Forests glow under UV light and deer can see it

When a predator like a lion walks through the grasslands, it makes its presence known by roaring, which can be heard for miles. Since the lion is at the top of the food chain, it doesn’t really need privacy. But for animals like deer, making loud noises isn’t exactly the safest way to communicate. To get noticed, deer mark areas with scuffs and scratches, known as signposts. Now, a new study in Ecology and evolution reports that these signposts have a hidden glow and other deer can see them.

In the study, the researchers describe this hidden glow as photoluminescence, the process by which an organic object absorbs short-wavelength light and re-radiates it at a longer wavelength.

“This study provides an interesting first look at how UV light emissions might appear on deer signposts in the field,” says Jonathan Goudenbergan ecologist at the University of Oslo, who was not involved in the research.

Most previous research on photoluminescence in mammals has focused on the animals themselves, such as in their fur or skin, with little attention to environmental photoluminescence. To investigate how the environment might glow in response to animal behavior, the study’s lead author, Daniel DeRose-Broeckert, and his colleagues surveyed an 800-hectare area in the Whitehall Forest and located white-tailed deer (Odocoileus virginianus) active signposts.

The team marked these signposts with marker tapes and GPS points. Of the locations, they analyzed 109 antler rubs, 37 abrasions and 20 urination spots. The light measurements showed that these markers emit light in wavelengths that stand out from the surrounding environment when exposed to ultraviolet (UV) radiation.

In an interview, DeRose-Broeckert told us that deer are more sensitive to blue wavelengths and UV light than humans. The sensitivity of a deer’s eyes picks up the wavelengths emitted by the glow, i.e. the range of 450-460 nm and 537 nm. Also, deer are most active at dawn and dusk, when sunlight is low and visible light fades, making ultraviolet wavelengths more prominent.

White-tailed deer rub their antlers and brow gland on the tree bark to remove the outer layer of the trunk. These exposed “underbark” parts of the tree glow under the UV light. However, the researchers are not sure whether it is the friction that causes the shine or the exposed part of the tree.

Another signpost followed during the study was deer scratches. This is when deer pass under a low-hanging branch, probably one to two meters above the ground, and grip the ground below with their hooves. When they do this, they release scent particles from their interdigital gland, which is located between the hooves. The compounds released by this gland are known to be photoluminescent. Additionally, deer urinate in the same area. Urine also glows due to the presence of porphyrins and amino acids.

“Signposts are a kind of community sign,” DeRose-Broeckert told us in an interview. “Deer go, and they smell it just to see who’s around and also to check their breeding status.”

The team noticed that the signposts glowed brighter during the breeding season. The scientists hypothesize that more intense rubbing generates this brighter glow.

DeRose-Broeckert told us that white-tailed deer make these environmental signposts as a form of cryptic communication. Actually, we still don’t know exactly what is being communicated.

Goldenberg cautions that “any interpretation of communication must be approached with the moderate understanding that photoluminescence can be widespread without being functional.”

The research was published in Ecology and evolution.