The platypus is a strange animal. It has a bill like a duck and lays eggs. Its fur looks brown in normal light. But under ultraviolet light, it glows green and blue.

Scientists found this by looking at three old platypus bodies in museums. Later, other scientists found that 125 kinds of mammals also glow. Nobody knows why they glow. Maybe it has no special reason.

In 2019, researchers in the United States noticed that flying squirrels glow pink under ultraviolet light. This made them wonder whether other mammals glow too, so they examined three platypus specimens in museums. In normal light, the fur was brown, but under ultraviolet light it glowed green to blue-green. This was the first report of fluorescence in an egg-laying mammal.

However, in 2023, a team at the Western Australian Museum found fluorescence in 125 species from all 27 orders of living mammals. It was especially common in nocturnal animals. So the platypus is not as special as it first seemed, and scientists still do not know what purpose, if any, this glow has.

The platypus already holds a reputation as one of nature's oddest mammals: it has a duck-like bill, lays eggs, and can sense electric signals from prey. In 2021, researchers added another surprise. Inspired by their earlier discovery that North American flying squirrels fluoresce pink, Paula Spaeth Anich and colleagues shone ultraviolet light on three museum specimens of platypus.

The uniformly brown fur glowed green to cyan, with a peak around 500 nanometers, the first such report in a monotreme. It seemed like a unique trait, but a 2023 survey by Kenny Travouillon and colleagues tested a wide range of museum specimens and detected fluorescence in 125 species spanning all 27 living orders of mammals, especially among nocturnal species.

By checking that the emitted color did not shift with the wavelength used, they found evidence, in a subset of specimens, that the glow was genuine fluorescence rather than reflection. Its purpose remains a mystery. Ideas range from communication to protection from ultraviolet light or concealment from predators, but none has been tested, and fluorescence may simply be a chemical side effect. The platypus findings themselves rest on only three preserved specimens, so preservation chemicals might have influenced the result.