An increasing number of animals are being found to fluoresce. Fluorescence happens when a chemical fluorophore absorbs of shorter-wavelength light and them emits some of the absorbed energy as light at a longer-wavelength. In some biological examples, blue light is absorbed and green light is emitted
Why animals fluoresce is the subject of active research. Does the change to a more noticeable colour mean that fluorescence is sometimes involved in signalling to other animals of the same sex, or of different sex, or of the same species, or to a predator, or to prey? Is it assisting in camouflage, the fluorescent emission matching, say, the emission from plants in the background?
Perception of fluorescence depends on the properties of the photoreceptors of the animal exposed to the fluorescence, so that in the case of proposed signalling between animals of the same species, it would be a requirement to demonstrate that the visual pigments of the eyes of that species can actually pick up the colour.
There is, of course, the possibility that there is no function of the fluorescence at all, that it is just a by-product of a particular molecule used as a pigment or for some other purpose in the skin or other tissue. This is the same argument as to why some creatures from the depths of the sea have bright colours; the production of brightly coloured molecules used by the animal are simply not selected against since there is no predator present that can see them.
A very well argued paper appeared in 2017 in Philosophical Transactions of the Royal Society by Justin Marshall (University of Queensland) and Sonke Johnsen (Duke University, North Carolina). The authors explained the background to explaining any function of fluorescence in terms of communication between individuals. They set out criteria that must be satisfied in order for such a role for fluorescence to be accepted. The Budgerigar, Melopsittacus undulatus, they found from descriptive and experimental studies, fulfilled all the criteria. In short, both males and females prefer to associate with potential mates that fluoresce.
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| Fluorescence characteristics of the budgerigar (a,b) White light and UV fluorescent excitation photographs of front and back of head showing fluorescent cheek and crown feathers from Marshall & Johnsen 2017 |
More recent studies have shown fluorescence from the bony tubercles of the skull of many species of chameleon—just one more feature added to the list of properties of those extraordinary animals. The phenomenon is particular evident in those chameleons living in humid forests ‘known to have a higher relative component of UV light’. The blue light emitted would be in sharp contrast to the brown and green colours reflected by the surroundings.
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| from Prötzel et al 2018 |
Even more recently came photographs showing fluorescence in a number of amphibians, this time a green emission from blue light (with some species showing emission in response to UV (ultraviolet)).
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| from Lamb & Davis 2020 |
The accumulating evidence for fluorescence in a wide range of animals (and plants) raises so many questions as to the molecular mechanisms involved and possible functions, that the only conclusion at present is the inevitable: more research is required.
In the meantime enjoy the pictures.
Lamb JY, Davis MP. 2020. Salamanders and other amphibians are aglow with biofluorescence. Scientific Reports 10, 2821. https://doi.org/10.1038/s41598-020-59528-9
Marshall J, Johnsen S. 2017 Fluorescence as a means of colour signal enhancement. Phil. Trans. R. Soc. B 372: 20160335. http://dx.doi.org/10.1098/rstb.2016.0335
Prötzel D, Heß M, Scherz MD, Schwager M, van’t Padje A, Glaw F. 2018. Widespread bone-based fluorescence in chameleons. Scientific Reports 8, 698. DOI:10.1038/s41598-017-19070-7
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