The Glowing Secret That Mammals Have Been Hiding

At first, it seemed to be another caprice of two already unusual animals: Flying squirrels and platypuses were found to be fluorescent, absorbing invisible ultraviolet light and re-emitting it in shocking pink or bright cyan.

But they are far from alone. According to a paper published in the journal Royal Society Open Science this month, lions, polar bears, scaly-tailed possums and American pikas also fluoresce. So does every mammal species a group of scientists could get their hands on.

While this large survey of museum specimens doesn’t reveal any broad evolutionary benefit, it overturns the view of mammal fluorescence as an occasional and mysterious quirk. Instead, it appears this trait is “basically the default,” said Kenny Travouillon, curator of mammalogy at the Western Australian Museum and the paper’s lead author.

While scientists have documented fluorescent mammals for over a century, there has been a flare-up of interest in the topic in the past several years. Researchers shining blacklights into backyards, forests and museum cabinets have come away with a crayon box of discoveries.

Most of the resulting studies focused on one species, or a few, “trying to better understand the nuances of the trait” in a single mammal type, said Erik Olson, an associate professor of natural resources at Northland College in Ashland, Wis., who helped to uncover fluorescence in flying squirrels, platypuses and springhares.

He was not involved in the new study, in which researchers investigated museum specimens of 125 species belonging to more than half of existing mammal families, from Antilocapridae (pronghorns) to Vespertilionidae (vesper bats).

They found some fluorescence in all of them. The surveyclearly establishes a broad distribution of the trait within mammals,” Dr. Olson said, “something I did not expect.”

Dr. Travouillon said the idea for this survey blinked on in 2020 when the platypus discovery spurred researchers at the Western Australian Museum to point a UV lamp at their own collections. They found turquoise-tinged wombats and bright-flanked flying foxes. But were these stuffed specimens really glowing? Or could something else, like preservatives or fungus, be to blame?

Joining up with colleagues from Curtin University in Perth, the team used a spectrophotometer to expose the specimens to UV light and analyze any emitted fluorescence. They also tested newly acquired specimens of several species — including a platypus, a koala and an echidna — before and after they were preserved.

Preservation with borax and arsenic affected the intensity of fluorescence, increasing it in certain cases while dampening it in others. But it never created fluorescence where there wasn’t any.

This before-and-after testing is “a great contribution to understanding the effects of museum preservation on fluorescence,” said Linda Reinhold, a zoologist at James Cook University in Australia who served as a peer reviewer of the study.

As they were doing these tests, the researchers noticed a pattern: Light-colored areas of fur and skin uniformly fluoresced.

Wondering if this was universal across mammals, they decided to expand their inquiry, taking advantage of the museum’s collections to includeas many species as possible on the mammal family tree,” Dr. Travouillon said.

One by one the mammals went under the spectrophotometer. A koala’s light belly and ears fluoresced greenish. A ghost bat’s bare wings, ears and nose leaf gave off a pale yellow. Even a house cat’s white fur emitted a faint gleam.

Eventually, “it started to be a bit boring,” Dr. Travouillon said. “We were checking them like, ‘Yeah, it’s glowing.’”

In the end, specimens from all 125 species they tested showed some degree of fluorescence. Most often, it came from structures made of unpigmented keratin, such as white fur, the bare skin of pouches and paw pads, or accouterments like quills, claws and whiskers. A wallaby with albinism, a condition in which production of the pigment melanin is interrupted, shone a “super intense” blue, Dr. Travouillon said, while the least glowy specimen, a dwarf spinner dolphin, fluoresced only in the teeth.

In some cases, pigmented fur also fluoresced, suggesting other materials can be involved — as previously seen in springhares, whose fluorescence does not match their color pattern and has been traced to pigments called porphyrins.

As in the past, the discovery of UV-fluorescent living things brings up a tricky question: Can mammals even detect these glows in nature?

For the most part, the photographs of splotchy springhares and radiant polar bears in articles like this are taken in artificial conditions that maximize their effect. They don’t reflect appearances in the real world, where the power of the rest of the light spectrum drowns out these hidden colors.

When the team looked for trends, they saw that nocturnal animals had more fluorescence in terms of surface area than diurnal ones, although the difference was small.

In addition, “prey species tend to have it on the belly, but carnivores tend to have it on their backs,” Dr. Travouillon said, suggesting a potential brightening effect under moonlight that could help predators recognize their own species. Other experts, like Ms. Reinhold, question whether moonlight would provide enough UV to make this happen.

But it’s difficult to imagine any utility for some animals newly added to the glow chart, such as the Southern marsupial mole, which is blind and spends life entirely underground, Dr. Travouillon said.

Innes Cuthill, a professor of behavioral ecology at the University of Bristol in England who was not involved in the paper, said it should put to rest the idea “that fluorescence in animals is necessarily a signal.”

But we may not beat the rainbow’s end. Given the study’s findings on the potential confounding effects of preservation, examining live animals of these species might be “mind-blowing,” Ms. Reinhold said. “I hope this study will inspire others to go forth into the wilds with a UV flashlight (and an appropriate permit, of course).”