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 dormice shine with bright pinks and greenish-blues.

Photoluminescence occurs when a substance absorbs photons of ultraviolet light and re-emits them at longer wavelengths. 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) would display photoluminescence too. She made a high-stakes bet with her colleague: “The losing one would bake a cake,” Nummert says.

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

Most of the mice’s fur shone a bright reddish pink. Without the yellow filter, the fur appears more of a purple colour. The feet and the nose, on the other hand, were bluish green.

To expand their observations, the researchers examined dormice stored at the Estonian Museum of Natural History under the UV light. These old 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 them camouflage among plants that reflect light in a similar way, depending on what kind of creatures are looking.

It’s 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 UV light.

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

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