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 1-3Chapter Summaries & Analyses
Chapter 1 Summary: “Leaking Sacks of Chemicals: Smells and Tastes”
The common narrative that dogs have a strong sense of smell appears in the first chapter. While humans’ sensory perception of smell happens similarly to that of dogs, dogs have more of everything that enables smell. In addition, their noses have slits that send some air to the olfactory bulb, splitting it off from the main passage to the lungs, unlike human noses. The dog’s nose is designed so that sniffing does not scatter a scent trail but circulates it so that scent can be picked up better.
Yong makes the point that humans have recognized that dogs have a superior sense of smell to their own, but human recognition of this has generally resulted in exploiting that sense for people’s purposes, not in understanding what that sense of smell creates for the dog. As a result, people force dogs’ sense of smell to be of use within humans’ umwelt, rather than trying to understand how that sense of smell helps to create the dog’s umwelt.
Yong also challenges the myth that humans are not good smellers, tracing this false narrative back to the 1879 contention by Paul Broca that human olfactory bulbs are smaller than other animals’ and that smell is “animalistic” and not part of “higher” thinking. Textbooks still commonly teach this prejudicial approach to smell. In addition, smell is undervalued in the West, yet other human cultures place a much higher value on smell and can even distinguish people by smell.
More important, however, is to think about the kinds of questions that are posed when considering any sense. While a dog’s sense of smell may be used for humans’ own purposes, the umwelt of each species is suited to its needs. Thus, better questions to pose than “How good is this animal’s sense of smell?” are “How important is this sense for this animal?” and “How is the umwelt for this animal created in its sense of smell?”
The chapter also explores the sense of smell in the umwelten of army ants, African elephants, and shearwaters (sea birds). All these animals smell through nostrils or antennae and, thus, smell through two openings and “in stereo,” but snakes smell through their tongues, usually the sensory organ of taste.
Taste functions more simply than smell. For humans, taste is determined by five criteria: salty, sweet, sour, bitter, and umami, or savory. Other animals may have a few more criteria. Taste is almost always used to make simple decisions about food, with the most basic one being whether something should be eaten. Snakes don’t need to make this determination through taste, but through smell with their tongues. In this example, the absence of a sensory system points not to deficiency but to difference.
Flavor is different from taste and more equated with smell; taste is described as a “coarse” sense. Very small animals can often taste with their legs or feet. For example, bees can taste the nectar that they stand on. Mosquitoes taste DEET with their feet, and the taste is so bitter that they leave before biting. Obligate carnivores—animals with a diet of at least 70% meat—have generally lost their ability to taste sweetness, as this is not necessary in eating animal flesh. What can be tasted varies according to diet and necessity.
Chapter 2 Summary: “Endless Ways of Seeing: Light”
This chapter begins with jumping spiders and their two pairs of eyes: one primary and one secondary. The central pair enables a very focused field of view. The secondary eyes on either side of the primary eyes allow for sensitivity to motion. If these eyes are covered, then the spider cannot detect motion at all.
Humans’ paired eyes that are on the head, face forward, and are of equal size are just one iteration of the diversity of placement and kinds of eyes among animals. Eyes are not always on the face or the head; some animals have eyes on their mouths and arms, for example.
Animals see differently, but all vision begins with light. It is difficult to define what exactly light is, however. Photoreceptors detect light. All photoreceptors, regardless of species, contain opsins. Opsins connect with chromophores, which absorb energy from light. The chromophore changes shape with this absorption; since it is partnered with the opsin, the opsin also changes shape. This change of shape results in an electrical signal sent to a neuron.
While eyes are extremely different, they all detect light through opsins. These different types of vision seek different information about the light that they sense. This is why eyes are so varied.
Humans have sharp or acute vision, though not as acute as that of many diurnal raptors, the category that includes eagles and hawks. Acute vision is the result of densely packed photoreceptors and is a trade-off, however, as eyes cannot have both high resolution (i.e., acute vision) and light sensitivity. Humans cannot see in the dark, but other animals without acute vision often can.
Alternatively, scallops have up to 200 eyes. Each eye may have excellent resolution, but scallops may not have actual vision; they may not see “scenes” as humans do. Yong compares scallops’ sight to the human sense of touch, in that people feel via touch but do not create tactile worlds. Touch functions merely as a detection system for humans, perhaps similarly to the way that scallops’ sense of sight also functions as detection.
