Widespread bone-based fluorescence in chameleons

Abstract

Fluorescence is widespread in marine organisms but uncommon in terrestrial tetrapods. We here show that many chameleon species have bony tubercles protruding from the skull that are visible through their scales, and fluoresce under UV light. Tubercles arising from bones of the skull displace all dermal layers other than a thin, transparent layer of epidermis, creating a 'window' onto the bone. In the genus Calumma, the number of these tubercles is sexually dimorphic in most species, suggesting a signalling role, and also strongly reflects species groups, indicating systematic value of these features. Co-option of the known fluorescent properties of bone has never before been shown, yet it is widespread in the chameleons of Madagascar and some African chameleon genera, particularly in those genera living in forested, humid habitats known to have a higher relative component of ambient UV light. The fluorescence emits with a maximum at around 430 nm in blue colour which contrasts well to the green and brown background reflectance of forest habitats. This discovery opens new avenues in the study of signalling among chameleons and sexual selection factors driving ornamentation.

Figure 1

Chameleons of the genus Calumma with fluorescent tubercles of bony origin. (A) Male C. globifer (ZSM 141/2016) showing congruent tubercle/fluorescent patterns (from left to right); top row: alive in the field under sunlight, micro-CT scan of head surface (probable edge artefact in cheek region), micro-CT scan of the skull; bottom row: alive in the field under UV light, ethanol-preserved under UV light. (B) Excitation-emission matrix (intensity in arbitrary units) of fluorescent tubercles on right temporal region of C. globifer (ZSM 221/2002). (C) Fluorescent tubercles (FTs) on temporal region (right body side) of a male C. crypticum (ZSM 503/2014) under visible (above) and additional UV light (below); framed area of the skin, including a FT, was histologically sectioned (Fig. 2). (D) Distribution of FTs on the head surface (left) and micro-CT scan (right) of the head of C. guibei (ZSM 2855/2010) shows the bony origin of the FTs.

Figure 2

Histological sections of skin of a male Calumma crypticum (ZSM 503/2014) from the temporal region (framed in Fig. 1C). (A,B) 3D-reconstruction (volume rendering) of 279 histological sections of skin (without tubercle) (A) and of tubercle (B), i.e. upper margin of frame in Fig. 1C). (C) Section of skin and underlying bone in adjacent skin, stained (Richardson). (D) Section of FT (centre, stained). (E) Detail of skin near FT (stained). (F) Detail of FT (stained). For detailed views of the chromatophores, see Supplementary Fig. S5.

Figure 3

Analysis of distribution of FTs on adult individuals of Calumma species groups. (A) Schematic chameleon phylogeny of Tolley et al. updated with genus Palleon, light blue lines indicate genera where fluorescence occurs (* indicates genus where no data was available). (B) Mean value of fluorescent tubercles (FTs) per head side and species, males plotted against females of 126 individuals/24 species assigned to four species groups; dashed diagonal line shows 1:1 ratio, samples below it show more tubercles in males than females. (C,D) PCA scatter plots assigned to four species groups, factor loadings are given in Supplementary Tables S2, 3; (C) Distribution of FTs per head side based on 12 characters (Supplementary Table S4) of 140 individuals/29 species. (D) Number of FTs per cranial bone from lateral view of 25 adult males of different Calumma species based on 6 characters (Supplementary Table S5). (E) Bar charts of number of FTs per cranial bone (M, maxilla; PRF, prefrontal; Flat, frontal seen laterally; POF, postorbitofrontal; JU, jugal; SQ, squamosal; see Fig. 1D) from lateral view of 25 adult males of different Calumma species.

Figure 4

Fluorescent tubercles showing sexual dimorphism under UV light at 365 nm (A–D) and fluorescence in further chameleon genera (E–G). (A) Male Calumma crypticum ZSM 32/2016. (B) Female C. crypticum ZSM 67/2005. (C) Male C. cucullatum ZSM 655/2014. (D) Female C. cucullatum ZSM 654/2014. (E) Brookesia superciliaris, male (only UV light at 365 nm). (F) Bradypodion transvaalense, male (dim light and additional UV light at 395 nm). (G) Furcifer pardalis, male (daylight and additional UV light at 365 nm). For details see ‘fluorescent photography’ in Materials and Methods.