Ultraviolet Fluorescence

The fur of the platypus (Ornithorhynchus anatinus), one of only five extant species of monotremes (egg-laying mammals), absorbs ultraviolet light at wavelengths of 200-400 nm and re-emits visible light, making it fluoresce, according to new research from Northland College and the Warner College of Natural Resources at Colorado State University. This is the first observation of biofluorescence in a monotreme.

This image is a digital reproduction of a painting by John Lewin of a platypus in 1808.

This image is a digital reproduction of a painting by John Lewin of a platypus in 1808.

Biofluorescence, in which short wavelengths of light are absorbed and longer wavelengths are re-emitted by living organisms, has been observed in a wide range of fishes, reptiles and amphibians and birds.

Within mammals, marsupial opossums and placental flying squirrels are known to have fur that biofluoresces under UV light.

“It was a mix of serendipity and curiosity that led us to shine a UV light on the platypuses,” said Northland College’s Professor Paula Spaeth Anich, lead author on the study.

“But we were also interested in seeing how deep in the mammalian tree the trait of biofluorescent fur went.”

“It’s thought that monotremes branched off the marsupial-placental lineage more than 150 million years ago. So, it was intriguing to see that animals that were such distant relatives also had biofluorescent fur.”

In the study, Professor Anich and colleagues examined three specimens of the platypus from Tasmania and New South Wales, Australia, housed in the Field Museum of Natural History and the University of Nebraska State Museum under visible and UV light.

The fur of the animals appeared uniformly brown under visible light and green or cyan under UV light, due to fluoresced wavelengths that peaked around 500 nm.

A male platypus (Ornithorhynchus anatinus) museum specimen collected from Tasmania, Australia, photographed under visible light and 385-395 nm UV light without and with a yellow camera lens filter. Cyan to green biofluorescence of 500 nm is seen in the middle panels. UV absorption is indicated by dark areas in the far right panel. Image credit: Anich et al., doi: 10.1515/mammalia-2020-0027.

A male platypus (Ornithorhynchus anatinus) museum specimen collected from Tasmania, Australia, photographed under visible light and 385-395 nm UV light without and with a yellow camera lens filter. Cyan to green biofluorescence of 500 nm is seen in the middle panels. UV absorption is indicated by dark areas in the far right panel. Image credit: Anich et al., doi: 10.1515/mammalia-2020-0027.

“Like the marsupial opossum and the placental flying squirrel, platypuses are most active during the night and at dawn and dusk,” the researchers said.

“It may be that these mammals — and possibly others — developed biofluorescence to adapt to low light conditions.”

They suggest this may be a way for platypuses to see and interact with each other in the dark.

“The discovery of biofluorescence in the platypus adds a new dimension to our understanding of this trait in mammals,” they said.

“Biofluorescence has now been observed in placental New World flying squirrels, marsupial New World opossums, and the monotreme platypus of Australia and Tasmania.”

“These species, inhabiting three continents and a diverse array of ecosystems, represent the major lineages of Mammalia.”

“Biofluorescence in mammals is not restricted to a few closely related specialists; instead, it appears across the phylogeny, which begs the question: Is biofluorescence an ancestral mammalian trait?”

A paper on the findings was published in the journal Mammalia.

_____

Paula Spaeth Anich et al. Biofluorescence in the platypus (Ornithorhynchus anatinus). Mammalia, published online October 15, 2020; doi: 10.1515/mammalia-2020-0027

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