The Curious Case of the Pink Squirrel

One night in May 2017, Jonathan Martin stepped into his backyard in northern Wisconsin. He was carrying a special flashlight that emitted ultraviolet (UV) light. Martin knew that shining these invisible energy waves on the leaves and flowers of certain plants can reveal interesting patterns, which are normally undetectable to the naked eye. But before he could begin his search, he was distracted by the squeak of a flying squirrel, an animal that’s typically brown with a cream-colored underside. Martin turned his flashlight toward the sound—and couldn’t believe what he saw. In the beam of UV light, the squirrel’s belly gave off a neon pink glow!

Martin is a forestry scientist at Northland College in Ashland, Wisconsin. He immediately told some of his colleagues what he had witnessed in his backyard. “Nobody really believed him,” says Allie Kohler, who was a biology student at Northland at the time. Many animals—including birds, fish, frogs, insects, and lizards—can fluoresce, or absorb and then emit light. But this ability had almost never been observed in mammals like flying squirrels.

One night in May 2017, Jonathan Martin stepped into his backyard in northern Wisconsin. He carried a special flashlight that emitted ultraviolet (UV) light. Martin planned to shine these invisible energy waves on leaves and flowers. He knew doing this can reveal interesting patterns on certain plants. The naked eye normally can’t see these patterns. But before he could begin his search, he heard the squeak of a flying squirrel. Martin turned his flashlight toward the sound. He couldn’t believe his eyes. Flying squirrels are usually brown with a cream-colored belly. But in the beam of UV light, the squirrel’s belly glowed neon pink!

Martin is a forestry scientist at Northland College in Ashland, Wisconsin. Right away, he told some of his colleagues what he had seen in his backyard. “Nobody really believed him,” says Allie Kohler. She was a biology student at Northland at the time. Many animals can fluoresce, or absorb and then emit light. They include birds, fish, frogs, insects, and lizards. But scientists had almost never seen this in mammals like flying squirrels.

Kohler and her professor, biologist Erik Olson, started bringing a UV flashlight on research trips into the woods, just in case. Early one morning, while it was still dark, they spotted a flying squirrel and pointed the flashlight at it. Its fur glowed bright pink in response—just like Martin had claimed. “That was all we needed to launch a full investigation,” says Kohler.

Kohler and her professor, biologist Erik Olson, took research trips into the woods. They started bringing a UV flashlight, just in case. Early one morning, it was still dark. They spotted a flying squirrel and pointed the flashlight at it. The result? Its fur glowed bright pink, just like Martin had said. “That was all we needed to launch a full investigation,” says Kohler.

Fluorescence occurs when light is absorbed at one wavelength and then re-emitted at a different wavelength (see Borrowed Glow). Wavelength is the distance between two identical points on a wave. UV light has a short wavelength, making it invisible to humans (see The Electromagnetic Spectrum). Things that fluoresce soak up UV light and release visible light, which people can see because it’s made up of longer wavelengths. These wavelengths correspond to different glowing colors, like pink, green, orange, yellow, and blue.

Living things fluoresce for a variety of reasons. Some fish species, for example, use fluorescent patterns to attract mates. Other fish emit light that seems to match that of the anemones and corals among which they live. This may help them camouflage themselves to avoid being seen by predators.

Some materials absorb light at one wavelength and then emit it at a different wavelength. That’s when fluorescence occurs (see Borrowed Glow). Wavelength is the distance between two matching points on a wave. UV light has a short wavelength. That makes it invisible to humans (see The Electromagnetic Spectrum). Things that fluoresce soak up UV light and release visible light. People can see visible light because it has longer wavelengths. These wavelengths appear as different glowing colors, like pink, green, orange, yellow, and blue. 

Living things fluoresce for different reasons. For example, some fish species use fluorescent patterns to attract mates. Other fish live among anemones and corals. The fish emit light that seems to match that of these animals. This may help camouflage them, so predators won’t see them. 

To learn more about whether fluorescence is common in flying squirrels, Kohler headed to the Science Museum of Minnesota. In a dark basement, she examined dozens of preserved flying squirrels from the museum’s collection and photographed them under UV light. Every single specimen glowed hot pink, even if it had been dead for decades. This confirmed that the glow-in-the-dark animals Martin, Kohler, and Olson spotted in the wild hadn’t been a fluke. “Someone’s pet or science experiment didn’t run away,” jokes Kohler. Flying squirrels really are naturally fluorescent!

The team published their findings in a scientific journal in 2019. But they still had many questions. Was flying squirrels’ fluorescence an adaptation that helped them survive in their environment? And did other mammals have this eerie ability to glow too?

