Fluorescent Mammals Are Much More Common Than Scientists Thought

Mammals have been discovered to have a secret superpower: nearly all species glow in the dark.

Before now, it was thought that only Australian marsupials such as platypuses, wombats, Tasmanian devils and echidnas, or spiny anteaters, were fluorescent under ultraviolet (UV) light. However, new research has found that a huge number of other mammals also glow.

Of 125 species from across all 27 orders of mammal—including primates, rodents and cetaceans—107 of the species, or 86 percent, showed some form of fluorescence, according to a new study published in the journal Royal Society Open Science.

"Fluorescence was present in every group. It was evenly spread out, suggesting that it is a common feature in all mammals," Kenny Travouillon, curator of mammalogy at the Western Australian Museum and co-author of the paper, told Newsweek.

Some of the animals studied had fluorescent fur, while others had fluorescent teeth or nails, with many possessing a combination of these traits, including marine mammals.

"The dwarf spinner dolphin only had its teeth glow. We only were able to test a couple of seals, a dugong and a dwarf spinner dolphin that we had access to at the time. The highly pigmented skin of the dolphin didn't glow, but the less-pigmented skin of the dugong did glow. I suspect that most marine mammals without hairs will have their pale skin glow. The pale fur in seals did glow, as did their whiskers," Travouillon said.

Fluorescence is caused by chemicals absorbing light and then emitting it at longer and lower-energy wavelengths.

"One example is when an animal's surface absorbs high-energy, short-wavelength UV light and emits the fluorescence as a lower-energy, colored, often pink, green or blue, glow," Edward Narayan, a senior lecturer in animal science at the University of Queensland, Australia, told Newsweek. "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."

The reason for these mammals glowing in the dark under UV is unclear, although the authors said that it may be an adaptation for being seen from a distance.

"We think that the parts of the animals that fluoresce will appear brighter than the rest of their body, and so it will give them an advantage for being seen from a distance," Travouillon said. "In some species, this would be a disadvantage as predators would see them, but most prey only have fluorescence on their belly, which won't be easily seen by a predator. Predators are the ones with the bright spots and stripes on their back. They obviously want to be seen by their counterparts potentially for finding mates during the mating season."

Other experts say that the fluorescence has deliberately evolved for other reasons.

"It's also possible that animals might have UV patterns that confuse predators, and break up their usual outline (a bit like the stripes of zebras)," Matthew Wills, a professor of evolutionary palaeobiology at the Milner Centre for Evolution, University of Bath, southwest England, told Newsweek. "Or the UV glow may be a side effect of excreting certain waste products into the fur and claws. The fact that monotremes, marsupials and placentals all exhibit the trait may suggest that it's rooted very deep in the mammal family tree."

Alternatively, it could have evolved to provide some benefit that we have not determined yet.

"There are some observations which suggest it is adaptive at least in some species," Michael Lee, a professor of evolutionary biology at Flinders University and the South Australian Museum, told Newsweek. "First, it seems to be more developed in nocturnal animals, and in particular species, the glow is restricted to the belly (which allows it to be exposed only when desired). These trends would not be expected if UV fluorescence was not adaptive; it should appear randomly across species, and randomly across individuals."

However, it is also possible that fluorescence in mammals is a side-effect of evolution, arising by accident and somehow sticking around. These sorts of traits are known as spandrels, evolving as a byproduct of the evolution of a different characteristic.

"It really seems that for most, if not all, mammals, the fluorescence is a side-effect of chemicals that are present in the hair and skin, and that is not adaptive," Alistair Evans, an associate professor of evolutionary developmental biology at Monash University, Australia, told Newsweek.

"Certain chemicals in hair and skin, particularly keratin, seem to naturally fluoresce. As they found fluorescence in nocturnal and underground-dwelling mammals, it is very unlikely that there is a functional reason," Evans said. "If fluorescence is due to chemicals that are found in all mammals, including their ancestors, and there is no disadvantage to it, it is not surprising to find it across so many mammals. But it's a new and fascinating way to look at all these mammals that we've never seen before."

Fluorescence might not even be a by-product of a selected trait, but simply the result of a property of the tissue.

"Our teeth fluoresce under UV. No one is claiming that this was acquired for a specific function," Eli Amson, a paleontologist at the State Museum of Natural History Stuttgart, Germany, told Newsweek. "Natural conditions are very different from what happens when you flash a UV light at an object in a dark room. So, from my perspective, keratin, and teeth, fluorescence is trivial."

This kind of phenomenon has been seen to happen in many other species.

"As an analogy, the redness of blood is not an adaptation; rather, the redness is due to hemoglobin, which contains a red-colored compound vital for carrying oxygen," Lee said. "Scorpions also glow in the dark, and this is at least partly a side-effect of a compound that protects against parasites."

More research will need to be done to investigate the fluorescence of mammals, looking into how it benefits the animals, if at all, and if the mammals themselves can even observe the trait.

"The key question is not whether *we* can notice any mammal florescence, but whether those mammal species can notice very faint fluorescence amongst their own kind," Lee said. "Nocturnal species will typically have more-sensitive eyes than humans, attuned to different wavelengths such as UV. These results should catalyze some interesting behavioral research on nocturnal mammals."

So, while it's possible that fluorescence evolved to provide mammals with some benefit, it may also be an accident of natural selection, or simply a result of keratin's natural properties.

"We would not completely dismiss the possibility that UV fluorescence is an adaptive trait across mammals, as the living world is an endless source of surprises, but that does not seem likely to me given the available evidence," Timothée Bonnet, a researcher in evolutionary biology at the Australian National University, told Newsweek.

"There could be some value in the idea that mammals active at night or dusk would show signals that are more visible thanks to UV fluorescence, but the idea needs more investigation. Also, that does not explain why UV fluorescence is common in mammal species with other habits, or even species living all their lives underground. It may be useful to note that some minerals also fluoresce in UV light but, of course, nobody would suggest that it is useful to a rock.

"So, to us, the most likely explanation to the widespread UV fluorescence of mammals is an incidental property of the molecules and molecular structure present in the common ancestor of all living mammals and has persisted to this day," Bonnet said. "Those molecules and structures that make up skin, fur, nails and teeth were probably selected for other of their properties like being flexible, resistant to abrasion, or visible in wavelengths other than UV, and it is likely that those properties are still adaptive in many species, which would help maintain them."

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