Deer may see hidden glowing signs in forests

The forest may be shining, at least for the deer

By Anirban Mukhopadhyay edited by Sarah Lewin Frasier

At dawn and dusk, forests often appear muted and almost monochrome to human eyes. But white-tailed deer might see a very different landscape: for them, the forest might be illuminated.

Since the 1970s, biologists have understood that deer leave signposts — places where they have rubbed their antlers on trees or left urine on the ground that they have scratched with their hooves — for communication based on smell. But a group of researchers wondered whether these brands also convey visual messages.

Researchers scanned and analyzed 146 such signs in Georgia’s Whitehall Forest using ultraviolet lights. They found that although the panels appear unremarkable under daytime lighting, they re-emit a blue-green light that deer can see when exposed to the ultraviolet wavelengths common at dawn and dusk. This can happen because the wood rubs the bark off the strip, revealing a lignin-rich interior wood that re-emits the longer wavelengths in a way that the bark does not.

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“If someone saw a reflector on a tree, a friction is similar to that, but obviously not as brutal,” says Daniel DeRose-Broeckert, study co-author and biologist at the University of Georgia.

The visual contrast also intensifies as the breeding season approaches. In their study describing the results, published in Ecology and evolution, The authors argue that these later marks are brighter because the antlers of male deer have completely hardened and their rubbing can expose more of the inner antler. The increased gland activity that accompanies increased testosterone can also add incidental biological residue to the surface.

This change in light is particularly relevant, the authors say, because white-tailed deer eyes have increased sensitivity to short- and mid-wavelength colors, especially in low-light conditions.

Traditionally, biofluorescence in mammals, such as the shiny fur of wombats and bare-nosed bandicoots, has been studied as a property of the animal itself. Biological materials such as semen also fluoresce under UV light. This work suggests that biofluorescence might also be integrated into the environment, even if only subtly, adding a largely invisible layer to how animals might communicate.

However, not everyone is convinced that these fluorescent contrasts would be visible to deer under natural conditions. “If humans don’t see these marks in natural light, it’s unlikely that deer will see them either,” says Almut Kelber, an ecologist at Lund University in Sweden, who was not involved in the study. Demonstrating a visual role would require behavioral tests separating sight from smell, she adds.

DeRose-Broeckert describes the study as a first step: It demonstrates that rubbing and scraping can generate contrast at wavelengths that deer can see and lays the groundwork for future experiments with natural lighting.

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