One summer night in his bungalow on the Maine coast, as Vincent Dethier lay in bed, he became aware of a presence in the darkness. It was hinted at by only the slightest sounds, and perhaps a stirring in the air. This signaled, he surmised, that a brown bat had made the bungalow its summer home and was now engaged in an aerial battle with a moth. The bat hunted, probing the darkness with ultrasound, listening to and deciphering the returning echoes, while the moth, registering the sound, might abruptly shift course to evade the predator. To Dethier, an entomologist, the experience was profound:
To realize that a whole world of life and death is being enacted before a person’s eyes and he cannot see it, and around his ears, and he cannot hear it is to diminish whatever feelings of superiority and arrogance one may have. It is to feel humble in the knowledge that there are other worlds and other perceptions. It is to appreciate that we are surrounded with an infinity of wonder and beauty. (The Ecology of a Summer House, 1984, p. 39-40.)
That knowledge of other animal worlds, captured in the concept of Umwelt, is the focus and core of science writer Ed Yong’s masterful new book, An Immense World: How Animal Senses Reveal The Hidden Realms Around Us (2022).
As Yong writes, zoologist Jakob von Uexküll first used the word Umwelt in 1909 to define that part of an animal’s environment it “can sense and experience — its perceptual world.” (p. 5) Yong explores what science can tell us about the worlds that different animals perceive, and the senses they employ to map, navigate, and survive in those worlds. Put aside the notion of just the traditional five senses being the portals through which animals understand their worlds, or a similarly limited number of stimuli that animals have evolved to register, respond to, and employ. The overarching world in which we and all other animals live contains each of those perceptual worlds, but no animal perceives them all. Different groups of animals are at home in one or more of these worlds in markedly different ways and degrees. Our challenge is to move beyond the limitations of how we humans engage in the world and stretch our understanding to catch a glimmer of those perceptual worlds alien to us, in order to better understand the one world in which we and all other animals live.
There is an urgency, Yong writes, to gaining that understanding and acting on it before human-generated environmental changes – such as light pollution and noise pollution – render entire perceptual worlds barely habitable or, indeed, uninhabitable for their denizens.
Our journey in the book is through several different kinds of perceptual worlds accessible through specific senses. I came away with a set of critical guideposts that helped me appreciate, though, perhaps, not understand, the worlds Yong was exploring for me. He writes, “The first step to understanding another animal’s Umwelt is to understand what it uses its senses for.” (p. 61) Essential to that effort is an appreciation that the world in which we all live is full of “invisible currents of information that flow around us, and which animals can detect with the right sensory equipment." (p. 173) Further, we must recognize that all animals are engaged in an evolutionary balancing act – strengthening any particular sense in any particular way carries a cost. Writ large, an animal’s resources are finite and, so, what is gained to enhance survival is often compensated for by something lost. Further, given that the overarching world is dynamic, change is the watchword and every animal group’s Umwelt can remain static only at its peril. Finally, we humans must recognize that our particular Umwelt prejudices us in favor of those senses at which we excel – sight in particular – and limits our ability to understand, intuitively or otherwise, another animal’s perceptual world. There is no hierarchy of superior and inferior Umwelten, just different ones. A heady brew of insights, indeed.
Yong devotes chapters of the book to the sensing of smell and taste, light, color, pain, heat, contact and flow, surface vibrations, sound, echoes, electrical fields, and magnetic fields. The reality of the senses across the animal kingdom is staggering. Consider just a single aspect: where the organs or cells that work on the traditional ones might appear. Eyes are not just for heads, but adorn the inner edges of a scallop’s shells and are on each of a starfish’s five arms; ears can be found on the joints of certain insects, the abdomens of others, the mouths of still others and on antennae; smell is helped by the tongues of snakes and lizards; some insects taste with their feet and legs. Clearly, this affects how those senses work and what they perceive.
The book is replete with amazing examples of the ways animals understand and live in their Umwelten. Consider the tiny treehopper which uses its abdomen to vibrate the surface of plants to generate myriad sounds, some deep throated and others shrill, to communicate (the young to tell mom they perceive a threat, adults to call a group together, adults to find mates). These sounds are generally inaudible to us but not to others in the taxon. When insects that vibrate plant surfaces to communicate gather together, the cacophony must be impressive. Or consider the whiskers of harbor seals that stand up from the animal’s nose and eyebrows. These are sensitive sensory organs that are touched by the wakes left by objects moving through water. This hydrodynamic touch sense is fine tuned to follow a wake with great precision. Harbor seals in the wild will lie in wait, their whiskers able to signal not only when a fish swims past but its size as well. As Yong observes, our own sense of touch is tied to the present, but the seal’s whiskers which respond to the touch of the wake capture the recent past.
One sense that resonated (pun intended) with me in particular was echolocation as practiced by bats and dolphins. Echolocation differs from the other senses because it’s adding to the environment and using the response to that added energy to define a perceptual world. For the bat, this sense poses a host of significant challenges – outgoing sounds need to be distinguished from incoming echoes, it must protect its ears from the deafening volume of the ultrasonic calls it emits, the interplay of call and response occurs while bat and prey are on the move – and bats have evolved different mechanisms and behaviors to cope with the complexity of those challenges.
Actually, air is a rather poor medium for echolocation because sound loses energy quickly in it and so this sense can define the contours of a perceptual world only over a short distance. In contrast, water is an excellent medium for sound conduction and dolphins take full advantage of that. Indeed, sound retains enough energy as it travels through water that dolphins use echolocation to probe inside of objects. Animals’ skeletons because perceptible to them as do fish swim bladders, allowing a dolphin to distinguish among potential prey.
At the same time, evolution has fueled responses to the powerful echolocation sense. Although most insects are deaf, some, including half of moth species, have ultrasonic hearing, an ability that appeared in the insect world after bats came on the scene, perhaps some 65 million years ago. Thus, a moth might well hear a bat’s outgoing call and take evasive action before the echo reaches the predator’s ears. Other moths have evolved the ability to make ultrasonic clicks, potentially confusing bats. Still others evolved long, elaborate tails, incredibly beautiful to the human eye, but misleading to the echolocating bat who may well interpret returning echoes as describing a much larger prey. The attacker may come away with a mouth full of moth tail while the insect survives to carry on its kind.
Yong’s prose is graceful, carrying the reader along easily in this journey of exploration. That he has mastered an immense world of information and anecdote is abundantly clear. I must acknowledge that the cumulative effect of being exposed to these multiple perceptual worlds can be overwhelming, leaving a reader (well, this reader) mostly intuiting what is there, and not understanding it in any meaningful way. That may be unavoidable in a work of this depth and breadth. Despite that, I suspect I am now more open to the incredible diversity of the perceptual worlds around me whether I can understand them or not. (On a very immediate level, this has added a new dimension to my appreciation of the two cats currently sharing our house.)
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