If you’ve ever watched fireflies glowing on a summer evening, then you’ve seen bioluminescence. Fewer people, however, have likely witnessed a different phenomenon: biofluorescence. Put simply, this is what happens when a living thing absorbs outside light and then re-emits it as light of a different color. Sometimes the trick goes unnoticed; most people can’t see ultraviolet (UV) wavelengths with the naked eye. But at least we can use blacklights to recognize the biofluorescent beasts who lay hidden in our midst—including these 11 animals you’d never suspect.
1. Flying squirrels
Outside Rocky and Bullwinkle cartoons, flying squirrels never fly. Instead, they glide, using skin flaps to drift gently downward after leaping off elevated surfaces—an ability the gray and red squirrels who probably stalk your backyard definitely don't have. One other thing that sets some species of flying squirrels apart from their grounded counterparts? A glow. In 2019, scientists discovered that three species of New World flying squirrel all have fur that looks bright pink under a blacklight. They announced their discovery in a paper published in the Journal of Mammalogy that year. Nobody knows what purpose (if any) the squirrels' biofluorescence serves.
Under UV lights, most scorpions adopt a “soft blue-green glow,” according to an expert at Australia's Queensland Museum. The majority of the arachnids are nocturnal, and seem to avoid UV light—including moonlight. It’s been proposed that the glowing adaptation is a built-in alert system meant to discourage scorpions from wandering around on bright, moonlit nights, when they’re more likely to be seen by a predator.
They may look like plants, but corals are animals—and some are biofluorescent animals housing symbiotic algae that supply them with nutrients. These corals are capable of throwing psychedelic light shows to the benefit of nature documentary crews and the algae. When the animals reabsorb light, they can shield the single-celled algae from harmful UV rays or help them photosynthesize.
A 2014 study identified more than 180 biofluorescent fish species, from eels to rays to gobies. Certain catsharks—small-bodied predators with elongated eyes—made the list, too. Though the human eye can't see their hidden colors, research involving a “shark’s eye camera” has shown that two species, the chain catshark and the swellshark, can see members of their own kind biofluoresce. Interestingly, the skin patterns on wild, glowing swellsharks become clearer as the fish dive deeper.
5. Sea Turtles
While searching for biofluorescent sharks off the Solomon Islands in 2015, marine biologist David Gruber and his team passed a hawksbill sea turtle. The blue light from their camera set the turtle aglow, its shell brightening the ocean with neon colors. “It almost [looked] like a red and green spaceship,” Gruber told National Geographic. Never before had anyone documented biofluorescence in a wild reptile. Since the turtles live around glowing reefs, the adaptation could be a way for them to blend into their surroundings. Or maybe it’s just the byproduct of the algae that reside on their shells.
Given their duck bills, beaver tails, and venomous feet, you’d think platypuses couldn’t get any weirder. But in 2020, biologists revealed these egg-laying Australian mammals are biofluorescent as well: Their fur glows blue, teal, and green under UV lights. The reason behind their colorful glow remains a mystery.
Found on the craggy coastlines of Ireland, Scandinavia, Russia, France, Northern Canada, and Maine, the Atlantic puffin has a colorful beak designed to carry mouthfuls of fish. When ornithologist Jamie Dunning shone a UV light onto the corpse of one bird in 2018, some of the ridges on its bill glowed neon blue. Scientists theorize that potential mates might find the display alluring.
Popular with first-time bird keepers, budgerigars are sold in pet stores around the world as “budgies” or “parakeets.” Not only are the creatures friendly and personable, but they look pretty cool when lit with a blacklight. Experiments have shown female budgies use plumage biofluorescence to judge male suitors.
Teddy bears were famously named after U.S. President Theodore Roosevelt. Toy manufacturers tried to duplicate their success in 1909 with the launch of—wait for it—“Billy Possums.” Named in honor of Roosevelt’s successor, William Howard Taft, the marsupial dolls didn’t last very long. By the time Taft made his reelection bid in 1912, they’d become a forgotten fad. Who knows? Maybe Billy Possums would’ve sold better if the fact that real opossums are biofluorescent was common knowledge back then. As with other surprisingly radiant creatures, the reason opossums glow remains unknown.
For years, springhares baffled scientists. The nocturnal, rabbit-sized rodents, which look a little bit like kangaroos, were lumped together with other animals before finally being given their own taxonomic family. And the creatures weren't done perplexing researchers yet: In February 2021, scientists announced that South African and East African springhares turn pinkish-orange in the presence of a UV light. “We were equal parts shocked and excited. We had so many questions,” Eric Olson, one of the scientists behind the discovery, told The New York Times. Unlike all the other nocturnal animals with biofluorescence, the springhare is an Old World placental mammal—the first known species within this group to glow.
11. Pac-Man Frogs
In 2020, biologists Jennifer Y. Lamb and Matthew P. Davis took a close look at members of 32 amphibian species, representing 14 distinctive families. Every single one was found to be biofluorescent, including the eastern tiger salamander and Ceratophrys cranwelli, a big-mouthed predator nicknamed the “Pac-Man frog.” There's still much to learn about why so many amphibians glow. Speaking to Mental Floss last year, David said, “Now that we know that this phenomenon exists across amphibians, there’s all kinds of interesting applications that future researchers may head toward.”