How To Attract A Mate The White-Tailed Way: With Ultraviolet Glowing Urine

That’s the findings of a recent paper from researchers at the University of Georgia, who spent three months analyzing deer markings across hundreds of acres of nearby Whitehall Forest. Such evidence of the animals’ presence is plentiful in the fall breeding season, when hormones surge and the deer instinctively spar, scrape, and spread their scent around in their best displays of dominance.

So, how does a hopeful young buck attract a nice doe? Well, there are a couple of ways to announce your availability: you could use your antlers to tear up the outer layers of trees – a tactic known as a rub; if you’re feeling very feisty, you could try a scrape, where you rub your head on an overhanging tree branch – perhaps you give it a chew too – then dig a little hole in the ground underneath and urinate into it.

To humans, these markings are fairly subtle. To other deer, however, they’re a massive glowing “XXX” sign: thanks to secreting glands in the deer’s heads and mouths, even those signposts that aren’t covered in pee are wafting out the aroma of stag-in-rut. “If you’re a one-and-a-half-year-old buck and you see a torn up tree […] you know there’s a big gentleman in the area so be cautious,” Daniel DeRose-Broeckert, University of Georgia Deer Lab graduate student and coauthor of the new paper, told Cool Green Science last month. 

But while it’s long been known that deer use these techniques to communicate via scent, the new study takes that “glowing” description and makes it literal. As the animals progress first into and then through their rut period, it turns out these rubs and scrapes start to glow under ultraviolet light – first dimly, and then very brightly indeed.

“The urine definitely glows,” DeRose-Broeckert told Science. To him, it “looks like spilled white paint,” he said, though he is colorblind – to his colleagues, the rubs and scrapes are greenish-blue, or violet if the deer has urinated on them. Either way, though, “it’s pretty striking.”

And for any female in the area, the combination of smell and visuals must be impossible to ignore. “You get stimulus from two senses,” DeRose-Broeckert said. “You have the sense of smell telling you something, and then, oh boy, it’s also superbright.”

Of course, this was all found under UV lights, which deer are not particularly known for carrying. Does that put the kibosh on the whole “mating signals” hypothesis? Well, not quite: in fact, deer vision is very well-adapted for low-light environments, and they can see ultraviolet hues without a problem. 

“Their vision is vastly different from ours,” explained DeRose-Broeckert in a statement this week. “Once the Sun is slightly gone around dusk and dawn, the UV light dominates for deer since it’s not being washed out by the visible light spectrum from the sun.”

There’s just one catch: just because the deer can theoretically see these markings, doesn’t mean they’re actually interacting with them in the real world. “To be certain that any observed fluorescence has a biological function one would also need to observe a behavioral response to it in the animal,” Ron Douglas, a biologist at City St George’s, University of London, who wasn’t involved in the research, told Science – and the new study reports no such thing.

That’s not a failing on the researchers’ part – that question simply wasn’t what they were studying. But it does mean that future research should seek to confirm that the deer are actually affected by the bucks’ glowing pee-stains – a question the team proposes to investigate by potentially painting over rubs and scrapes, or otherwise manipulating them to stop them from lighting up.

Either way, though, the discovery stands as proof that there’s more going on under our noses than we ever realize. And that, if you’re a randy deer, it can’t hurt to carry a tin of glow-in-the-dark paint.

The study is published in the journal Ecology and Evolution.