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.

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.”

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.

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).”

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