Many birds have visual fields that are completely panoramic, with no blind spots. Mallards, for example, can simultaneously see what is behind and in front of them, unlike humans, who cannot see what is behind them as they move forward. Birds often use different eyes for different tasks, too. Cows have a perceptual field that is broad and all around them, almost like a stripe.
Photoreceptors not only determine the acuteness of vision but also determine the critical flicker-fusion frequency (CFF), which measures how quickly the brain processes visual information. Humans’ CFF is 60 frames/second, while the average fly’s is 350. What people perceive as a movie would appear to a fly like separate images, one after another. The fly’s very fast vision requires a lot of light, which is why flies are active during the day. This vision is so fast that if a person approaches a fly slowly, the fly may not even detect the movement.
Chapter 3 Summary: “Rurple, Glurple, Yurple: Color”
Yong begins this chapter with a consideration of dogs’ perception. This time, instead of smell, he considers their perception of color. Although dogs were once thought to be color-blind, scientists now believe that dogs see colors.
Color is determined by the perception of the varying wavelengths of light. All animal vision, as discussed in the previous chapter, depends on opsin proteins, and different opsins absorb different wavelengths of light. Humans have three opsins, with a different “cone” in the retina that contains each of them. The three cones in human eyes are referred to as red, green, and blue. They detect these different wavelengths, and neurons then compare them in a process called opponency. Without opponency, there is no conscious perception of any kind of spectrum of color. This means that color is subjective and is a sensation. The color “red” is a sensation; there is nothing inherently red about the wavelengths humans perceive as red.
All color vision depends on opponency. This means that having one cone is like having no cones, as it provides no way to compare wavelengths. Many animals are monochromats, which means that they see in shades of gray, and color vision is not necessary for them. A fundamental question for this chapter is “What, then, is the point of seeing colors at all?” (87).
Most mammals, including dogs, are dichromats, meaning that they have two rods and see in shades of blue, grey, and yellow. If most mammals are dichromats, why are primates trichromats? Again, this chapter unpacks assumptions that complexity—simply put, seeing more colors—is superior to seeing fewer colors, without definitively answering the question about trichromats.
Most animals can see ultraviolet (UV) light, but humans generally cannot. Flowers have UV patterns on their petals, which function as signals to pollinators, and many birds have UV patterns on their feathers.
Most birds are tetrachromats, as they have four cones. Dichromats (for example, dogs) see 1% of what trichromats (primates) see, and trichromats see about 1% of what tetrachromats (birds) see. Birds do not only have a cone that detects ultraviolet light in addition to the three cones humans have. Many colors they see are “cocktail colors,” which means that they are colors that are not represented through a single wavelength of light but are the result of multiple cones being stimulated and creating a color that is combination of wavelengths. Many birds see cocktail colors that are combinations of UV and red, green, and/or yellow.
The chapter explores in depth animals’ detection of color and their reactions to this detection. Yong begins to flesh out, though, that while researchers are beginning to understand what animals detect, it is always much less clear what animals experience and feel in this detection, and this may never be known. The experience of detecting, after all, is only a small part of the umwelt of each animal.
Chapters 1-3 Analysis
Yong begins the book with what have long been considered the “primal” senses of smell and taste before moving into the sense that largely defines the human umwelt and matters most to humans: vision.
The author begins this exploration of the senses with a discussion of dogs as a starting point for contemplating the sensory worlds of nonhuman species. Yong uses dogs to introduce the new philosophical landscape of umwelten, presenting a familiar animal as a guide into the lives of animals that are less familiar.
These initial chapters also begin to unpack assumptions about the value of complexity in and of itself. Uexkull does not value complexity. Rather, for him, necessity is the pertinent issue. Thus, he attempts to discover what needs perception meets and how sensory systems can efficiently meet these needs. Sensory systems require energy, so the simpler and more efficient they can be while meeting an animal’s needs, the better.
Yong also demonstrates that human sensory systems are not always as complex as people assume they are. For example, vision is arguably humans’ primary sense, but in this case, humans’ sensory system is not as complex as that of other species, such as birds.
Yong also begins to tease out the difference between perception and the experience of that perception, even as he withholds judgment of any experience as better than another. For example, humans may use the sense of touch similarly to the way that scallops use their sense of vision, purely for detection rather than as an experiential phenomenon. People generally do not have “touch worlds,” but vision creates visual worlds; scallops, however, may have touch worlds in which their vision may be purely a means of detection.
