Photo by jayhem on flickr, used via a Creative Commons license.
Sea urchins do more than you might expect from a spiky ball. They seek out holes to hide in, travel in search of food, cover themselves in costumes of seaweed and rocks, and flee their slower predators. (Even the speediest urchin can’t flee a sea otter, but it has a chance against a sea star.)
All of this is a bit astonishing for an animal that has no eyes. How do they spot their hidey-holes? How do they see the sea stars in time to run away?
Humans see faces everywhere. We see a face in the craters of the moon, in wall sockets, sideways in punctuation :-) and just about anywhere else two dots and a line are arranged in even approximately the same positions as two eyes and a mouth.
Don’t those drawings of outlets look like faces?
Once we recognize something as a face, we process it differently from other visual stimuli. Certain parts of the brains are triggered preferentially by faces. We are especially good at perceiving faces: we can pick out matching faces faster than matching abstract patterns, and distinguish non-matching faces more easily than other images. This only works, however, when our brains recognize the faces as faces: if you flip faces upside-down, they no longer trigger the “face” switch for us, and we become much worse at distinguishing them. The same thing happens if you digitally scramble facial features, so that there’s an ear in the middle and an eye on the chin and a mouth slanting across the forehead, or any other mix-up that makes the face no longer be arranged like a face. Our brains are specialized to perceive face-shaped patterns much better than other patterns.
Animals interact visually all the time. Males try to look big and scary to rivals, or sexy to females. Prey animals try to look inedible—or better, invisible—to predators. Sometimes these animals use visual trickery to assist their cause.
You’ve probably encountered visual illusions before. Here are some classic ones:
a) The vertical line looks longer than the horizontal line, even though they’re both the same length. b) The top line looks longer than the bottom line, even though both are the same length. c) The middle circles are both the same size, but the one on the left looks bigger. d) The middle grey rectangle is just solid grey, but against the gradient background, it looks like a gradient. e) Both circles are the same shade of orange, but the one surrounded by black looks brighter.
Animals can use visual illusions a) and b) to appear bigger by changing their posture. Vertical stances make you look bigger than horizontal ones, and making a Y with your limbs looks bigger than letting them fall down. So if you’re a male peacock spider trying to look big and sexy to a female, you can raise a pair of back legs up in a Y to look bigger than you really are.
I once saw a talk by a scientist who works on jumping spiders—those colorful, fuzzy, big-eyed teddy bears of the spider world—in which the speaker paused, after discussing the spiders’ excellent vision (courtesy of their many eyes, which are of several different types and see in various ways) and their sensitivity to vibrations (which they perceive through their legs and through the many fine hairs covering their body), to wonder, “What does the world feel like to these animals? What is it like to be a jumping spider?”
What is it like to be something other than human? There is so much research touching on this question, studies asking everything from “How does a bee navigate?” to “Are rats kind?” It’s a fascinating question, and it’s incredibly difficult.
Even in the visible spectrum, birds can discriminate more subtle color distinctions than we can, thanks to their at-least-five functional cone photoreceptor types (we only have three). But it’s in the ultraviolet (UV) part of the spectrum where they literally can see what we can’t.
Somewhat disappointingly, birds don’t generally have secret UV patterns the way that, for example, some flowers do (Andersson 1996). Instead, they seem to use UV to augment signals we can already see: bluebirds turn out to reflect UV, as do the spots on some thrushes, and so on. But the UV can still contain information invisible to our eyes. In the Alpine Swift and the European Starling, better-fed chicks reflect more UV from their skin; their parents can use this information to give more food to scrawny chicks in good times, or to cut their losses and favor the healthiest chicks in lean times (Bize et al. 2006).
Starling adult and fledglings – who may be too old and feathered to reflect much UV from their skin now, but are definitely still hungry. Photo by Tina S. White.
When certain birds (not owls) look out at you from their left eye, they can’t see you with their right eye on the other side of its head. So what do birds with eyes on both sides of the head actually see? Two different scenes? Or some sort of panorama distillation?