Have you ever been so shocked that all you could do was stand there – some might say with a deer in the headlights look? If you've ever used that phrase or witnessed a deer doing this in real life, you might find yourself asking "why do deer freeze in headlights?" Well, we're here to explain.
“Deer of several species tend to 'freeze' (i.e. stand very still without apparent reaction) to a stimuli when they cannot readily ascertain what they are seeing or hearing,” zoologist Dr Jochen Langbein, founder of Langbein Wildlife and head of the National Deer-Vehicle Collisions Project, told IFLScience.
Deer in headlights freeze “mostly during complete darkness when there is little ambient light to illuminate either surrounding countryside or the approaching vehicle, so that when staring into the bright headlights on full beam the animal is unlikely to be able to make out any object aside from the bright beam of light. Whilst even humans might be blinded in a similar manner when looking directly into a very bright light, deer are far more susceptible to this,” Langbein explained.
This is due to deer’s eyes being adapted to low-light conditions. Deer are most active at dawn and dusk, or twilight, making them crepuscular, so they have to be adept at navigating in the dark. Deer have “close to 10-fold the number of rod cells (that are x1,000 more sensitive to light than cones) to cone cells (which detect a wide spectrum of light photons and [are] responsible for color vision) than humans have”, Langbein said.
“Furthermore, the well-developed tapetum lucidum in deer (and other nocturnal mammals) acts as a reflector in the back of the eye that reflects light back over the retina, which further enhances the ability of photo receptors to form an image despite low light,” Langbein added.
If you’ve ever wondered why some animals appear to have glowing eyes in the dark, the reflective tapetum lucidum is the answer. In fact, reindeer are the only known animals that change their eye color in winter, with permanently dilated pupils both letting in more light and changing how tightly collagen is packed in the tapetum.
Deer also have a broad field of vision, described as part of their “security system” by Assistant Professor of Deer Ecology and Management at the University of Georgia, Dr Gino D’Angelo, on the Deer Season 365 podcast.
This wide-screen vision is thanks to eyes on the side of their heads and large, wide pupils: “We can go back to some of that work we did right in the deer’s retina actually labelling their cells. It showed us that they’ve got this horizontal visual streak,” D’Angelo explained. “They’re gonna pick up less detail, but it’s across this wide band that corresponds to that wider pupil they have to allow light in, corresponds to their wider-set eyes.” This allows them to bring in more information from their environment, scanning for things such as predators or potential mates.
“The downside of having such eyes situated on the side of the head is that you lose binocular vision and ability to focus clearly on objects directly in front, which leads to limited capability of depth perception and hence poor ability at judging the distance of how far away an approaching vehicle (which a deer may not even regard as a danger) actually is,” Langbein said.
In fact, researchers estimate that for a deer to see details visible to humans at a distance of 200 meters (656 feet), the deer would need to be much closer at 20 meters (65 feet) away.
How to prevent deer-vehicle collisions
Sadly, deer-vehicle collisions (DVCs) are all too common. In the UK, it is estimated that between 40,000 and 74,000 deer are injured or killed on the roads per year. In the US, over 1.3 million insurance claims for DVCs are filed every year, with the actual number of collisions estimated to be near 2.6 million. So how can we prevent other deer in the headlights from suffering the same fate?
“The main advice to drivers in order to reduce likelihood of deer freezing ahead of them in the road is to ensure dipping vehicle headlights and slowing down, so that the deer is more likely to be able to distinguish the shape of the vehicle through the glare of lights and see its escape route,” Langbein explained. “Overall, DVC prevention should not look for any single failsafe solution, but combine a number of differing complementary approaches best suited to the local situation, road type, speed and habitat.”
Deer can get used to roadside deterrents, making them less effective. Instead, Langbein says, “measures more likely to be effective at reducing DVCs are ones able to alter driver behavior.”
These could include: “i) digital deer / speed warning signs illuminated when entering road sections with high potential of deer crossing; or triggered when animals are ahead on/close to the road, ii) other traffic calming measures, including local publicity campaigns, iii) fencing to guide deer to safe crossing areas (ideally underpasses or bridges) or else at least to safer areas with good visibility where drivers can see animals well ahead of them, and vice versa.”
A study, published in 2020, on free-ranging white-tailed deer at NASA’s Plum Brook Station testing facility investigated whether an LED bar illuminating the front of vehicles could decrease the risk of DVCs. The researchers found that the LED bar reduced the likelihood of a dangerous DVC from 35 to 10 percent, which they write was “driven by fewer instances of immobility (freezing) behavior by deer in response to the illuminated vehicle.”
Another study, published in 2022, analyzed 1,012,465 DVCs in the US, finding that a switch from daylight saving to standard time leads to a 16 percent jump in DVCs, with collisions 14 times more likely to occur 2 hours after sunset than before sunset.
The researchers estimate that a permanent switch to daylight savings time could prevent 33 human deaths and 36,550 deer deaths a year. “It surprised me how striking this pattern was, of how much more likely deer are to get struck in the hour or two after darkness," first author Calum Cunningham said in a statement. “This one-hour shift in human activity could have such a significant effect.”