“A selection of glowing mammal specimens in the WA Museum collection: A) polar bear, B) southern marsupial mole, C) greater bilby, D) mountain zebra, E) bare-nosed wombat, F) six-banded armadillo, G) orange leaf-nosed bat, H) quenda, I) leopard, J) Asian palm civet, K) red fox, L) dwarf spinner dolphin”
From glowing cats to wombats, fluorescent mammals are much more common than we think
by Kenny Travouillon, Christine Elizabeth Cooper, JemmyBouzin, Linette Umbrello and Simon Lewis / October 3, 2023
“Recently, several mammals have been reported to “glow” under ultraviolet (UV) light, including our beloved platypus. But no one knew how common it was among mammals until now. Our research, published in Royal Society Open Science today, found this glow – known as fluorescence – is extremely common. Almost every mammal we studied showed some form of fluorescence. We also examined the glow to determine if it was really fluorescence and not some other phenomenon. Then, we tested if the fluorescence we observed in museum specimens was natural and not caused by preservation methods. We also searched for links between the type and degree of fluorescence and the lifestyle of each species, to gain insights on whether there are any benefits to glowing under UV if you’re a mammal.
Nightclub visitors will be familiar with white clothes, or perhaps their gin and tonic, glowing blue under UV light. This is a great example of fluorescence – when the energy from UV light, which is a form of electromagnetic radiation invisible to humans, is absorbed by certain chemicals. These chemicals then emit visible light, which is lower-energy electromagnetic radiation. In the case of gin and tonic, this is due to the presence of the quinine molecule in the tonic water. In the case of animals, this can be due to proteins or pigments in their scales, skin or fur. Fluorescence is quite common among animals. It has been reported for birds, reptiles, amphibians, fish, corals, molluscs and most famously scorpions and other arthropods.
However, it has been described less frequently in mammals, although recent studies have provided several examples. We already knew that bones and teeth glow with fluorescence, as do white human hair and nails. Some rodents have a pink glow under UV light and platypuses glow blue-green. Our team came together because we were curious about fluorescence in mammals. We wanted to know if the glow reported recently for various species was really fluorescence, and how widespread this phenomenon was. We obtained preserved and frozen specimens from museums and wildlife parks to study.
“A male platypus (Ornithorhynchus anatinus) museum specimen (FMNH 16612) collected from Tasmania, Australia, photographed under visible light and 385–395 nm ultraviolet (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.”
We started with the platypus to see if we could replicate the previously reported fluorescence. We photographed preserved and frozen platypus specimens under UV light and observed a fluorescent (although rather faint) glow. To make sure it was fluorescence and not some other effect that looked like it, we used a technique called fluorescence spectroscopy. This involved shining various sources of light at the samples and recording the specific “fingerprints” of the resulting glow, known as an emission spectrum. This way, we could confirm what we saw was indeed fluorescence.
Left: how humans see starlings.
Right: how starlings may see each other – with bolder markings and more colour.
The amazing world of UV vision, seen through the eyes of…. birds. pic.twitter.com/GWi9ZGdGL0
— Massimo (@Rainmaker1973) October 22, 2023
We repeated this process for other mammals and found clear evidence of fluorescence in the white fur, spines and even skin and nails of koalas, Tasmanian devils, short-beaked echidnas, southern hairy-nosed wombats, quendas (bandicoots), greater bilbies and even cats. Both fresh-frozen and chemically treated museum specimens were fluorescent. This meant it wasn’t preservation chemicals such as borax or arsenic causing the fluorescence. So, we concluded this was a real biological phenomenon.
Using specimens from the Western Australian Museum’s collection, we took the experiment to the next stage. We recorded every species of mammal that was fluorescent when we exposed the specimens to UV light. As a result, we found 125 fluorescent species of mammal, representing all known orders. Fluorescence is clearly common and widely distributed among mammals. In particular, we noticed that white and light-coloured fur is fluorescent, with dark pigmentation preventing fluorescence. For example, a zebra’s white stripes fluoresced while the dark stripes didn’t. We then used our dataset to test if fluorescence might be more common in nocturnal species.
