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Photo of a platypus glowing green under ultraviolet light. // Photo courtesy of Jonathan Martin/Northland College.

Platypuses get even more bizarre by glowing green

By Elaine Pham, November 18 2020—   

A recent discovery shows that platypuses glow in the dark.

Platypuses are bizarre looking monotremes that inhabit streams, lakes and lagoons across eastern Australia. They can spend time out of the water as well. Platypuses are quite unique as they use their mechanoreceptors and electroreceptors to locate prey underwater while swimming with their eyes closed. The platypus’ fur is a uniform dark brown colour, and it is extremely dense for insulation in water. A recent discovery has shown them to be biofluorescent and glow green since their fur can absorb short UV wavelengths of 200–400 nanometers and re-emits visible light of 500–600 nanometers. 

On Jan. 23, 2019, Allison Kohler, an undergraduate and her colleagues at Northland College in Ashland, Wisconsin shared in the Journal of Mammalogy that flying squirrels will glow bright pink under UV light. This was a very interesting find since only two mammals — flying squirrels and opossums, which glow purple — were found to have biofluorescence. Furthermore, this glowing trait is commonly observed only in some fishes, reptiles and amphibians and some birds. 

Erik Olson and colleagues were investigating fluorescence and testing the flying squirrel specimens at the Field Museum in Chicago. With curiosity and the knowledge that platypuses were also nocturnal like flying squirrels, this made platypuses a possible candidate for exhibiting this trait as well, so the scientists decided to examine the platypus specimen. To their amazement, the platypus specimen glowed green under UV light.

The scientists behind this discovery first examined two platypus specimens collected in Tasmania, Australia. The platypuses were photographed under visible light first on both dorsal and ventral sides. They were then photographed under UV light with and without a filter that blocked shorter wavelengths to only allow longer wavelengths to pass through. Finally, using SmartUV camera technology, they created an average spectrum for both specimens. There was a fluorescence peak around 500 nm, then a decrease which showed light absorption by the specimen. The final result was an absorption of short UV wavelengthsand re-emission of visible light, which revealed that platypuses exhibit biofluorescence. 

The intensity and pattern of the glow was both similar in the female and male specimen, therefore, this was not a trait specific to sex. Similar glow patterns allowed scientists to conclude that the role of biofluorescence was not for attracting mates. Since the discovery of biofluorescence was seen in nocturnal mammals, the trait might actually be an indication for adaptation in low-light. Nocturnal animals have developed adaptations to see in the light and to adjust to the colour changes as the day turns into night. When examining nocturnal animal vision, they have UV receptors. Specifically, nocturnal animals with dichromatic vision have evolved UV light sensitivity. Furthermore, for platypuses, the absorbance of short wavelength and emission of long wavelength might be a defensive adaptation since it might reduce its visibility to UV-sensitive predators. However, more field research must be done to understand the purpose of this unique trait. 

So what does this discovery exactly mean for the scientific community? Biofluorescence is now seen in new world placental flying squirrels and marsupials opossums and monotreme platypus. The diversity of these taxa in terms of locations and ecosystems gives evidence on how the trait is observed across the phylogeny and not specific mammals that are closely related. It then poses the ultimate question of whether or not evolution played a role in possibly the emergence of biofluorescence in ancestral mammals even before the mammals we see today. 

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