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

When writing about animal senses, it is easy to emphasize the weird or the superlative. Ed Yong is not interested in comparisons or lists but in trying to understand animals on their terms. One touchstone is of course Thomas Nagel's famous 1974 essay but the lodestar of this book is a concept defined in 1909 by the Estonian-German zoologist Jakob von Uexküll: that of an animal's "Umwelt". Yong sensibly opens with it and offers a crisp definition: every animal "is enclosed within its own unique sensory bubble, perceiving but a tiny sliver of an immense world" (p. 5). This book is a sensory smorgasbord spiced up with some particularly fine writing in which Yong discusses the work of scientists past and present, and mixes literature research with interviews.

Smell, the detection of molecules in your environment, is a skill so useful and basic that even single-celled organisms do it. But it is also hard to study: we cannot predict smell from molecular structure, and it is hard to capture and play back smell in an experimental setup. Vision is easier, but eyes come in a bewildering variety. Jumping spiders dedicate one pair to sharp vision and another pair to motion detection. Scallops have hundreds of eyes lining their shell, but absent much of a brain they might very well see without experiencing vision. Other Umwelten include ultraviolet light, polarized light, and colours we cannot comprehend. But what haunted me was Yong's description of the Umwelt of the squid that dwell in the deep and have obscenely large eyes. The sperm whales that hunt squid do not glow, but they do trigger bioluminescent flashes as they collide with plankton. The squid, it seems, "scan one of the darkest environments on Earth for the faint sparkling outlines of charging whales" (p. 82).

The chapters on mechanoreception were particularly delightful. The star-nosed mole uses its tentacled nose as a touch organ: "with every press, its environment comes into focus in a starburst of textures [...] like a pointillist image appearing dot by dot" (p. 162). Animals can sense flows of air or water (e.g. the hydrodynamic wake left by fish), such that: "touch extends into the recent past [...] whiskers can feel what was, rather than simply what is" (p. 175). Treehoppers—this blew my mind—produce vibrations that travel through plants, inhabiting a world that can be made audible with contact microphones. Even sound is more multifaceted than just the infra- and ultrasonic calls beyond our hearing. Some birds completely retune their ears seasonally depending on what information matters most. Yong nicely builds towards Umwelten that are increasingly hard for us to understand. I have a renewed respect for bats after reading his list of ten challenges of echolocation. Meanwhile, some fish can both produce and sense electric fields to find prey, detect obstacles, and communicate—and might not even distinguish between these activities. And magnetoreception has us flummoxed despite decades of research as we still have not identified a clear organ for it.

The chapter on pain, in particular, exemplifies the maturity and nuance that Yong brings to his exploration of sensory biology. You might think that, as a progressively-minded person, he is going to convince you that, of course, animals feel pain. Well, it is complicated. For one, it is an emotive and controversial topic and Yong spots an artificial dichotomy: animals are frequently either assumed to experience pain exactly as we do or not experience it at all. It is conceptually challenging for us to imagine what an intermediate state might be like, but there is no reason to assume it feels the same across the entire animal kingdom. Though pain is a useful mechanism warning of danger and injury, animals "differ in what they must avoid and what they must tolerate" (p. 120). For instance, male praying mantises continue mating while being eaten by the female. Yong introduces the distinction between nociception (the sensory process of detecting damage) and pain (the suffering that ensues); a distinction some people object to. After all, for other senses "we rarely distinguish between the raw act of sensing and the subjective experiences that ensue". However, Yong argues, that is not "because such distinctions don't exist. It's because they usually don't matter" (p. 124). Pain is unique in being the unwanted sense, the one we try to avoid, he adds. Finally, what hinders a better understanding is the instrumental attitude that hides behind these debates. "When we ask if animals feel pain, we're asking less about the animals themselves, and more about what we can do to them. That attitude limits our understanding of what animals actually sense" (p. 133).

Studying animal senses is hard and asks for humility. Why? Because we do not understand the stimuli that we do not experience, and risk misunderstanding the ones we do, interpreting them through our own senses. Because most sensory biologists spend their careers studying a single sense while animals rely on multiple senses simultaneously. Because sensory experiences could very well blend. Octopuses, for example, have arms lined with both taste and touch receptors. Does that mean they touch and taste at the same time or is it a single, fused experience? And because our experiences appear to us as "purely mental constructs divorced from physical reality". In myths and stories, we imagine transferring our consciousness into the body of another animal to experience their world. Instead, "an animal's sensory world is the result of solid tissues that detect real stimuli and produce cascades of electrical signals. It is not separate from the body, but of it. You can't simply imagine how a human mind would work in a bat's body or an octopus's, because it wouldn't work" (p. 333). I found it to be one of the most profound messages of this book. On the back of all of the above, the last chapter, where Yong discusses sensory pollution, hits extra hard. During the time that we learned more about animal senses, human activities have increasingly flooded animal Umwelten with stimuli, with light and noise pollution being the best documented.

The only recent book that comes close to what Yong has done here is Martin Stevens's Secret Worlds, which is a somewhat briefer affair, though Stevens brings his expertise as a sensory biologist (he also authored the textbook Sensory Ecology, Behaviour, & Evolution). Yong, however, brings his exquisite writing and skill at capturing scientific nuance for which his reporting is famous. Strictly speaking, it would not be correct to say this book is out of this world, as it is so fully steeped in it. Let me just say An Immense World is a sensory revelation that had me so utterly captivated that I did not want it to end.