In the grasslands, top predators such as lions loudly announce their presence with roars, and they have less need to hide their whereabouts. However, for smaller deer lower in the food chain, making loud noises can be dangerous, so they rely more on quiet and secretive ways to communicate with each other. A new study published in the journal "Ecology and Evolution" shows that the "marking points" left by deer in the forest (such as horn scratches and ground digging marks) will produce fluorescence that is difficult to detect with the naked eye under ultraviolet light, and the deer themselves can see this "invisible light."
The research team pointed out that this phenomenon belongs to "photoluminescence" - that is, organic matter absorbs short-wavelength light and re-emits it in the form of longer wavelength. Previously, research on mammalian photoluminescence mostly focused on the animal's own fur or skin, with little attention paid to the environmental "luminescence" caused by animal behavior. This study turned the focus to environmental changes after deer activity, trying to figure out how the forest "lights up" under ultraviolet light and what potential role it plays among deer herds.
Daniel Dross-Brockett, the first author of the paper, and his colleagues systematically searched for white-tailed deer (Odocoileus virginianus) activity markers in an area of about 800 acres in the Whitehall Forest in the United States. They marked the locations with ribbons and GPS and sampled different types of marks, including 109 antler scrapes, 37 gouging marks and 20 areas of deer urine. Optical measurements show that when these markers are exposed to UV light, they emit a light signal in a specific wavelength band that is significantly stronger than the surrounding background.
In an interview, Dross-Brockett said that compared to humans, deer are more sensitive to blue light and ultraviolet bands, and their eyes are particularly sensitive to light in the range of about 450–460 nanometers and 537 nanometers. At the same time, the peak activity of white-tailed deer occurs at dawn and dusk. At this time, visible light weakens and ultraviolet light is relatively more prominent in the environmental spectrum, which is conducive to the capture of such covert fluorescent signals by similar species.
Researchers found that white-tailed deer rub their antlers and forehead glands on tree trunks, peeling off the surface bark to expose the inner "inner bark", and this exposed part will shine significantly under ultraviolet radiation. It's unclear whether the rubbing action itself or the uncovered tree tissue causes the glowing effect, but what is certain is that these "scratched trees" are likely to stand out to deer's eyes.
Another type of key mark is the "gouge mark". Deer dig up soil with their hooves under low-hanging branches (about one to two meters above the ground), leaving behind chemicals secreted by the interdigital glands between their hooves. These secretions are thought to possess photoluminescent properties themselves. In addition, deer also urinate in the same area, and the porphyrins and amino acids in the urine also fluoresce under ultraviolet light, making this small piece of land a signal center for the superposition of multiple "luminous messages."
Dross-Brockett described these marking points as a "community bulletin board" for deer. Individuals will sniff these marks to confirm what similar activities are nearby and determine the other party's breeding status and other information. The research team observed that during the deer's breeding season, the brightness of these marks under ultraviolet light was significantly enhanced. The researchers speculated that this may be related to the deer rubbing and marking more frequently and vigorously during this period.
In his opinion, white-tailed deer achieve a kind of "stealth communication" by creating these fluorescent signatures in their environment: barely visible or intelligible to predators, but a conspicuous signaling system to deer whose vision is adapted to the ultraviolet spectrum. However, scientists are currently unable to explain exactly what information these light signals carry, such as whether they distinguish gender, individual identity, or more complex social status.
Jonathan Goldenberg, an ecologist at the University of Oslo who was not involved in the study, cautions that caution should be exercised as to whether such "light signals" actually have a communication function. He pointed out that photoluminescence phenomena may be very common in nature, but this does not necessarily mean that all luminescence phenomena are actively used by animals for communication, so the interpretation of "light language" must be based on more experimental evidence.
This study, published in the journal Ecology and Evolution, provides a new entry point into understanding how deer and other mammals use a combination of vision and chemical signals to communicate covertly. It also prompts researchers to pay more attention to the "luminous traces" left by animal behavior in their habitats.