Biofluorescence in Mammals: Green Platypuses & Pink Hedgehogs
Of all the fluorescent animals out there some of the mammals have to be the most striking. Seeing a hedgehog or a wombat fluoresce a vivid bright pink is something you will never forget. There has been an explosion of interest in fluorescence in mammals in the last 5 – 10 years, particularly during the COVID-19 pandemic. With most museums being closed and curators having some down time, it was the perfect recipe to start shining a UV torch on their taxonomy specimens in search of fluorescence. It didn’t take long for some very interesting discoveries to be made, probably the most famous of which was a story that took the internet by storm in November 2020 – The supposed green fluorescence of a platypus.
A Biofluorescent Hoax: The Platypus
Platypuses have got to be one of the weirdest creatures still alive today, having a duck-like bill, being venomous, laying eggs and yet being mammals that feed their young milk. When it was discovered that platypuses housed in the Field Museum of Natural History (Chicago, Illinois, USA) and the University of Nebraska State Museum fluoresced bright green it seemed to add another layer to this bizarre animals story. Unfortunately it was too good to be true as many Australians quickly found out after reading the article on the internet. Time and time again observation of live specimens in the wild provided little to no fluorescence. Why did the museum specimens glow so brightly but live ones not? The answer is quite simple – It wasn’t the platypus that was fluoresceing bright green but rather the chemicals that had been used to preserve it.
Linda Reinhold, an expert of fluorescence in mammals has tried to warn against relying on museum specimens when checking for fluorescence. The problems of inconsistent results go beyond errors caused by preservation chemicals. There is also photobleaching to consider, not to mention the potential differences between live and dead animals.
History of Biofluorescence in Mammals
The first observation of biofluorescence in a mammal goes back a lot further than you might think. The invention of the Woods lamp in 1903 made UV light sources available to researchers. It was around this time that the first work was done looking at fluorescence in sheep. Throughout the 20th century there were a number of pioneers who looked at fluorescence in animals, including mammals. These pioneers have largely been forgotten from the modern day story of fluorescence. In the 1950’s researchers in the US looking at ringworm realised that metabolites of tryptophan cause rats to fluoresce blue. Similarly in 1971 blue fluorescence from trytophyan metabolites were observed in possums and tree kangaroos.
Mammals That Glow Pink
Vibrant barbie pink is a colour that will stand out in any environment, and it is certainly not the colour you expect to see mammals glowing. In the largest comparative study done so far on biofluorescence pink was the dominant colour observed in fluorescent mammals. What causes this strange pink glow? – Porphyrins.
Porphyrins are a group of organic compounds that are found in many living things. They are involved in a variety of important biological processes, such as carrying oxygen in the blood and making chlorophyll in plants. Porphyrins are essential for life. They play a vital role in many important biological processes.
They also fluoresce pink under UV light, interestingly they are one of only a few flurophores that fluorece brighter under 395nm UV light, rather than the more common 365nm. The pink fluorescence of porhpyrins is broken down by UV light in a process known as photobleaching. Researcher Linda Reinhold has found this happens in a matter of a few minutes of exposure to sunlight. This is why the mammals that glow pink are primarily nocturnal.
What’s the function of Biofluorescence in Mammals?
This is a question that people have been pondering ever since a human first saw a mammal light up the night with it’s bright fluorescence. It is a question that we still don’t have the answer for, from what we know so far it appears that in the large majority of cases the fluorescence of mammals likely has no function and is simply a by-product of chemistry.
Visual ecologists Marshall and Johnsen suggested that assigning a biological function to observations of fluorescence requires five criteria:
- The absorption spectra of fluorescent pigments absorb available light wavelengths
- Under what natural lighting condition it is
- Emitted wavelengths contrast against typical backgrounds
- Fluorescent areas of animals are visible
- Intended observers have an appropriate spectral sensitivity.
They concluded that it can be rarely demonstrated that all these criteria are met for most examples of fluorescence, and so there was little evidence that most of the fluorescence described has a biological function.
Research in this area as with most aspects of biofluorescent research is still in it’s infancy and there may well be a case out there of a functional fluorescent mammal.