I was out riding my bike when a squirrel ran across the trail in front of me. The closer I got, the slower it ran. I was expecting it to turn around and go back the way it came, even though it was almost to the other side, but it made a final mad dash and crossed the trail.
Squirrels are legendarily bad at crossing roads. In the fall their little corpses litter the streets, almost as plentiful as dead leaves. There's even an Internet meme about how bad squirrels are at this.
You'd think they'd be good at evading cars. They can run pretty fast, and they evolved to avoid predators swooping down from the skies. But they're terrible at avoiding cars. So I got to wondering why.
When (most) people cross the road, they look both ways before they cross, then they go. Squirrels don't do this, I noticed on my ride. Their head faces forward, perpendicular to the road. Because squirrels are prey animals, their eyes are mounted on either side of their heads.
This is an advantage when you're in the middle of field, because they have a 360 degree field of view: if anything moves, they can see it and flee it.
Humans are predators, so their eyes are mounted on the front of their heads. They have a field of view of about 150 degrees. This requires us to turn our heads to do things like cross the street. This might seem like a disadvantage, but it's an advantage in this case: two eyes provide binocular vision, and therefore depth perception. This makes it much easier to estimate the distance and speed of oncoming cars or creatures.
When crossing the road, squirrels just charge out. They don't turn their heads to look with both eyes: they keep their heads pointing forward. This allows them to see traffic in both directions, but it hinders their ability to accurately gauge the distance and velocity of oncoming vehicles. They can see the cars, but they can't really tell how far away they are or how fast they're going.
Also, because squirrels have a field of view of almost 360 degrees, and not just 150, they're getting a lot more data all at once, which means more to process, which slows down decision making.
Finally, squirrels were evolved to evade predators. If a hawk is bearing down on you it's probably not a bad idea to retreat to your latest safe location.
My hypothesis, therefore, is that the combination of monocular vision, information overload and millennia of evolutionary programming have conspired to make squirrels really bad at crossing the road.
Raccoons are equally bad at crossing the road, based on the roadkill count. But their eyes are front-mounted, so they don't have the squirrels' excuse. They're just slow and fat.
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