“I’ll believe it when I see it.”

Try telling that to your dog or a fly. See what exactly? That hummingbird visiting flowers sees things quite differently than you or I, even if between individual humans our vision can be quite varied. Understandably, we tend to get a bit caught up in ourselves, because despite good science we still only really know definitively what the world looks like to us (and by that I mean down to each of us individually).

What does the world look like to some of the animals that we share the Hill with?

First, a bit about “eyes.”

Any eye, no matter the structure, detects light and processes it into neurons that help the organism with said eyes “see.” Enough said. But without getting too metaphysical about it, what exactly is seeing? For some organisms, it means merely detecting light and dark. For others it means seeing colors, shapes, and movement. There are simple eyes which consist of a lens or multiple lenses within a single structure (like our eyes or an eagle’s) and compound eyes are arrays of many lenses (think of fly eyes). The arrangement of these lenses and the photoreceptor cells are quite diverse and complex (even in the case of “simple” eyes) and although Darwin famously said that “the eye to this day gives me a cold shudder,” it’s fairly apparent why this is the case: organisms have deeply different needs from their eyes.

To attempt to briefly explain the evolutionary development of eyes, or even try to explain all the details of the different eye structures of even the few creatures below is laughable. So I’ll settle down into my childhood comfort zone and regurgitate some animal facts. I do this for your sake, because while eye evo-devo is a vast and fascinating subject, the real joy is knowing that slugs have no focus, and that birds can see an entire spectrum of light invisible to us.

Cats — Cats are obviously good at seeing at night. If you’ve ever caught your cat’s eyes in a flashlight, you also know their eyes are not like ours. Cats have a reflective layer called the tapetum lucidum, allowing them to see with much less light than us (around 1/6th the amount we’d need to see well). This is also why they have irises that can go from narrow slits to wide moons; they are extremely sensitive and need to protect themselves as they move between day and night.

However, while cats also have a better field of view than people, they don’t have very good visual acuity. Things aren’t very sharp to them and what we can see clearly at a distance is difficult for cats to discern. You know this too, cats sneak up and pounce, but when they try to leap onto a distant shelf they sometimes miss, which is why the internet was invented.

Because cats are crepuscular, seeing things in color is also less useful. But seeing shapes and movement is deeply important, both to eat and not be eaten. Cats have less cones (the photoreceptors responsible for seeing color) and way more rods (which are more sensitive to light) than us.

Pigeons — When you throw pigeons (specifically Columba livia) in the mix, you realize how lame our eyesight is. First off, pigeons can see 340º around them without moving their heads. That’s hard for us to comprehend, even if we’ve all seen 360º videos, but it is an understandable trait if you are a plump little sky chicken. We also have pigeons to thank for realizing that most birds see a broader spectrum of color than us, perceiving all those reds, yellows, greens, and blues but also beyond into the ultra-violet.

The discovery that pigeons had tetrachromatic vision didn’t create waves initially (it was an accidental realization), but when researchers started to explore bird vision more fully a whole host of implications for how birds see the world was discovered. Species that seem to display no obvious sexual dimorphism to us are distinguishable when they reflect UV light – sporting sex specific reflective patches. The Red-tailed Hawks sitting on the side of I-5 aren’t just hoping rodents will appear – they can see the ultraviolet reflecting urine of rats living in the highway median and know where to look.

Flies — That fly might appear to have two eyes symmetrically arranged on their heads, just like us, but they actually have two groupings of compound eyes. Somewhere in your brain you already know what compound eyes are but in case you forgot: each of the tiny hexagons that make up a fly “eye”, is an arrangement of individual apertures called ommatidia. Many species have thousands of them all with their own photoreceptors, all pointed in slightly different directions. It’s not quite like they have thousands of eyes, but it’s close.

Most compound eyes don’t have great resolution because the ommatidium all focus on slightly different places but they take in a lot of information, fast. (Insects that need to see particularly well, like dragonflies, bees, and drone flies have sections of their eyes that are flattened, allowing some of their compound eyes to see in the same plane, giving them better acuity.) Each ommatidium senses light individually, meaning they can collectively create an image in a fly’s brain faster than two individual eyes. Depending on the species color isn’t a strong part of a fly’s vision, but clearly emphasizing quick response time is worth sacrificing for.

Last and also least, slugs and snails — Terrestrial slugs and snails don’t need to see much. They are protected by a plethora of slimes and shells and their food doesn’t move. Slugs and snails have the tiniest of dark spots on the end of tentacles on their heads, which also smell things in their extremely myopic world. Their visual world is entirely black and white and without focus; like how we felt this morning after realizing it was daylight savings. These terrestrial gastropods need to find dark places to hide from predators and dehydration, and can also sense sudden changes in light that might mean a predator is looming over them. Not that they can do much about it besides curl up on themselves and hope for the best.

Vision is just one in a suite of senses that doesn’t exist in a vacuum – so quite possibly plucking it away from the rest of these creatures’ realities isn’t all that illuminating. But I still think it provides us with some wonder and joy at how diverse vision is, even within a small sample size. Now I’m going to go to sleep and see if I dream in color or not, because all this talk of eyes has made mine pretty tired.

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