Field observations of putative bone-based fluorescence in a gecko

A diversity of animals are biofluorescent, absorbing short-wavelength electromagnetic radiation, and re-emitting it at longer wavelengths, giving a distinctive pattern (Lagorio et al. 2015). Among terrestrial organisms, biofluorescence has been commonly recorded in arthropods, but rarely from vertebrates (Lagorio et al. 2015). It has long been known from parrots (Hausmann et al. 2003) and has recently been recorded from a frog (Taboada et al. 2017) and from chameleons (Chamaeleonidae: Prötzel et al. 2018).

In their recent paper, Prötzel et al. (2018) reported that chameleon fluorescence is based on the properties of bone visible through the skin, the first known case of externally visible bone-based fluorescence in vertebrates. They suggest that bone-based fluorescence could be widespread, especially in other squamates, which often use bony protuberances as ornamentation. I here report a further possible case of bone-based fluorescence in another squamate group, the geckos (Gekkonidae), discovered accidentally while searching for scorpions in South Africa using an ultraviolet (UV) light.

Bibron’s gecko, Chondrodactylus (=Pachydactylus) bibronii (Smith), is a widespread South African gecko, which is night active, occurring among rocks and also associated with buildings (Barts 2010). During searches at night using a UV torch (CREE Q5, 395–410 nm wavelength, adjustable focus) around a building in the Karoo National Park, South Africa (32.3151°S, 22.3443°E) in early August 2017, this species was observed to exhibit fluorescence. Pictures were taken of the geckos with and without UV illumination using a digital camera (Sony Cybershot DSC-HX400V) with a flash used for non-UV photos. Illustrative pictures are shown in Figure 1.

Figure 1.

Three views of C. bibronii under normal light and UV (395–410 nm wavelength) light. (A) Under normal light (camera flash). (B) The same gecko under intense UV light (focused beam) showing the strongly fluorescent head elements and fluorescent trunk elements (arrowed, I and II). (C) The same gecko again under more diffuse UV light, with just the head fluorescing.

Several C. bibronii individuals were observed: although it was not possible to unequivocally identify their life stage or sex, these included larger individuals presumed to be adults. The head was quite strongly UV fluorescent anteriorly (Figure 1B, C), extending behind the eyes. The florescence did vary in its intensity but did not obviously correspond with particular skin elements visible under normal light (compare Figure 1A, B). There appears to be a good correspondence with the skull structure of the gecko (e.g., see figures in Rieppel, 1984). The fluorescence is strongest anteriorly where the bony covering of the skull is greatest; it is absent from the large eye-sockets and at the back of the head where there is no bony covering (Figure 1B, C). In at least one individual, when intensely lit (Figure 1B) it was also possible to see faint additional fluorescence coincident with skeletal elements in the trunk of the body, notably the vertebral column, limb-bones, and pelvis, although these were not visible at lower intensities (Figure 1C) and were never as bright as the head.

As these observations were made on live individuals in the field, and anatomical and spectral emission studies were not carried out, further work is required to examine this phenomenon in more detail. However, despite their limited scope, these observations appear to support Prötzel et al. (2018) in their contention that other squamates might also possess bone-based fluorescence. Its adaptive value remains open to question, as in some other groups (Lagorio et al. 2015). Geckos use visual displays in part for intraspecific signaling (Marcellini 1977) and in least some species are able to distinguish colors even under night-time conditions (Kelber and Roth 2006). Chondrodactylus bibronii may use signaling to aggregate (Meyer and Mouton 2007), while males are territorial and behave aggressively toward each other (Barts 2010). Potentially, therefore, UV fluorescence could serve a number of signaling purposes, but further work is required to confirm or refute this role.