Is fluorescence common in flying squirrels? Kohler headed to the Science Museum of Minnesota to learn more. The museum’s collection includes preserved flying squirrels. In a dark basement, Kohler examined dozens of them and photographed them under UV light. Every single specimen glowed hot pink, even if it had been dead for decades. So the glow-in-the-dark animals Martin, Kohler, and Olson spotted in the wild hadn’t been a fluke. “Someone’s pet or science experiment didn’t run away,” jokes Kohler. Flying squirrels really are naturally fluorescent!

The team published its findings in a scientific journal in 2019. But the team still had many questions. Why did flying squirrels fluoresce? Was this an adaptation that helped them survive in their environment? And could other mammals glow too? 

In 1985, scientists documented blue, purple, and pink fluorescence in several species of opossums. But not much else was known about fluorescence in mammals. So Martin, Olson, and colleague Paula Anich decided to visit the Field Museum of Natural History in Chicago. Armed with UV lights, the scientists combed through the museum’s enormous collection of specimens, searching for other mammals that glowed.

The researchers started by examining a relative of flying squirrels: the gray squirrel, a common sight in North American cities and suburbs. The scientists found that these animals don’t fluoresce like flying squirrels do. But another drawer of specimens the team opened contained springhares—large, jumping rodents that live in grasslands in Africa. “Sure enough, there’s a fluorescent springhare [species],” says Olson. “That was an exciting chance discovery.”

In 1985, scientists found blue, purple, and pink fluorescence in several species of opossums. But not much else was known about fluorescence in mammals. So Martin, Olson, and colleague Paula Anich visited the Field Museum of Natural History in Chicago. The scientists looked through the museum’s huge collection of specimens. With UV lights, they searched for other mammals that glowed.

The researchers started with a relative of flying squirrels. The gray squirrel is common in North American cities and suburbs. The scientists found that these animals don’t fluoresce like flying squirrels do. But then the team opened another drawer. It contained springhares. These large, jumping rodents live in grasslands in Africa. “Sure enough, there’s a fluorescent springhare [species],” says Olson. “That was an exciting chance discovery.”

The team also decided to investigate the platypus, a strange stream-dwelling animal from Australia. Platypuses are monotremes—mammals that lay eggs. Monotremes aren’t closely related to rodents, so the scientists weren’t expecting much. But under UV light, the normally brown platypuses lit up in stunning shades of fluorescent blue and green.

The team also looked at the platypus. This strange animal lives in streams in Australia. Platypuses are monotremes, mammals that lay eggs. Monotremes aren’t closely related to rodents, so the scientists weren’t expecting much. Platypuses are normally brown. But under UV light, they lit up in stunning shades of fluorescent blue and green. 

Though they live on different continents, all the fluorescent mammals the Northland team has studied have something in common: They don’t come out much during the day. The fluorescent species the scientists found are either nocturnal—active mainly at night—or crepuscular—most active at dawn and dusk. When it’s darker out, UV light emitted by the sun or reflected off the moon isn’t drowned out by visible light like it would be in the daytime, says Martin. That could make fluorescent patterns more likely to show up.

As with other fluorescent animals, being able to glow could help mammals communicate with other members of their species or blend in with their surroundings. To find out for sure, scientists would need to observe their behavior in the wild. That’s difficult to do with animals that hide during daylight hours, says Olson. But he and his colleagues hope that other scientists will pick up where they left off and discover more about mammals that glow.

For Martin, who first noticed the fluorescent flying squirrel in his backyard, the project goes to show what can come from curiosity. Kohler, who is now a graduate student at Colorado State University, agrees. “It’s made me realize that there’s so much that is yet to be discovered,” she says. “To see the secrets that nature is hiding, sometimes all it takes is looking at things in a new light.”

The fluorescent mammals that the Northland team has studied live on different continents. But they all have something in common. They don’t come out much during the day. The scientists found some fluorescent species that are nocturnal. They’re active mainly at night. The others are crepuscular, meaning they’re most active at dawn and dusk. UV light is emitted by the sun or reflected off the moon. When it’s darker out, this light isn’t drowned out by visible light like it would be in the daytime, says Martin. That could make fluorescent patterns more likely to show up. 

Other animals fluoresce to communicate with other members of their species or blend in with their surroundings. Do glowing mammals do the same? To find out for sure, scientists would need to watch them in the wild. That’s hard to do with animals that hide during daylight hours, says Olson. But he and his colleagues hope that other scientists will continue the research and discover more about glowing mammals.

Martin was the one who first noticed the fluorescent flying squirrel in his backyard. For him, the project shows what can come from curiosity. Kohler, now a graduate student at Colorado State University, agrees. “It’s made me realize that there’s so much that is yet to be discovered,” she says. “To see the secrets that nature is hiding, sometimes all it takes is looking at things in a new light.”