FEATURE Lindsay Thomas Jr.
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In the light of day, our eyes see the pale yellow or white wood exposed by a buck that has shredded the bark off a small tree. But in twilight woods, deer eyes likely see a blue or purple light emanating from fresh rubs like glow sticks drawing them to a hub of social information about their deer neighbours. According to brand new science, rub lines appear to deer like a line of illuminated trail markers guiding them along an avenue of breeding opportunity.
We’ve known for a while through studying the anatomy of deer eyes that their vision is more sensitive than ours to the blue end of the spectrum. Where we have the visual receptors or ‘cones’ for interpreting red, blue and green hues, deer lack red cones altogether. Instead, they are especially sensitive to the deepest blues and purples of the ultraviolet range, where our eyes can’t follow. Why might deer need to see ultraviolet light that our eyes can’t receive?
Previous research revealed that many animals produce a ‘photoluminescent’ glow visible to other animals with UV sensitivity, but the University of Georgia (UGA) Deer Lab recently conducted the first-ever study of environmental photoluminescence used for communication in animals. They looked for a glow coming from rubs and scrapes, and they couldn’t believe their eyes.
Midnight Science
Much of what we know about deer visual capabilities comes from research at the UGA Deer Lab, including that deer have dichromatic vision with two peak sensitivities, one at 450 nanometers (dark blue) and another at 537 nm (green). Our trichromatic vision has three peak sensitivities: 420 nm (very dark blue-violet), 530 nm (green), and 565 nm (orange-red). Below 400 nm, known as ultraviolet light, deer are 20 times more sensitive than humans. Our lenses filter out almost all UV light, preventing us from seeing shorter wavelengths that are visible to many other animals, including deer.
UV light is with us during the day but is overpowered by visible light, so these shorter UV wavelengths make up much of the available light after sunset. Though it lingers deep into the night, and the moon sheds some, UV light is most abundant around sunrise and sunset. Deer eyes are adapted to these low-light conditions at dawn and dusk when UV light is dominant, which matches up well with their typical peak hours of activity.
Researchers at UGA wondered if there might be more to UV sensitivity than just seeing well in the twilight hours. For example, some natural materials like certain mushrooms are photoluminescent. They don’t produce their own light, but when exposed to shorter wavelengths like UV, they emit longer wavelengths, which changes their color. Different photoluminescent materials can emit a variety of different colors. If you’re thinking about the glow of stains revealed by a blacklight, you’re coming out of the dark.
Daniel DeRose-Broeckert, a graduate research assistant at the Deer Lab, designed an experiment to look beyond deer into their habitat. Throughout fall 2024, UGA student volunteers and faculty, including Daniel’s supervisor Dr. Gino D’Angelo, hunted for fresh rubs and scrapes on Whitehall Forest near the UGA campus, an 840-acre research site with wild, free-ranging deer. Each fresh rub or scrape was marked with a GPS waypoint so Daniel could find them when he returned well after sunset in complete darkness. He needed to be in control of light sources and not be subject to variables at dawn and dusk like cloud cover. Working sometimes until midnight and beyond, he shined 365 and 395 nm UV lights on rubs, scrapes and their surroundings, and he measured emitted wavelengths with a spectrometer.
“I chose to use 365 and 395 nanometer lights because those fall within the UV range that is mostly present during crepuscular hours,” said Daniel. “Having more than one light source was important because photoluminescent materials can react to one set or a single wavelength of UV light but not another.”
Overall, Daniel collected data from 109 fresh rubs and 37 fresh scrapes. He took measurements on numerous nights in two rut phases based on previous research into rut timing at Whitehall Forest:
- Pre-rut: September 8 to October 2, which includes peak rubbing activity and early scrape activity at the study site.
- Peak Rut: October 14 to November 15, which includes late rubbing activity and peak scraping activity at the site.
Radiant Rubs
The results for rubs were significant, Daniel told me. At each site, he tested rubs, undisturbed bark of the same tree, plus dirt and leaf litter on the ground. Rubbed trees produced significant photoluminescence of a different wavelength than background light or light reflected off surrounding materials. Incredibly, the photoluminescence was in a range that closely aligned with a deer’s visual sensitivity. In particular, the 365 nm light source produced photoluminescence that almost perfectly aligned with deer sensitivity, though 395 nm also produced photoluminescence uniquely visible to deer.
Daniel wanted to understand the source of this photoluminescence, so he dug into previous research. He found that chemical compounds like terpenes in tree sap, including limonenes that are largely responsible for pungent odors such as pine scent, are known to be photoluminescent.
“Cambium and inner sapwood layers of wood are also known to produce chemicals that photoluminesce,” said Daniel. “The photoluminescence of wood is so characteristic that it’s used for identifying wood from different tree species.”