To do this, we correlated the total area of fluorescence with ecological traits such as nocturnality, diet and locomotion. Nocturnal mammals were indeed more fluorescent, while aquatic species were less fluorescent than those that burrowed, lived in trees, or on land. Based on our results, we think fluorescence is very common in mammals. In fact, it is likely the default status of hair unless it is heavily pigmented. This doesn’t mean fluorescence has a biological function – it may just be an artefact of the structural properties of unpigmented hair. However, we suggest florescence may be important for brightening pale-coloured parts of animals that are used as visual signals. This could improve their visibility, especially in poor light – just like the fluorescent optical brighteners that are added to white paper and clothing.”
Fluorescent mammals are far more common than earlier thought, study suggests
by Donna Lu / 4 Oct 2023
“Fluorescence in mammals is much more common than previously thought, new research suggests. A luminous property, fluorescence has been described in recent years in Australian marsupials including platypuses, wombats, Tasmanian devils and echidnas. But scientists now believe the quality is widespread across mammals after researchers studied 125 species and found all of them showed some form of fluorescence. The researchers found 107 of the 125 species (86%) had fur that glowed under UV light. The 125 species represent all 27 living mammalian orders and about half of all living mammal families.
Fluorescence was most common and most intense among nocturnal species, the researchers found, but it was also present in diurnal animals, which are active during the daytime, including the mountain zebra and the polar bear. “We didn’t know that so many mammals had glowing skin or glowing fur,” said Dr Kenny Travouillon, curator of mammalogy at the Western Australian Museum and the study’s lead author. Animals that are fluorescent appear to glow under UV light because proteins in their fur or skin absorb the radiation and emit it as visible light. “The UV light hits the hair or skin, and proteins … shift the light into the visible range,” Travouillon said. “That’s why they appear blue or green, or even red.”
All mammals including humans have fluorescent teeth, and many also have fluorescent nails, Travouillon said. The dwarf spinner dolphin was the only animal the researchers analysed whose sole fluorescent body part was its teeth. The researchers analysed a combination of preserved and frozen animal specimens – mainly from the Western Australian Museum collection – to try to rule out that the glow wasn’t merely an artefact of the preservation process. The study, published in the journal Royal Society Open Science, noted that “preservation may play a part in the intensity of the fluorescence observed for some specimens”. Unpreserved frozen platypus specimens were least fluorescent, the team found, with the glow more intense for specimens preserved with borax and even more again for those preserved with arsenic. “But we did that for other animals and we actually found the opposite – the brighter ones were actually the frozen ones and preservation actually decreased the intensity of the fluorescence,” Travouillon said, citing the koala, Tasmanian devil and echidna as examples.
Linda Reinhold, a zoologist at James Cook University who was not involved in the research, said “if fluorescence of the fur can be significantly underrepresented in [some] museum specimens … it boggles the mind as to what the phenomenon is like in these species in the wild. Although only 86% of the mammals studied had fluorescent fur, they all had at least fluorescent claws or teeth, as we do when we go to a disco.” Reinhold noted that several of the specimens analysed – including brushtail possums, kangaroos, black rats and European hedgehogs – displayed less colourful fluorescence compared to specimens in previously published research. She questioned whether this discrepancy was geographic or the result of preservation differences. The exact purpose of fluorescence in mammals is still a mystery, but the study’s authors believe it could be a means to make animals appear brighter and “enhance visual signalling, especially for nocturnal species”. “Carnivores usually have spots or stripes on their back. I think this could be potentially a way for them to recognise each other within their own species,” Travouillon said. “But a lot of animals only have this glowing on their bellies, because that’s not visible to predators … maybe members of their species, when they get close, will be able to recognise them.”
The scientists concede that in some species fluorescence is unlikely to play any functional role. The southern marsupial mole, for example, covered in yellow-white fur, was one of the most fluorescent species they analysed. But the animal is also blind, and the researchers believe the fluorescence is merely a byproduct of not having pigmented fur. White but not other coloured hair in humans is also innately fluorescent. “Fluorescence is unlikely to be important for some flying mammals such as microbats that use echolocation to navigate and locate prey,” the researchers added. Other scientists have expressed reservations about the significance of biofluorescence, given that the amount of UV light reaching Earth decreases at night-time. Biofluorescence in non-human mammals was first reported in 1911, in European rabbits. “A lot of people might want to get a UV light and try it on their pets,” Travouillon said. “Just one word of caution – they shouldn’t put the UV light in the eyes of their pets, because that does damage the eyes.”
ARCTIC REINDEER SEE in ULTRAVIOLET
NOW in SHARK VISION
NON SPECTRAL COLORS