Scientists have a new understanding of the Australian platypus. Illuminated by ultraviolet lamps, this strange-looking creature looks more peculiar than usual, emitting a soft blue-green color instead of the typical brown we usually see.
This recent discovery has not been found in other monocular animals, a primitive mammal. This makes scientists wonder: Are we seeing an ancient world with fluorescent fur?
"Biofluorescence has now been observed in flying squirrels of the New World of placenta, marsupial New World possums, and monocular platypus in Australia and Tasmania," the authors write.
These taxa, inhabiting three continents and a diverse ecosystem array, represent the main lineage of mammals.
Biofluorescence has been found over the centuries in a variety of plants, fungi, fruits, flowers, insects and birds. Only recently, however, have scientists begun to actively track examples of animal kingdoms. So far, many of the discoveries have been accidental.
For example, in 2015, scientists stumbled upon the first fluorescent turtle while searching for glowing corals. Two years later, the first fluorescent frog was accidentally discovered, and the team suggested that others "start carrying ultraviolet flashlights into the wild."
Among mammals, the first example of biofluorescence was discovered in 1983 in the only Marsupial in North America, the Virginia possum. But it wasn't until 2017 that researchers completely accidentally found something similar in the North American flying squirrels (Glaucomys), which are classified as placental mammals.
During a nightly survey of the lichen, the researchers were surprised to point the LED flashlight at a bright, bubblegum-shaped pink shrew.
The only thing possums and squirrels have in common is their nocturnal lifestyle. This is also when ultraviolet light is strongest, suggesting that this trait is common in mammals, where they are most active at night, dawn or dusk.
Like flying squirrels and possums in North America, The Australian platypus is nocturnal. However, after 150 million years of evolution, they have separated from other animals.
Although Australia is home to some of the most primitive mammals on earth, little attention has been paid to biofluorescence in Australian animals. But if they also have glowing fur, this trait may be older than we think and more common among mammals.
Paula Spaeth Anich, a biologist from Northland College, recalls: "It was serendipitous discoveries and curiosity that led us to irradiate platypus with ultraviolet rays at the Field Museum."
"But we're also interested in how deeply the characteristics of biofluorescent hairs are in mammalian lineages."
The researchers started with two museum specimens collected from Tasmania, one male and one female. Studies have found that the fur of these creatures can absorb short-wave ultraviolet light, which then emits visible light, emitting green or cyan fluorescence.
The researchers examined another platypus specimen taken from New South Wales and found the same thing.
"The fur of this specimen is brown in visible light and biofluorescent green in ultraviolet light," the authors wrote.
To their knowledge, the team says this is the first time single-hole biofluorescence has been reported. However, in June, a member of the Queensland Mycological Society claimed they had found a platypus that had been killed by a road crash and had similar fluorescence on its body.
Linda Rhinehold wrote in the association's non-peer-reviewed briefing: "Most of the platypus fur is dark purple under ultraviolet light, but some turn moss-green, although not so bright."
Reinhold also spotted two large northern brown lizards in fluorescent pink fur on the road, which she managed to catch.
While scientists have some ideas, our sample is too small, so it's too early to say what benefits this feature would bring to nocturnal mammals.
In 2017, when the fur of a flying squirrel was found to be biofluorescent, some thought it might be related to camouflage, as many trees were covered with biofluorescent moss and lichens.
However, the bandicoots discovered by Reinhold are terrestrial mammals, and their fluorescence may have made them stand out.
Standing out can be an advantage, depending on the situation. For some birds, the biofluorescence of their feathers plays a role in mating ceremonies. Fish use this property to communicate with each other.
However, in platypus, samples of both males and females showed similar fluorescence, suggesting that this feature is not dimorphic. What's more, because the platypus usually swims with its eyes closed, the light on its fur may not be used to communicate with its peers.
Instead, the researchers believe it could help the platypus avoid other UV-sensitive nocturnal predators by absorbing rather than reflecting it.
Before we know exactly what's going on, further research in the wild is needed. We don't even know how biofluorescence of this fur works, and the benefits of this property may vary from species to species.
However, the fact that this strange glow is present on the fur of egg-laying monotremes, marsupials, and placental mammals suggests that it has deep roots.
If nothing else, this discovery is a good reminder that we are completely ignorant.