To human eyes, flying squirrels of the Glaucomys genus have soft brown fur, with a snowy white underside. To one another, however, and to other animals which can detect ultraviolet light, they are a vivid candy pink.
The squirrels’ pinkiness was discovered quite by chance. Jonathan Martin of the US Department of Natural Resources was conducting a survey of fluorescing lichens with a handheld UV light in a Wisconsin forest. Martin shone the light at a flying squirrel that he happened upon and was stunned to see it glowing pink.
A surprising number of vertebrates, including birds, fish, frogs, reptiles fluoresce variously in blue, pink, red, yellow and green colours. Despite the wide occurrence across taxa, the ecological significance is not always clear.
In budgerigars, the brightness of the fluorescent feathers is an important signal in mate choice. As with budgies, fluorescence in fish is used for communication between individuals, and also indicated in camouflage and predator avoidance.
In mammals, fluorescence is not widely documented or studied, with only a couple of known examples. The Virginia opossum (Didelphis virginiana) has fluorescence of the skin and fur, and the eastern fox squirrel (Sciuris niger) has high incidence of fluorescence in teeth and skull, but not their external features.
After Martin’s discovery of the fluorescing flying squirrel, a team of researchers led by Allison Kohler, also from the Department of Natural Resources, conducted a formal investigation into the prevalence of fluorescence in the Glaucomys genus, and some related North American species.
Museum specimens of all three members of the genus from a wide geographical range across the US, Canada and extending south to Mexico and Guatemala, across a temporal range of 130 years, were exposed to UV light.
All 109 specimens, male and female, exhibited pink fluorescence. For comparison, museum specimens of three co-occurring diurnal squirrel species were also placed under UV light – with no fluorescence observed.
The researchers thus reported in the Journal of Mammalogy that external UV fluorescence occurs across all species, individuals, sexes, time scale and habitats of the Glaucomys genus, and speculate that is an ecologically significant trait.
Glaucomys squirrels are the only ones that are active at dawn and dusk, and at night, and UV wavelengths are relatively abundant during twilight periods. The animals also stay active year round in the UV-rich environments of snowy forests instead of hibernating like other small mammals.
The forests abound with fluorescing lichens, and several co-occurring species of owl also fluoresce pink on their pale feathers. “In short,” the researchers suggest, “fluorescent flying squirrels may blend into their UV-saturated fluorescent environment.”
New World flying squirrels are only distantly related to opossums – the only other mammals with a distinctly pink pelage – and both have twilight activity patterns that “may be key to understanding the ecological significance of their fluorescence”, the paper concludes.