wave their legs around because they have heat sensors, not smell sensors, on them. Thus, their sense of heat, rather than smell, draws them to humans. The pesticide DEET works by disrupting ticks’ ability to sense body heat.
Chapter 6 Summary: “A Rough Sense: Contact and Flow”
This chapter begins with a discussion of sea otters’ sense of touch. The somatosensory cortex is the part of the brain that processes touch, with different sections of it receiving signals from different parts of the body; the larger parts of the cortex engage with the most tactile parts of an animal’s body. For example, in mice, this is the whiskers, but for naked mole rats, it is the teeth. For humans, it is hands, lips, and genitals.
Sea otters are extremely sensitive to touch and are much faster at determining textures than humans are. They do this with their hands—which people typically refer to as paws—that look like mittens. To humans, these appear to be crude versions of their own hands and, therefore, ill-equipped to be sensitive to touch. This is an entirely biased perception, however, that misreads the animals’ sensory organs and systems. Their refined sense of touch allows them to discriminate carefully as they dive for food.
Humans, like sea otters, have a sense of touch that comes primarily through direct contact with what is being sensed. Touch is generated through movement over the surface of what is being touched. Other animals, however, can “touch” through indirect contact via phenomena such as breezes and waves.
There is a broad range of sensitivity in touch organs as well as in what these organs are used to feel and the body parts that house them. For example, star-nosed moles have a pink, fleshy “nose” made of what look like small tentacles or even red roots, but this is an organ of touch, not smell. Their “noses” function almost like hands. The star-nosed mole’s sense of touch is incredibly fast, faster than any processing of light—vision—that occurs in animals. The “nose” “works like a hand and scans like an eye” (165). These moles sense by pressing and lifting with their “noses.”
Birds’ beaks are often very sensitive, even though they seem inanimate to humans, almost like fingernails. Although, like human nails, birds’ beaks are sheathed in keratin, they are not used only for prying or pecking. Rather, they are tactile sensory organs. Many water birds can filter very small particles through their beaks, determining what is edible through touch, rather than taste.
Crocodiles, too, are incredibly sensitive to touch. They have sensors all over their bodies, from head to tail, that are 10 times more sensitive to fluctuations in pressure than human fingertips are.
Insects and spiders also have airflow sensors. The filiform hairs of crickets are 100 times more sensitive than any visual receptor. Yong points out that they are so sensitive that they could not be more so without breaking the laws of physics. Responding to only meaningful frequencies prevents them from being overwhelmed by these receptors.
Fish can feel very small movements of water around them. All fish have a system of sensors that provides them with a hydrodynamic awareness of their surroundings; this is called the lateral line. This allows them to feel at a distance through the flow of water around them, providing another example of being able to “touch” at a distance. Fish “school” because of the lateral line: When the fish closest to a predator feels the water it displaces, that fish quickly swims away. This further distorts the flow of water, and each successive fish picks up on this disruption. These fish form a cohesive group because of the lateral line.
Chapters 4-6 Analysis
These chapters deviate from earlier ones in that they no longer begin with the familiar dog. Instead, Yong increasingly introduces animals who are less likely to be familiar and may even seem strange to his readers. Increasingly, he approaches these animals in the labs in which they are confined.
As the book moves into an exploration of less familiar animals, the umwelt becomes more relevant and important in bringing up ethical questions and challenges to the status quo. Finn, the dog introduced in the first sentence of the book, appears in a cognitive lab, but he is not confined to that lab. The less familiar animals that now populate the book, however, are increasingly confined to tanks and cages that they are never allowed to leave.
Earlier in the book, Yong challenges assumptions about color, exploring the reasons why people might take it for granted that a particular color is an intrinsic reality of the world, such as a blue sky or green grass. Here, he uses other animals’ umwelten to challenge readers to consider the differences in senses of heat, touch, and pain across species. In the case of pain, this is particularly relevant to the field of ethics. For example, he shows that there is nothing inherently painful about acids, as experiments on naked mole rats have provided evidence that they do not feel pain when exposed to them. Yong makes the case that ethical treatment of animals depends largely on animals’ perception of pain. If they experience pain, then different ethical questions must be raised than in the absence of this sensation.
Yet animals’ umwelten can be diminished and polluted by many other factors in addition to pain, as Yong argues in his conclusion in Chapter 13. Uexkull’s theory, after all, is about the senses’ engagement with the world and how animals make meaning out of this engagement. Experiments that produce nociception but do not produce pain may be viewed as “better” than pain-inducing experiments, but they nonetheless deprive animals of full engagement within their umwelt. The consideration of animals as subjects raises more challenging questions about treatment of them than simply ones of pain.
Chapter 4, for example, begins with a naked mole rat. These animals normally live in underground tunnels that they “constantly expand, remodel and patrol” (117), but the rats Yong presents live in plastic cages with toilet paper rolls in what he refers to as a “replication” of their natural habitat. Chapter 5 also begins in a lab, with a 13-lined ground squirrel, and Chapter 6 begins in a marine laboratory. All three chapters begin with animals that are confined for the purposes of research. Past animal research in which animals are mutilated or killed is recognized by Yong as “cruel,” yet there is no discussion in his book of current research in which animals are also confined, sometimes mutilated, and sometimes killed.
In these chapters, Yong repeatedly presents material that he anticipates being counterintuitive to readers to challenge their assumptions. He anchors this section in the following argument: People cannot assume that all animals experience pain or experience it in the ways that humans do, though all animals experience nociception. At the same time, humans should not assume that it is possible to deny animals the experience of pain. He also attempts to challenge his readers’ assumptions about appearances. For example, people interpret sea otters’ hands (or paws) as unrefined because they seem to lack digits, but their hands are much more sensitive to touch than those of humans.
