Namib Sand Geckos: these nocturnal geckos have biofluorescent markings that emit a bright, neon-green glow when exposed to the moonlight
This species (Pachydactylus rangei) is found only in the Namib Desert, which stretches across Namibia, Angola, and South Africa; the geckos typically inhabit the arid, coastal region known as the Skeleton Coast.
In order to escape from the blistering heat of the desert, they use their webbed feet to burrow down into the sand during the day, and then emerge only at night, when the temperature has finally dropped. The webbing on their feet also enables them to run more easily across the dunes.
Namib sand geckos are covered in translucent scales, but they also have a strangely colorful appearance, as the colors/shades of their circulatory system, spinal column, internal organs, and optical membranes remain partially visible through the skin, producing various shades of pink, dark blue, purple, magenta, orange, and yellow.
They also have several distinctive markings running along their lower flank and encircling their eyes; these markings are known to fluoresce when exposed to UV light (including moonlight), emitting a bright, neon-green glow.
Some researchers have theorized that the biofluorescent markings may act as a signal to other geckos, allowing them to locate one another in the vast, desolate expanse of the desert, as this paper explains:
The fluorescent areas of P. rangei are concentrated around the eyes and along the lower flanks. This positioning is practically invisible to predators with a higher perspective (e.g. birds and jackals), but highly conspicuous from a gecko’s perspective. As P. rangei is sociable but generally solitary, and occurs at low population densities, such a signal might serve to locate conspecifics over greater distances ...
Encounters in P. rangei might serve purposes beyond mating opportunities: as the Namib desert has extremely low precipitation, fog is a key water source for its flora and fauna. Fog condenses on the bodies of the geckos, and they lick it from their faces. In husbandry, we have observed individuals licking water from conspecifics, taking advantage of a much greater available surface area.
Additionally, after short periods of isolation, the geckos run to meet each other. The combination of vital hydration with socialisation might reinforce signals that enable such meetings, and the cost of visibility to predators with higher vantage points, might constrain the signals to regions best visible from eye-level and below.
The Namib sand gecko is the only terrestrial vertebrate that is known to use an iridophore-based form of biofluorescence (you can find a more detailed explanation of that mechanism in the article mentioned above). The fluorescent dermal markings are also unique to this species.
Scientific Reports: Neon-green fluorescence in the desert gecko Pachydactylus rangei caused by iridophores
Animal Diversity Web: Pachydactylus rangei
Dr. Mark D. Scherz's Blog: A Neon-Green Glowing Gecko!
Australian Geographic: Skeleton Coast - Namibia's strange desert dwellers
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