Deer Create Mysterious Ultraviolet Signals That Glow in Forests
Did you know that deer can see ultraviolet light? A recent study has revealed an even more fascinating discovery: deer can also leave glowing trails visible in those wavelengths. This finding is shedding new light on how deer communicate with each other and perceive their environment.
During the autumn mating season, male white-tailed deer (Odocoileus virginianus) mark their territory in the forest by rubbing their antlers against trees and the forest floor. This process, known as 'deer rubs' and scent-marking scrapes, leaves behind scent marks in the form of glandular secretions, urine, and poop. These marks serve as signposts for other animals, warning rivals and attracting potential mates.
But here's where it gets controversial: scientists at the University of Georgia (UGA) have discovered that these marks glow in ultraviolet wavelengths, which deer eyes are capable of seeing. The resulting photoluminescence would be visible to deer based on previously described deer visual capabilities.
This is the first time scientists have documented evidence of any mammal actually using photoluminescence in their environment. While UV-induced photoluminescence in mammals has been studied for more than a century, this is the first time it has been observed in deer.
The study, conducted in a 337-hectare research forest called Whitehall, where deer roam freely, tracked down deer 'signposts' - 109 rubs and 37 scrapes - during two roughly month-long surveys in the fall of 2024. The team returned to each one at night with ultraviolet flashlights peaking at 365 nm and 395 nm, wavelengths that abound in the skies at twilight and dawn, when deer are most active.
Both of these wavelengths are visible to deer, and anything that glows bright enough under these torches would be easily visible to a deer's eye. The scientists used a tool that measures irradiance values to determine how much light is reflected or emitted at each wavelength from a given spot.
The team reports that rubs and urine found on scrapes exposed to 395 and 365 nm had greater average irradiance values (i.e., brighter) than the surrounding environment, and exhibited photoluminescence. It's unclear how much of this glow comes from the trees and shrubs and how much is from remnant deer fluids.
When deer damage plants, they expose woody lignin and plant terpenes, types of compounds also known to exhibit photoluminescence. The team notes that whether the photoluminescence is the result of deer forehead glandular secretions or wood properties, the fact remains that rubs visually contrast the surrounding environment in a way that is uniquely suited for deer vision.
The photoluminescence emitted by the deer signposts was the right kind to be registered by the cones within a deer's eye that are sensitive to short- and middle-wave visible light. This reaffirms that deer eyesight is adapted to the low-light conditions of dawn and dusk. More impressively, it suggests deer are communicating with light-up 'noticeboards' throughout the forest that the rest of us can't even see.
As to what the deer are saying? Until further research is conducted, we won't know for sure. However, the irradiance of rubs increased at the same time as deer hormone levels increased, and behavioral changes are known to change with the progression of the breeding season. This research was published in Ecology and Evolution.
