On Deer Vision (Part 1): Why Do They Get “Caught in the Headlights”?

Have you ever heard of the saying, “like a deer caught in the headlights”? This idiom didn’t come from nowhere: deer “freezing” in headlights is a fascinating phenomenon that raises questions about animal behavior. The typical scenario goes like this: a car goes down a road at night and suddenly encounters a deer, causing it to become paralyzed and stare straight into the car’s headlights. Why does this happen?

A Deer’s Night Vision: Explained 

Deer are crepuscular creatures, meaning they are most active during twilight hours (before sunrise and after sunset) — a low-light environment. Unlike humans, a deer’s pupils are elliptical, which means that their eyes can dilate so much it can potentially cover the entire width of the orb. This allows them to take in as much light as possible during low-light times, improving their vision during the times when they are most active. 

They are also adapted to this environment with high ratios of rod cells to cone cells. These are the two main types of photoreceptors in an animal’s retina. Rods are used for sensing motion, which works best in low-light conditions, whereas cones are optimized in bright-light conditions and are used to detect distances and colors. 

“Deer have close to 10-fold the number of rod cells to cone cells”, says zoologist Dr Jochen Langbein. This grants them incredible vision during the nighttime but sacrifices their color perception and visual acuity (the clarity or sharpness of vision). 

Rhodopsin and Flash Blindness

Rod cells hold a pigment called rhodopsin, which absorbs mainly green-blue and red-purple light. Because of this and its affinity to the colors that are most abundant during the night, it allows deer to gain the ability for powerful night vision. 

However, when this pigment absorbs light, it breaks down. This is the reason why exposure to continuous light allows us to be less sensitive to it due to the absence of rhodopsin, as opposed to that moment of pause as our eyes adjust to sudden brightness when we wake up in the middle of the night and turn on our bathroom lights. 

This phenomenon is called flash blindness. The same thing happens to deer, but on a way larger scale. Their high levels of rod cells and very dilated pupils, compounded with the incredibly high intensity of light from a car's headlights, create a perfect storm of temporary blindness, leading to a “paralyzed” state. 

Deer Behavior

Sometimes, deer may look like they’re purposefully putting themselves in danger by running onto the roads. However, this behavior can be linked to their hearing and subconscious flight responses. The sounds of vehicles trigger a sense of panic in them, causing them to jump into action, but they often become confused about which direction to flee in, resulting in them jumping in front of vehicles.

Impact on Deer and Humans

Unfortunately, deer-related vehicle accidents are more common than you think. According to a US study in 2022, more than 2 million deer-vehicle accidents occur every year, leading to: 

• 10 billion USD in economic losses annually

• 59,000 human injuries and 440 human deaths

These accidents generally peak during the autumn — a deer’s mating season. During these times, male deer look for potential partners, and females try to evade them, leading to a spike in nighttime movement. This activity is what leads to an increase in the risks of a deer-vehicle collision. 

Conclusion

Deer being “frozen” in headlights is a complex yet captivating phenomenon that opens doors to studies in animal behavior and anatomy. Understanding this can not only expand our knowledge about deer but also inform drivers, especially those in countries with high rates of deer-vehicle collisions, on how to navigate roads more safely. 

Works Cited

Benson, Francesca. “Why Do Deer Freeze in Headlights?” IFLScience, 2 Mar. 2023, www.iflscience.com/why-do-deer-freeze-in-headlights-67782. 

Brown, Forrest. “Forget Sharks and Bears – It’s Deer That You Should Worry about Hurting You.” CNN, 21 Apr. 2024, edition.cnn.com/travel/deer-deadly-vehicles-collision/index.html. 

Cunningham, Calum X., et al. “Permanent Daylight Saving Time Would Reduce Deer-Vehicle Collisions.” Current Biology, vol. 32, no. 22, Nov. 2022, www.cell.com/action/showPdf?pii=S0960-9822%2822%2901615-3, https://doi.org/10.1016/j.cub.2022.10.007.

Fancied Facts. “The Intriguing Phenomenon: Deer Frozen in Headlights.” Medium, 13 Sept. 2023, fanciedfacts.medium.com/the-intriguing-phenomenon-deer-frozen-in-headlights-3c582003b541.

Melissa . “Why Deer Freeze in Your Headlights.” Gizmodo, 13 Aug. 2014, gizmodo.com/why-deer-freeze-in-your-headlights-1620684474.

“Vision in Animals.” Animal Eye Care, animaleyecare.com/useful-info/vision-in-animals.