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

Uexkull compares an animal’s body to a house, with windows—the senses—that open to the world. In his example, the world is represented by a garden. The first 11 chapters provide an understanding of different senses and how they open up different worlds for different animals. But, of course, Uexkull is interested not only in the windows themselves but also in how they are integrated into the architecture of the house. This requires an understanding of how the windows work on their own, as well as how they might “open” and “close” in coordination with one another. Chapter 12 focuses on this coordination of the senses, and it also pays attention to how animals coordinate sensory information coming from outside their bodies with information coming from within their bodies.

Every animal relies on more than one sense, and animals use as many senses as their nervous systems can manage. Sense organs provide two forms of information whenever animals move: exafference and reafference. Exafference refers to stimuli from the outside world, while reafference indicates signals produced within an animal. Yong describes the former as “other-produced” and the latter as “self-produced.” Leaves rustle in the wind on their own, demonstrating exafference, but perception of their rustling is reafference.

“Corollary discharges” refer to the way that the senses anticipate what will happen with every move an animal makes. They explain, for example, why it is impossible for people to tickle themselves. Corollary discharges also create a stable visual field, even though the field is not stable. They also account for a fish’s ability to sense the flow of other fish swimming without being distracted by the flow related to its own movement.

The topics discussed above relate to the ability to distinguish between the self and the other, which is not only a philosophical question but also a neurological one. According to neuroscientist Michael Hendricks, “This is largely what sentience is […] and perhaps it’s why sentience is: It’s the process of sorting perceptual experiences into self-generated and other-generated” (328). Sentience does not require consciousness, and it is a “foundational condition of animal existence, which flows from the simplest acts of sensing and moving. Animals cannot make sense of what’s around them without first making sense of themselves” (328).

Yong concludes the chapter with a complex umwelt, presenting the possibility that an octopus exists within two umwelten. While octopuses have traditional brains, they also have eight arms, with each sucker on the arms having its own mini brain: the sucker ganglion. Each sucker ganglion connects to a brachial ganglion in the middle of the arm. The sucker ganglia don’t communicate with each other, but the brachial ganglia do. Thus, each arm controls itself. The central, traditional brain can coordinate these arms when needed. The arms have proprioceptors, but these may function specifically only within each arm; the octopus may not have a sense of its body as a whole. Unlike humans, whose bodies are much more limited in their motions, octopuses are bodies of “pure possibility,” according to philosopher Peter Godfrey-Smith. He notes that a body of possibilities may be too much for any single brain to manage. After all, octopuses can change color and texture. They can move in ways that are difficult for many people to imagine because of their extreme flexibility and changeability. More specifically, the arms may live in a world of taste and touch, while the head’s guiding sense is vision. The analogy Yong uses, then, is that the octopus lives in two semidetached houses that have completely different architectural styles and are connected by a small door between the two residences.

Yong emphasizes that as challenging as it may be for humans to imagine themselves as other animals, it is impossible for them to imagine how their brains would work within others’ umwelten. This is because human brains simply would not work in other sensory worlds.