The forest may be glowing—at least to deer
Deer antler rubs and hoof scrapes change how parts of the forest reflect short-wavelength light, perhaps leaving a glowing signal
Deer signposts may be extra reflective in twilight.
Stan Tekiela Author/Naturalist/Wildlife Photographer/Getty Images
At dawn and dusk, forests often appear muted and nearly monochromatic to human eyes. But white-tailed deer might see a very different landscape: to them, the forest could be aglow.
Since the 1970s, biologists have understood that deer leave signposts—spots where they have rubbed their antlers on trees or left urine on ground that they scraped with their hooves—for scent-based communication. But one group of researchers wondered whether these marks also provide visual messages.
The researchers scanned and analyzed 146 such signposts in Whitehall Forest in the state of Georgia using ultraviolet lights. They found that although the signposts look unremarkable in daytime lighting, they reemit blue-green light that deer can see when exposed to ultraviolet wavelengths common at dawn and dusk. This might happen because antler rubs strip bark away, revealing lignin-rich inner wood that reemits the longer wavelengths in a way bark does not.
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“If anyone’s seen a reflector on a tree, a rub is similar to that—although obviously not as stark,” says study co-author Daniel DeRose-Broeckert, a biologist at the University of Georgia.
The visual contrast also intensifies as the breeding season approaches. In their study describing the findings, published in Ecology and Evolution, the authors argue that these later marks are brighter because male deer antlers have fully hardened and their rubs can expose more inner wood. Increased gland activity that accompanies rising testosterone may also add incidental biological residues to the surface.
This shift in light is particularly relevant, the authors say, because the eyes of white-tailed deer have enhanced sensitivity to short- and middle-wavelength colors, especially under low-light conditions.
Traditionally, mammalian biofluorescence, such as glowing fur in bare-nosed wombats and bandicoots, has been studied as a property of the animal itself. Biological material such as semen also fluoresces under UV light. This work suggests that biofluorescence might also be embedded in the environment, even if only subtly—adding a largely unseen layer to how animals might be communicating.
Not everyone is convinced that these fluorescent contrasts would be visible to deer under natural conditions, however. “If humans do not see these markings under natural light, then deer are unlikely to see them either,” says Almut Kelber, an ecologist at Lund University in Sweden, who was not part of the study. Demonstrating a visual role would require behavioral tests that separate sight from smell, she adds.
DeRose-Broeckert describes the study as a first step—it documents that rubs and scrapes can generate contrast at wavelengths deer can see and lays the groundwork for future experiments with natural lighting.
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