Hopefully you’ve never hit a deer, or had an otherwise bad driving incident with one. But even if you haven’t had such an experience, you’ve probably heard the expression about stopping “like a deer in the headlights.” So what’s up with that; why do deer stop in the headlights?
Why Do Deer Stop in the Headlights?
Human eye anatomy and deer eye anatomy is actually kind of similar. You’ve got a pupil that lets in light to the eye. It gets reflected onto your retina, which is what sends all the visual input to the brain. Deer eyes are quite similar in principle, though there are some key differences.
First off, deer eyes are on the sides of their head. They lack a sense of depth perception like we have. You can kind of take your own depth perception away by covering one eye for a whole day. Probably isn’t going to turn you into a deer, though.
Anyway, deer eyes are on the sides of their head to give them almost 360 degrees of vision. Since prey animals like deer or rabbits don’t really chase things, they don’t really need to register how far away things are from them. They simply see something that could be dangerous and run the other direction. Conversely, predators like tigers or people need depth perception when they’re chasing their prey. They don’t need peripheral vision because there’s probably not a lot chasing them.
You might be thinking deer eyes must be particularly sensitive–and you’d be right. Deer pupils are not unlike cat eyes. Their pupils can narrow and dilate to cover almost the entirety of the eye. Well, not the eyeball. We mean the part of the eye that sticks out of the head. Think about it, you probably haven’t seen the whites of a deer’s eye.
Anyway, this allows deer to let in a lot of light, particularly useful at night.
You’ve probably heard of rods and cones before, they’re the photosensitive bits in the retina that are largely responsible for letting you see colors and stuff.
Cones are good at looking for color. You use them in high-light environments because there’s largely enough light to let you see stuff. So that excess lets you filter through different wavelengths. You have cones for looking at red, green, and blue light.
We don’t care about cones though. We’re interested in rods. These are responsible for your ability to see in the dark. They have no ability to distinguish between colors but they are really good at picking up minute details in the dark. That’s why your vision is essentially black and white in the dark.
Deer have way more rods, which is what gives them acute vision at almost all times–including nighttime. They even have this nifty thing called the tapetum. Basically it’s a wacky mirror that gets more light into the retina. We don’t have it, but animals like cats and raccoons that are active in the dark have them.
Deer are crepuscular by nature, which means they like to be active during twilight. Which is the time just before dusk and just after dawn. Which means deer eyes spend a lot of time almost fully dilated, picking up small scraps of light to get by.
Which means when you shove your headlights into their eyes, it’s obscenely bright to them.
Think about what happens in the dark when your friend puts a flashlight in your face. Nothing good. It takes a lot of time to get your eyes back in order, you don’t have a switch in your head that says “night vision time.” Plus, our eyes are far less sensitive than a deer’s. That shift is going to take a lot longer the more sensitive the eye is.
So that deer in the way of your car isn’t being stubborn or whatever, they’re actually just functionally blinded and waiting for their eyes to adjust.
The deer response mechanism to threats is also to freeze when scoping out a threat. If they don’t know what something is, they’ll stay still to identify it. You know, because at night, being still is a pretty good way to stay hidden. Blinded by light and trying to adjust, combined with the fact that a deer has no concept of what the heck a car is, they’re definitely not going to figure out your SUV is a threat before it hits them.
This blindness from going to extreme dark to extreme light has a name too. It’s called “bleaching.” Which is way better than putting bleach in your eyes (do not), but not as nice as r/eyebleach.
Deer don’t know headlights, but we do. See if you can’t identify car headlights here.