There are many ways to shine in the natural world. The best-known type of glow is bioluminescence, in which a chemical reaction occurs in the organism’s body to produce light. Only in this way can life shine in complete and utter darkness. But organisms can also glow through fluorescence, a process in which molecules absorb high-energy light and release that energy by emitting lower-energy light. When high-energy light shines on a molecule, it temporarily excites the electrons of that molecule. And when the molecule breaks free, it emits the excess energy as a photon of light with a longer wavelength. Organisms thus glow green, red, yellow, or orange under higher-energy UV light.
To many people, fluorescence may seem like a great party trick at an ice rink or club, because it often is. But some animals communicate with each other through fluorescence. In the oceans off Japan and South Korea, the charmingly named Flower Hat Jellyfish hunts prey with a fluorescent crown of bright pink and green tentacles. In 2015, researchers at the Monterey Bay Aquarium observed that juvenile rockfish were much more attracted to glowing jellyfish tentacles than non-fluorescent mimics. And the researchers found that the birds, called budgies, took less interest in mates whose fluorescent feathers were coated with UV-blocking sunscreen.
Some scientists believe that this covert communication could also occur on land, between creatures that are much more closely related to us. In the past few years, a group of researchers at Northland College in Wisconsin discovered that a number of mammals that appear uniformly brown in the daytime fluoresce under black light. These include hot pink flying squirrels and aquamarine platypuses, as reported by Cara Giaimo for New York Times. Curators at the Western Australian Museum began shining a black light on their specimens and found glowing bilbies, hedgehogs, porcupines and echidnas.
The glow was undeniable. However, other researchers have remained divided on what this means. Mammals communicate a lot with the patterns and colors of their fur; the stark black and white coat of a skunk warns predators of its noxious odor, and the white flash of a white-tailed deer warns the herd that danger is near. The scientists who discovered this proliferation of mammalian fluorescence therefore proposed that nocturnal or twilight animals could signal each other with their glow. However, it is not entirely clear whether these luminescent animals can even detect their own fluorescence, especially among the multitude of other light sources in the real world. For example, hot pink flying squirrels have lost the ability to see the UV light that makes them glow pink. And while our nails and teeth glow, we don’t see this fluorescence ourselves.
Now, a group of researchers from James Cook University in Australia has put this nocturnal hypothesis into practice in a paper recently published in Australian Journal of Zoology. They wanted to test whether predators would be more or less attracted to fluorescent rats, which could reveal whether the fluorescence is a way for the rats to camouflage themselves, or whether the glow is a way for similar species to communicate with each other.
The researchers bought 36 dead brown rats whose fur fluoresces whitish blue. They put the fur on a plastic rat model – presumably to make sure the experiments weren’t actually eaten – and sprayed half of it with UV-protected hairspray, then sprayed all the fur with regular hairspray to make sure they all smelled the same.
The researchers then placed their model rats in three different locations in the field: an avocado farm, a rainforest bordering a stream, and a forest. At each location, they placed two rat models, one lit and one not, in front of remote cameras triggered to record for 20 seconds. Rat models were chained to a camera to prevent predators from escaping with the experiment, which they often attempted. The team conducted the experiment on a new or full moon night because previous research had shown that full moons triggered the scorpions’ natural fluorescence in a way that flying insects could detect.
After reviewing videos from the test nights, the researchers found 11 species of marsupials, at least nine species of placental mammals and four species of birds interacting with the rat models. The team recorded which rat model the wild animals chose to interact with first to indicate their preference between the two. Dogs, cats, and snouted marsupials called northern long-nosed bandicoots interacted with mock rats at all three sites. Small carnivorous marsupials called quolls were perhaps the animals that most enthusiastically interacted with the rat models, often attempting to drag them away as prey. And although Eastern Barn Owls only arrived at the farm once per full moon, the two seemed to differ over their coveted prey.
But for all the animals captured in all the videos, the researchers found no significant preference for the glowing or non-glowing rats. All rat predators and similarly sized species approached the models with equal opportunity. The team offered two possible explanations for the finding: either that the full moon’s light wasn’t strong enough to elicit the rats’ fluorescence, or that the nocturnal animals simply weren’t interested in the fluorescence even though they could detect it.
Although other researchers have suggested that this fluorescence may offer animals camouflage in their environment, this group of researchers found little evidence for this line of thinking. In all environments—farm, forest, and rainforest—the fluorescent rats showed much more luminescence than the ground beneath them.
So has this new study solved the fluorescence puzzle? Not quite. Although the researchers found no evidence for existing theories about the function of fluorescence, more studies like this one would be needed to thoroughly unravel. It’s safe to say that scientists still don’t understand why this elusive coloration is so widespread among mammals, and what advantage it might offer the furry creatures roaming the land. When researchers recently shined UV light on 125 specimens representing more than half of the surviving mammal families, they found that every one of them fluoresced, suggesting that this kind of glow is not a joke but the norm. So the most likely explanation seems most likely – that all this fluorescence has no real function. After all, would a rat need a reason to glow ethereally at night? Sometimes beauty is a work of chance, we are lucky enough to learn to see.