The garden dormouse glows under UV light - but we don’t know why

Garden dormice may not seem particularly flamboyant. In fact, the small, brownish-white rodents spend much of their life trying not to be seen. But new research shows that under the right light, these animals shine with bright pinks and greenish-blues.

Photoluminescence occurs when a substance absorbs photons of ultraviolet light and re-emits them at longer wavelengths, often as visible colours. It occurs in many marine animals, some insects and millipedes and in the plumage of some birds. It encompasses two processes: fluorescence, when photons are re-emitted almost immediately, and phosphorescence, which can last for several minutes.

After seeing recent studies that found photoluminescence in nocturnal mammals such as flying squirrels and springhares, Grete Nummert at Tallinn Zoo in Estonia wondered if garden dormice (Eliomys quercinus) display it too. She made a high-stakes bet with a colleague that this would be the case: “The losing one would bake a cake,” says Nummert.

She and her team gathered a few of the garden dormice kept at Tallinn Zoo while they were sleeping and shone ultraviolet light on the animals.

Most of the mice’s fur glowed a bright purple colour, which looked reddish pink in photographs taken with a yellow filter on the camera lens. The feet and the nose, on the other hand, were bluish green.

To expand their observations, the researchers examined dead dormice stored at the Estonian Museum of Natural History under the UV light. These specimens also displayed photoluminescence, though their colours had faded over the years.

Nummert, who says the cake she won was “delicious”, isn’t sure yet why the dormice are photoluminescent. Some parrots use photoluminescence to signal to potential mates. Springhares have a patchy photoluminescence that may help camouflage them among plants that reflect light in a similar way, depending on what kind of creatures are looking.

It is also possible that the photoluminescence exhibited by dormice and other rodents is just a by-product of something they eat or some other natural process. It isn’t even clear if the dormice can perceive these colours themselves — humans certainly can’t without the help of an intense, artificial source of UV light.

“There is a whole world we cannot see,” says Nummert. “Animals perceive the world differently from us.”