The preceding paragraph could have been pulled from a high school textbook. But take a moment to consider the miracle of what it describes. Light is just electromagnetic radiation. Sound is just waves of pressure. Smells are just small molecules. It's not obvious that we should be able to detect any of those things, let alone convert them into electrical signals or derive from those signals the spectacle of a sunrise, or the sound of a voice, or the scent of baking bread. The senses transform the coursing chaos of the world into perceptions and experiences—things we can react to and act upon. They allow biology to tame physics. They turn stimuli into information. They pull relevance from randomness, and weave meaning from miscellany. They connect animals to their surroundings. And they connect animals to each other via expressions, displays, gestures, calls, and currents.
The senses constrain an animal's life, restricting what it can detect and do. But they also define a species' future, and the evolutionary possibilities ahead of it. For example, around 400 million years ago, some fish began leaving the water and adapting to life on land. In open air, these pioneers—our ancestors—could see over much longer distances than they could in water. The neuroscientist Malcolm MacIver thinks that this change spurred the evolution of advanced mental abilities, like planning and strategic thinking. Instead of simply reacting to whatever was directly in front of them, they could be proactive. By seeing farther, they could think ahead. As their Umwelten expanded, so did their minds.
An Umwelt cannot expand indefinitely, though. Senses always come at a cost. Animals have to keep the neurons of their sensory systems in a perpetual state of readiness so that they can fire when necessary. This is tiring work, like drawing a bow and holding it in place so that when the moment comes, an arrow can be shot. Even when your eyelids are closed, your visual system is a monumental drain on your reserves. For that reason, no animal can sense everything well.
Nor would any animal want to. It would be overwhelmed by the flood of stimuli, most of which would be irrelevant. Evolving according to their owner's needs, the senses sort through an infinity of stimuli, filtering out what's irrelevant and capturing signals for food, shelter, threats, allies, or mates. They are like discerning personal assistants who come to the brain with only the most important information. Writing about the tick, Uexkull noted that the rich world around it is “constricted and transformed into an impoverished structure” of just three stimuli. “However, the poverty of this environment is needful for the certainty of action, and certainty is more important than riches.” Nothing can sense everything, and nothing needs to. That is why Umwelten exist at all. It is also why the act of contemplating the Umwelt of another creature is so deeply human and so utterly profound. Our senses filter in what we need. We must choose to learn about the rest.
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The senses of animals have fascinated people for millennia, but mysteries still abound. Many of the animals whose Umwelten are most different from ours live in habitats that are inaccessible or impenetrable—murky rivers, dark caves, open oceans, abyssal depths, and subterranean realms. Their natural behavior is hard to observe, let alone to interpret. Many scientists are limited to studying creatures that can be kept in captivity, with all the strangeness that entails. Even in labs, animals are challenging to work with. Experiments that might reveal how they use their senses are hard to design, especially when those senses are drastically different from ours.
Amazing new details—and, sometimes, entirely new senses—are being discovered regularly. Giant whales have a volleyball-sized sensor at the tip of their lower jaw, which was only discovered in 2012 and whose function is still unclear. Some of the stories in these pages are decades or centuries old; others emerged as I was writing. And there's still so much we can't explain. “My dad, who is an atomic physicist, once asked me a bunch of questions,” Sonke Johnsen, a sensory biologist, tells me. “After a few I don't knows, he said: You guys really don't know anything.” Inspired by that conversation, Johnsen published a paper in 2017 entitled “We Don't Really Know Anything, Do We? Open Questions in Sensory Biology.”
Consider the seemingly simple question How many senses are there? Around 2,370 years ago, Aristotle wrote that there are five, in both humans and other animals—sight, hearing, smell, taste, and touch. This tally persists today. But according to the philosopher Fiona Macpherson, there are reasons to doubt it. For a start, Aristotle missed a few in humans: proprioception, the awareness of your own body, which is distinct from touch; and equilibrioception, the sense of balance, which has links to both touch and vision.
Other animals have senses that are even harder to categorize. Many vertebrates (animals with backbones) have a second sensory system for detecting odors, governed by a structure called the vomeronasal organ; is this part of their main sense of smell, or something separate? Rattlesnakes can detect the body heat of their prey, but their heat sensors are wired to their brain's visual center; is their heat sense simply part of vision, or something distinct? The platypus's bill is loaded with sensors that detect electric fields and sensors that are sensitive to pressure; does the platypus's brain treat these streams of information differently, or does it wield a single sense of electrotouch?