eli5, why do deer run at the slightest sound or movement, but just stand there and stare when a car is coming towards them at full speed?
In: 10337
I would disagree that they run at the slightest cause. Their primary defense is stealth, not being seen. Movement defeats that. For the most part they freeze for that purpose. Most of the deer I’ve seen killed ran in front of, or into the car when they panicked at the last moment.
When they feel they are being seen they freeze to become invisible to predators. Their instinct isn’t prepared for cars sadly.
Deer are generally prey animals and their only defence is their ability to run fast. As a herd animal they are sensitive to any dangers that may be in their environment, so slight rustling of dangers or gunshots will send them off. They also may freeze in other threading situations.
But cars are different because they travel so fast which deer don’t expect, the headlights generally dazzle their vision in the middle of the night when their eyes are focused for the dark. Any traffic coming is hard to judge due to the blinding and unfamiliar noise of a car coming at speed.
Basically, they evolved to be extra sensitive to their environment during the day and night but not for large speeding vehicles.
Bright lights at night don’t occur naturally and deer have not evolved seeing that stimuli as a threat. That said, I suspect if loud noises occurred simultaneously from multiple directions deer would react similarly since they don’t know which direction to run away from.
Source: I hunt deer
Deer either freeze completely or immediately bolt when they detect something, they are super sensitive to movement and sound. They also have a reflective layer on their retina that bounces light back through their photoreceptors so they can see better at night (this is why lots of animals eyes are shiny).
Car headlights completely blind them and they aren’t able to see that it’s a giant thing moving at quick speeds directly towards them, so their instinct is to freeze until they see movement / figure out what it is.
You’ll notice that they’re more likely to not freeze in front of a car during the day because they can see it moving closer.
One thing that others haven’t mentioned: prey animals like deer and squirrels and zebras escape predators by being nimble, not fast. They can’t outrun hawks and wolves and lions in the wild, so their defense mechanism is to stand still as the predator comes towards them, and then to abruptly change direction at the last second and run off. With any luck, the predator’s momentum will cause them to overshoot the deer, forcing the predator to stop, turn around, and restart the pursuit, giving the deer time to escape.
That works great when you have a predator who is tracking you and trying to intercept where you are. It works poorly when your “predator” is a huge car headed in a straight line roughly towards where you are. The car isn’t adjusting its course to hit you, so it’s not going to be fooled if you change direction at the last second.
Grad student here!
Deer protect themselves from predators by waiting until they’re a certain distance away, then bolting. This makes them very difficult to catch and lets them avoid exhausting themselves every time a potential predator is in the area.
This flight distance is not significantly affected by the speed of approach. In other words: the deer *absolutely* see that car approaching, but it’s often traveling faster than any natural threat. When that car is three seconds from impact, it may still be too far away to be inside that flight distance. Therefore, they wait another second or two before bolting–and get run over instead.
Also, deer have an unfortunate tendency to keep moving in whatever direction they were facing to begin with. If a deer is on the roadside and you cross into its flight distance, it might bolt straight forward–in other words, right in front of you.
Edit: Since there seems to be some confusion, I’ve given multiple presentations on whitetail deer behavior and how environmental factors affect risk. I also worked with a deer ranch prior to grad school, which is how I got interested in the first place.
[This paper](https://www.sciencedirect.com/science/article/abs/pii/S1361920919315330) and [this one](https://www.sciencedirect.com/science/article/abs/pii/S0001457519302052) both have some good background info.
I think especially at night animals get confused by headlights. I’ve also watched cats trying to cross the street at night and watched them wait for headlights to do so and almost get killed. It’s like they don’t know the light source is attached to a car and it probably causes their eyes to adjust and not see well in the dark so they need the light to see.
My father used to tell me that deer have three main senses. Sight, Hearing, and Smell. If you think about it they do have large eyes, large ears, and long noses. His theory was that if you triggered two of those senses at a given time they would take off. I think cars blind them and the can’t smell it or sometimes hear it coming.
Once they are startled they tend to go back the direction they came from. GENERALLY… I have my doubts sometimes because I’ve seen deer get shot at and just stand there but I do believe my father to be mostly correct.
Experiment: Wait until midnight. Let your eyes get well adjusted to the dark. Then, run through a forest of trees.
Shine a bright light into your face and stare at it for a few seconds. Run through the forest of trees again.
You will soon discover why deer stop running and stand in the middle of the road after shining bright lights into their eyes.
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