Ultraviolet light illuminates species-specific biofluorescent casque patterns in cassowaries (Casuarius)

Cassowaries possess prominent keratinous and bony cranial ornaments, called casques, which have been suggested to function in visual signaling. Despite possessing vividly colored head and neck skin, casque keratin appears dull, which has abated the casque’s proposed role as a visual indicator. Given that cassowaries, like many other birds, can perceive color in the ultraviolet (UV) range, we exposed heads of living and museum-preserved cassowaries to UV wavelengths (365; 385–395 nm) and discovered that casque keratin possesses biofluorescent patterns that differ significantly between species (Casuarius bennetti, C. casuarius, C. unappendiculatus). To assess whether casque surfaces reflect these patterns, we tested for the presence of UV reflectivity using a UV-sensitive camera. We found that casque keratin reflects 365 nm UV light, a wavelength within the range of cassowary eye sensitivity. However, our observations cannot confirm whether species-specific patterns can be detected by cassowaries in natural light conditions. Nonetheless, we recommend that biofluorescent signatures may prove useful to taxonomically identify incomplete specimens in museum collections as well as to more easily census living individuals and populations in the field. Altogether, our approach highlights the importance of future studies continuing to build upon the examination of UV reflectance, beyond the presence of fluorescence, as in many previous UV signaling studies.