However, Daniel also found previous research identifying the chemical components of buck forehead gland secretions. He cross-referenced some of those chemical components, such as phenols, with other research showing that they photoluminesce. The glow of fresh rubs was not as bright in the pre-rut as what Daniel measured in the peak of the rut, which is also when a buck’s forehead gland is most active.
“It could be the chemicals produced by removing the bark and exposing the cambium layer, or it could be forehead gland secretions, or it’s both,” said Daniel. “But either way, there is a glow that is uniquely visible to deer.”
Daniel also noted that both potential sources of photoluminescence on rubs – tree sap and buck forehead gland secretions – produce strong, unique scents. So rubs offer both visual and scent clues for deer that help them see, locate and learn from these signposts.
Shimmering Scrapes
Unlike rubs, scrapes were a disappointment in the pre-rut phase. Scratched dirt is dead and dull in UV light beams and did not give back any exciting wavelengths. Licking limbs proved too small and difficult a surface to measure, so Daniel did not have enough data for reliable analysis there.
As it turned out, pre-rut scrapes did not glow for a good reason – there was no urine in them. Though bucks were making fresh scrapes in the pre-rut, none of them contained urine until the second phase of the study. Then, scrapes started lighting up in the UV beams.
“I could see the urine in the scrapes with my own eyes,” said Daniel. “It looked like spilled milk. What’s cool about it is that the wavelengths produced by the urine in scrapes align very nicely with a deer’s peak visual sensitivity. If a deer did not have a nose, and they walked up to a scrape, they can see how much use it’s getting like it’s spilled white paint.”
Again, Daniel went looking for the source of photoluminescence in urine. It’s the same in deer as for humans: the likely source, based on abundant research, is organic compounds including porphyrins and amino acids. As with rubs, these materials produce a photoluminescent glow measurable by Daniel’s spectrometer that is not produced by dirt or leaves in and around the scrapes.
Another interesting observation Daniel made about urine in scrapes: “They always seemed to urinate on one side of the scrape or the other,” he said. “It wasn’t random or scattered all the way around the scrape, it was always on the same side. It’s as if they all approached a scrape from the same direction and stood in the same position. I think either they’re trying to refresh their own scent and visual signs, or they’re trying to cover up another buck’s urine.”
Science Illuminates Again
Deer are very likely seeing a different world than our eyes reveal at twilight. When UV light dominates at dawn and dusk, fresh rubs on trees likely glow blue to purple, as does fresh urine in scrapes, helping these features stand out from a distance even to deer that are not receiving any of the scent information on the wind. It’s possible there are even more environmental features that produce a glow deer can see.
“I think that rubs look like highway reflectors to deer,” said Daniel. “That’s how I imagine it. When the rest of the woods are dark, the rubs and the urine in the scrapes are highlighted because it’s throwing a different color, a brighter color, than the light that’s contacting it.”
Daniel’s advisor Dr. Gino D’Angelo agreed. “This really drives home how appropriate it is to describe these markings as ‘signposts’,” said Gino. “We now know that signposts glow like the neon lights of the Honky Tonk Highway in Nashville.”
Just as we hang flags and pin reflective markers in trees to help us find deer stands in the dark, a deer’s ability to detect the photoluminescent glow of signposts is likely about efficiency: in locating and evaluating other deer, advertising their own attributes, finding a mate, and even surviving.
“The glow of signposts could speed transit of deer like walkway lighting, adorn the woods like holiday lights to ring in the breeding season, serve to intimidate deer like graffiti from rival gangs, and make it easier for deer to locate signposts to check their scent from a distance without having to approach as closely,” said Gino.
Daniel’s findings also emphasize the notion that deer are crepuscular animals – active at dawn and dusk – and not nocturnal or diurnal. Though they certainly move in darkness and daylight, there is more visual information available to them when UV light is strongest. Deer movement studies consistently showing dawn-and-dusk peaks make even more sense now.
The next time you’re in a deer stand straining your eyes to see deer that are moving with ease and grace through the twilight woods, remember they’re not only seeing everything much better than you can in low light. They’re probably guided by glimmering photoluminescent features hidden to our eyes, which we can only imagine. To me, that’s one of the coolest deer facts science has illuminated in a long time.
About Lindsay Thomas Jr.
Lindsay Thomas Jr. is NDA's Chief Communications Officer. He has been a member of the staff since 2003. Prior to that, Lindsay was an editor at a Georgia hunting and fishing news magazine for nine years. Throughout his career as an editor, he has written and published numerous articles on deer management and hunting. He earned his journalism degree at the University of Georgia.