Fluorescence — the absorption of short wavelengths of light and re-emission of longer wavelengths of light — is widely distributed among mammals and is most intense among nocturnal species, according to new research by biologists from Curtin University, the Western Australian Museum and the Queensland University of Technology.
Travouillon et al. examined the phenomenon of mammalian fluorescence to determine the extent of apparent fluorescence among mammals and to identify the anatomical regions most likely to fluoresce. The images show mammal taxidermy under UV light: (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. Image credit: Travouillon et al., doi: 10.1098/rsos.230325.
Fluorescence is the process by which a chemical on the surface of an organism absorbs light and then emits the light at longer and lower-energy wavelengths.
One example is when an animal’s surface absorbs high-energy, short-wavelength ultraviolet (UV) light and emits the fluorescence as a lower-energy colored, often pink, green or blue, glow.
Fluorescence can make the previously invisible UV light visible by shifting it within the range of white light, so an animal does not necessarily have to see into the UV spectrum to detect fluorescence.
Numerous organisms have been reported to fluoresce including plants, corals, insects, spiders, scorpions, crustaceans, mollusks, fish, amphibians, reptiles and birds.
Fluorescent compounds have been identified in a variety of animal materials including bone, teeth, claws, fur, feathers, carapace and skin, and the visible fluorescent colors observed include red, yellow, green, blue and pink.
“Among mammals, the first published reports of fluorescence were in rabbits and humans in 1911 with more recent published observations of fluorescence for New World flying squirrels, springhares, platypus, dormice and a variety of other rodents, as well as an array of other mammalian species,” said lead author Dr. Kenny Travouillon, curator of mammals at the Western Australian Museum, and his colleagues.
“There is also a plethora of anecdotal reports of mammals glowing under UV light.”
In their study, the researchers examined a wide range of mammal species held in a museum collection for the presence of apparent fluorescence using UV light.
They then analyzed a subset of preserved and non-preserved specimens by fluorescent spectroscopy at three different excitation wavelengths to assess whether the observations were fluorescence or optical scatter, and the impact of specimen preservation.
They identified examples of the phenomena among 125 species representing 79 families and 27 orders from all three mammalian subdivisions (monotremes, marsupials and placentals).
“We started with the platypus to see if we could replicate the previously reported fluorescence,” the scientists explained.
“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.”
“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.”
“We noticed that white and light-colored fur is fluorescent, with dark pigmentation preventing fluorescence. For example, a zebra’s white stripes fluoresced while the dark stripes didn’t.”
“The only major mammalian clade missing from our dataset is the lemurs, a group that requires further investigation for the occurrence of luminescence,” they noted.
“We predict, based on the prevalence of white fur, that this clade will also contain fluorescent species.”
“Both fresh-frozen and chemically treated museum specimens were fluorescent,” they added.
“This meant it wasn’t preservation chemicals such as borax or arsenic causing the fluorescence. So, we concluded this was a real biological phenomenon.”
According to the authors, fluorescence was most common and most intense among nocturnal species and those with terrestrial, tree-dwelling and burrowing habits, with more of their body being more fluorescent.
To test if fluorescence might be more common in nocturnal species, we correlated the total area of fluorescence with ecological traits such as nocturnality, diet and locomotion,” they said.
“Nocturnal mammals were indeed more fluorescent, while aquatic species were less fluorescent than those that burrowed, lived in trees, or on land.”
Based on their results, Dr. Travouillon and co-authors think fluorescence is very common in mammals.
“It remains unclear if fluorescence has any specific biological role for mammals,” they said.
“It appears to be a ubiquitous property of unpigmented fur and skin but may function to make these areas appear brighter and therefore enhance visual signaling, especially for nocturnal species.”
The findings were published in the October 2023 issue of the journal Royal Society Open Science.
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Kenny J. Travouillon et al. 2023. All-a-glow: spectral characteristics confirm widespread fluorescence for mammals. R. Soc. open sci 10 (10): 230325; doi: 10.1098/rsos.230325
