Fluorescence, or biofluorescence, occurs in nature when electromagnetic wavelengths from the visible light spectrum are absorbed by fluorescent proteins in a living organism, and then emitted as light at a lower energy level. It differs from bioluminescence which is the natural production of light by chemical reactions within an organism. Bioluminescence occurs commonly in species such as fireflies, glow-worms and lantern fish. Fluorescence is less common than bioluminescence but still occurs widely across the animal kingdom, most commonly in aquatic saltwater species including a range of fish, crustaceans and jellyfish. Fluorescence was first discovered in aquatic seas turtles in 2015 (Gruber & Sparks, 2015) (Figure 1). It is rare in terrestrial vertebrate species but has been documented in a species of parrot (Hausmann et al., 2003). Amphibians exhibit a wide range of skin colourations and patterns but until 2017 no amphibian species were known to exhibit fluorescence (Taboada et al., 2017).
The South American Polka-dot tree frog (Boana punctata) lives in seasonal swamps of primary and secondary
forest in Ecuador. It is an unusual amphibian in that it possesses a
translucent skin. During fieldwork, Taboada et
al. (2017) observed that living adults and juveniles illuminated with UV-A
blue light produced a bright blue/green fluorescent emission. Through chemical
analysis the authors found that the fluorescence was traceable to a class of
compounds that occur in lymph and skin glands. Under natural conditions the fluorescence
is highest under low-light conditions and enhances the brightness of
individuals. B. punctata is a
crepuscular species, being most active at dawn and dusk. Studies of the eye
structure of closely related species suggests that B. punctata may be able to detect fluorescence and the authors propose
that fluorescence has an adaptive significance and may be used in the visual
communication in this species. This is the first study to document fluorescence
in amphibians and suggests that if may be more widely spread in tropical
Since 2017, fluorescence has been discovered in other amphibian species. Pumpkin toadlets are a group of small, terrestrial and diurnal frogs which live in the Atlantic forest of Brazil. Most species are brightly coloured yellow, orange or red and possess toxic skin compounds. Recently, research has found that two species, the pumpkin toadlet Brachycephalus ephippium and B. pitanga, exhibit fluorescence in the bones of the head and back, which is visible through their particularly thin skin (Goutte et al., 2019). Bone fluorescence in living animals is rare, but has been described in some species of chameleon. Brachycephalus ephippium and B. pitanga have recently been shown to be deaf to their own advertisement calls and are known to use visual communication signals involving hand-waving and mouth-gaping (Goutte et al., 2017). The authors propose that fluorescence in these pumpkin toadlets could be used to enhance visual communication between members of the same species such as attracting mates. However, the fluorescent patterns could alternatively be used to reinforce the warning coloration in these pumpkin toadlets which are highly toxic (Goutte et al., 2019).
This study adds to the growing list of documented cases of fluorescence in terrestrial amphibians. Although fluorescence is rare in amphibians, there is a high likelihood that a greater number of species utilise fluorescence, particularly in species which are crepuscular/diurnal and use visual communication. Further research is required to investigate the prevalence of biofluorescence across amphibian species and elucidate its ecological and adaptive significance.
Goutte, S., Mason, M. J., Christensen-Dalsgaard,J., Montealegre-Z, F., Chivers, B. D., Sarria-S, F. A., Antoniazzi, M. M., Jared, C., Sato, L. A. & Toledo, L. F. (2017) Evidence of auditory insensitivity to vocalization frequencies in two frogs. Nature Scientific Reports, 7: 12121. DOI:10.1038/s41598-017-12145-5.
Goutte, S., Mason, M. J., Antoniazzi, M. M., Jared, C., Merle, D., Cazes, L.,Toledo, L. F., el-Hafci, H., Pallu, S., Portier, H., Schramm, S., Gueriau, P. & Thoury, M. (2019) Intense bone fluorescence reveals hidden patterns in pumpkin toadlets. Nature Scientific Reports, 9: 5388. https://doi.org/10.1038/s41598-019-41959-8.
Gruber, D. F. & Sparks, J. S. (2015) First Observation of Fluorescence in Marine Turtles. American Museum Novitates, 3845, pp 7. DOI: 10.1206/3845.1
Hausmann, F., Arnold, K. E., Marshall, N. J. & Owens, I. P. F. (2003) Ultraviolet signals in birds are
special. Proceedings of the Royal Society London B, Biological Society, 270 (1510): 61–67.
Taboada, C., Brunetti, A. E.,Pedron, F. N., Neto, F. C., Estrin, D. A., Bari, S. E., Chemes, L. B., Lopes, N. P., Lagorio, M. G. & Faivovich, J. (2017) Naturally occurring fluorescence in frogs. Proceedings of the National Academy of Sciences, 114 (14): 3672–3677.