[Warning: this article contains images of stuffed animals conserved in natural history museums.]
Up until a few months ago, it was thought that the only mammals whose fur glowed in the dark were various species of American opossums. In November and December 2020, however, perhaps in an attempt to show that such a negative year could give us something to smile about, a number of institutions in the United States and Australia discovered that the biofluorescent properties of other mammals, including marsupials (wombats, Tasmanian devils and bilbies), cingulates (armadillos) and monotremes (platypuses and echidnas): give off a greenish yellow glow when illuminated with UV light.
Tweet from the WA Museum Boola Bardip in Perth (Australia)showing photos of a bilby and an armadillo under UV light;
and a response with a photo of a wombat.
Biofluorescence and Bioluminescence
It is important to distinguish between two different phenomena regarding the capacity of living beings to emit light. Biofluorescence, the phenomenon we are discussing in this article, occurs when an organism, exposed to a high frequency light (like ultraviolet light) reflects a lower frequency light (i.e. colours in the spectrum that are visible to human beings). This physical effect may be caused by special proteins or other organic substances that make up the living tissue. In the animal world, many species of fish, coelenterates, reptiles, amphibians and birds are biofluorescent. Scientists have speculated that, in certain conditions, glowing in the dark can be useful as it enables the animal to be recognised by its own kind while also helping it to hide from predators whose vision may not detect low frequency light.
If biofluorescence is activated only in the presence of external light, there are also organisms that can emit light independently from their own body thanks to a chemical reaction that produces light energy. This type of emission is called bioluminescence. The best-known example of this is the firefly, but various creatures, such as fish, cephalopods and crustaceans are also bioluminescent.
The study that triggered this wave of interest in the effects of UV light on the fur of certain mammals was a paper published in the international scientific magazine Mammalia
and written by Paula Spaeth Anich, Sharon Anthony, Michaela Carlson, Adam Gunnelson, Allison M. Kohler, Jonathan G. Martin and Erik R. Olson. In addition to being the first paper on this subject, the article is an in-depth study that includes a highly detailed description of the operations conducted to verify these results.
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. Mammalia 2020; 10.1515/mammalia-2020-0027
The researchers’ curiosity regarding this phenomenon was originally aroused by a previous discovery. This occurred while one of them was conducting a survey on lichen in a wood and happened to notice that when the flying squirrels there were illuminated with a UV torch, their fur shone with a bright pink glow
Having confirmed their discovery by examining other stuffed specimens of squirrels conserved in the Chicago Field Museum of Natural History, the researchers decided to extend the range of their survey to that unique animal, the platypus. This egg-laying mammal has a duck-like bill, a beaver-like tail, otter-like feet, and a spur on the hind foot of the male that injects venom. Two specimens of the creature, a male and female, were taken from the Chicago Field Museum and another male specimen was added from the State Museum of the University of Nebraska.
A post from the zoo in Toledo (Ohio) regarding biofluorescence in Tasmanian devils.
In a laboratory, photographs were taken of the animals by applying a filter to the lens that blocked wavelengths below a certain limit so the higher wavelengths (those emitted by the biofluorescent tissues) could be seen better. In visible light, the thick fur of the platypuses looks universally brown, but when exposed to ultraviolet light it gives off a glow between green and cyan.
At present, it is difficult to give a universal reason for these discoveries regarding biofluorescence in mammals. Without doubt, there is a reason, as all these animals are nocturnal or crepuscular, and often this type of animal can see UV rays. However, the platypus swims and hunts with its eyes closed, as it identifies its prey using mechanoreception
. This means that the light emanated by its fur cannot be used as a signal to other specimens of the same species.
In the paper in which they presented their discovery, the American scientists expressed a desire to work with their Australian colleagues in order to observe the phenomenon of biofluorescence in animals in the wild, too.
all these animals are nocturnal or crepuscular, and often this type of animal can see UV rays.