Fire salamanders glow green and cyan under ultraviolet light

Shine an ultraviolet lamp on a fire salamander and the animal does something that had not been recorded before: it glows. The black-and-yellow amphibian, found across Europe and known for the poisonous liquid it releases from its skin, lights up in green and cyan tones. The effect had gone unnoticed despite decades of study of the species.

The glow sits on the yellow parts

The brightest glow appears on the salamander's yellow underside and along its sides. Researchers traced it mainly to glands in the skin and to the substances those glands release. Those secretions keep glowing for more than 24 hours after they leave the animal's body.

The glowing material also turns up in the salamander's blood and in its granular glands, which suggests the fluorescent molecules are spread through the body rather than sitting in one place. That kind of body-wide glow is rare. The researchers note it had previously been found only in tree frogs.

"It is fascinating that such a well-studied species still harbors unknown phenomena like this," says Bernat Burriel of the Museum of Natural Sciences in Barcelona, the first author of the study.

The salamander is not making the light itself

Fireflies and other glowing creatures produce their own light through chemical reactions inside their bodies. A fire salamander instead absorbs ultraviolet light, which human eyes cannot see, and sends it back out as visible colour. That is why the animal looks ordinary in daylight and only glows under a lamp that gives off ultraviolet light. The institute behind the work describes the colour as turquoise-blue.

In the forests where fire salamanders live, the only night-time light comes from the moon and stars, and full moonlight carries a larger share of ultraviolet and violet wavelengths than daylight does. The researchers suggest this could add small green and cyan flecks to the salamander's yellow markings.

For years, glowing animals were thought to live almost entirely in the sea, with scorpions as a rare land exception. Recent work has found the same trick in reptiles, birds and other amphibians. The new study, published in Royal Society Open Science, was led by researchers from the Museum of Natural Sciences in Barcelona, the Institute of Evolutionary Biology in Spain and the Max Planck Institute for Chemical Ecology in Jena, Germany.

What the glow is for is still unknown

One idea is that it helps salamanders find each other in dark or crowded places, or choose a mate. Another is that it strengthens a warning the animals already give. Fire salamanders advertise their poison with bold yellow and black patterns, a strategy called aposematism: bright colours tell predators that an animal is dangerous. A faint extra glow could make that message easier to spot.

"Fluorescence meets several criteria that suggest a communicative function," says Martin Kaltenpoth, a director at the Max Planck Institute for Chemical Ecology and a co-author of the study.

The link between salamander warning colours and their toxins was worked out more than a century ago, when scientists identified samandarines, powerful poisons the animals build from cholesterol-like starting materials. The glow was a surprise even so. "The presence of this fluorescent compound was surprising because salamander skin secretions have been studied chemically for decades, and we were unaware of any published reports on fluorescence," says Andrés Brunetti of the Max Planck Institute for Chemical Ecology.

The species is listed as vulnerable on the IUCN Red List of Threatened Species, with habitat destruction and fragmentation among the threats it faces.