Fire salamanders, the black-and-yellow amphibians that are among Europe's most recognisable animals, glow green and cyan under ultraviolet light, according to research published in Royal Society Open Science.
The study, announced by the Max Planck Institute for Chemical Ecology and reported by ScienceDaily, traced the fluorescence mainly to glands in the animals' skin and to the toxic substances those glands release. The secretions can continue to glow for more than 24 hours after leaving the salamander's body.
The glow is most pronounced on the yellow underside and along the flanks. Humans need a UV lamp to see it at all.
Biofluorescence occurs when compounds absorb ultraviolet light, which human eyes cannot detect, and reemit that energy as visible light. It requires an outside source of illumination, and should not be confused with bioluminescence: fireflies make their own light through chemical reactions, whereas a fluorescent animal must first absorb light from its surroundings.
Scientists once believed biofluorescence was almost exclusively a marine phenomenon, with scorpions standing out as an unusual example on land. More recent work has identified it in reptiles, birds and amphibians.
“It is fascinating that such a well-studied species still harbors unknown phenomena like this,” said Bernat Burriel of the Museum of Natural Sciences in Barcelona, the study's first author. “It reminds us that even the most familiar organisms can hide secrets that are only revealed when they are observed with new tools.”
The purpose of the glow is unknown. One possibility is communication, helping individuals recognise one another or assess potential mates; another is that it makes the animals' existing warning colouration more conspicuous to predators.
“Fluorescence meets several criteria that suggest a communicative function,” said Martin Kaltenpoth, director of the Department of Insect Symbiosis at the Max Planck Institute for Chemical Ecology and a co-author. “It could help salamanders detect each other in nocturnal or particularly dense environments, or act as an additional defense signal.”
In the damp forests where fire salamanders live, night-time light comes mainly from the moon and the stars, and full moonlight contains a greater proportion of ultraviolet and violet wavelengths than daylight does. Under those conditions, the researchers note, fluorescence could produce small green and cyan highlights against the yellow markings.
The chemistry is older than the discovery. Scientists connected salamander warning colours to chemical defence more than a century ago, when they identified samandarines, highly toxic steroidal alkaloids produced from cholesterol precursors. “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,” said Andrés Brunetti of the Max Planck Institute, a co-first author.
The compound responsible has not been identified. “We still don't know what the compound responsible for this fluorescence is, but everything indicates that it is a molecule unknown until now in this species,” said Salvador Carranza of the IBE in Barcelona, another co-author. The species is classified as vulnerable on the IUCN Red List.
The detail that merits attention is the 24 hours. A defensive signal that goes on working after the animal has gone is not a signal from an animal at all. It is a notice, left in place, maintained by nobody.
Taken literally, and this newspaper sees no reason not to, the forests of Catalonia contain warnings that outlast the thing being warned about. A salamander deposits its secretion, glows for a moment and departs. The secretion stays behind and goes on advertising a salamander that is no longer there, for a full day, in two colours.
The arrangement has obvious administrative advantages. The notice requires no supervision, cannot be withdrawn, and continues to deter predators from an animal that has already left the area. Our analysts observe that it is also unsigned: because the compound has not been identified, nobody can say what is issuing the warning, only that it is still issuing it.
There is one condition. Fluorescence needs an outside light source, so the notice is legible only when something shines ultraviolet on it, which in a forest at night means the moon. The Department of Unnecessary Research has drawn the only conclusion available. For twenty-four hours after a salamander has passed, its warning remains in force, and the moon is responsible for switching it on.