A study of more than 100 mammal species found that the vast majority of them, with the exception of the dwarf spinner dolphin, are able to glow externally in the presence of ultraviolet (UV) light. Although to varying degrees, fluorescence is present in many species, with white fur being a strong indicator of this trait when exposed to black light. Fluorescence is the process by which longer wavelength light, particularly UV light, is absorbed and re-emitted, producing brightness in the human visual range.
This bare-nosed wombat shines. So does its hairy-nosed cousin and most other mammal species. (WA Museum)
The discovery of fluorescence in mammals, especially marsupials, is relatively recent, mainly because for decades few people were interested in studying the phenomenon. UV light is limited to the daytime, when any fluorescence would be invisible due to the presence of other, brighter wavelengths. But in 2019, researchers found that flying squirrels glow pink in UV light, followed two years later by the discovery that platypuses glow green under the same conditions.
During the shutdown, Dr. Kenny Travouillon of the Western Australian Museum conducted more research on fluorescence in mammals. He shone a UV light on some museum specimens and found that fluorescence is very common. Travouillon and his colleagues then conducted a more systematic study to determine which mammal species fluoresce and how much. They recorded 125 mammal species in the museum's collections that showed some form of fluorescence. It turns out that fluorescence is universal in white fur, although it has not been confirmed in lemurs. Therefore, polar bears are probably the champions when it comes to fluorescence. Animals with dark fur also show some fluorescence, such as around the toes or toenails.
Fluorescence occurs mainly in the color white, followed by yellow. Colors such as green, blue, red, pink, and orange have also been observed, although less frequently. In order to confirm that this was indeed fluorescence and not optical scattering, the researchers shifted the wavelength of the UV light that was applied to the samples and observed to see if the emission spectrum shifted. This was confirmed. It showed that what was observed was indeed fluorescence and not an effect of the preservation techniques.
Fluorescence appears to be more common in nocturnal marsupials than in diurnal marsupials, but its ubiquity suggests that it may not be an evolutionary advantage. Rather, it may be a byproduct of molecules in the fur that went unnoticed until humans invented UV lights. However, it is possible that some animals have evolved enhanced versions of fluorescence for sexual signaling purposes.
It's not just mammals that glow. Birds, such as parrots, and some reptiles also glow. This has led to speculation that dinosaurs may have glowed as well. Awareness of fluorescence in human mammals was first documented in a scientific paper in 1911, but it has not been widely disseminated because blond hair glows weakly and white hair is often covered by chemicals in white shirts.
The study of fluorescence in mammals was published in Royal Society Open Science.
Links:
- https://museum.wa.gov.au/research/departments/terrestrial-zoology/terrestrial-zoology-curator-and-contacts/kenny-travouillon
- https://museum.wa.gov.au/about/latest-news/researchers-explore-phenomenon-fluorescence-mammals
- https://link.springer.com/article/10.1007/BF01680690
- https://royalsocietypublishing.org/doi/10.1098/rsos.230325
- https://twitter.com/IFLScience/status/1709199119867122017
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