Photoluminescence (encompassing both fluorescence and phosphorescence) is the absorption and re-emission of light, usually converting photons from lower to higher wavelengths. Since this phenomenon occurs vividly in some, but not all, mammals, the question emerges of whether fur photoluminescence is optically meaningful for those species that possess it. Despite sporadic accounts of photoluminescent mammal species in the literature, there have been no dedicated studies of the prevalence of this trait in any region of Australia. The photoluminescent characteristics of fur have never been examined for most mammal species worldwide. Only a handful of fur luminophores (fluorophores and/or phosphors) have been identified to date, with more suspected to be present in fur. The nature of photoluminescence in fur is also little understood, but has been noted as brighter in live and recently dead animals, with recent museum-based studies flagging, but not accounting for, the chemical changes that fur undergoes in different conditions. Since its detailed documentation in European rabbits (Oryctolagus cuniculus) more than 100 years ago, most studies have assumed that photoluminescence is a dormant by-product of some unknown physiological function. However, potential visual functions have recently been hypothesised because of a resurgence of interest coupled with colour photographs of mammals photoluminescing.
In this thesis, I studied photoluminescence in Australian mammals from the Wet Tropics of Far North Queensland. I addressed gaps in the literature associated with prevalence, the luminophores responsible, retention of photochemical properties, and the function of photoluminescence in the field. Firstly, I investigated how prevalent the phenomenon of photoluminescence is among mammals of the Wet Tropics, Australia, using fresh roadkill animals and frozen specimens from three collections. Although only a subset of Wet Tropics mammal diversity was studied here, I present the most comprehensive account to date of the occurrence of fur photoluminescence across taxa using fresh roadkill animals. Ninety-five per cent of mammals displayed at least a subtle photoluminescence in the fur at some wavelengths. Forty-two per cent of marsupial species and 29% of placental species displayed noticeably bright photoluminescence. Both monotreme species exhibited subtle photoluminescence. There appeared to be no pattern associated with specific diet or lifestyle factors based on species life history characteristics. My findings suggest that photoluminescence is more common than previously known, and that the biochemical basis of fur photoluminescence may be common among mammals.
Secondly, I collected fur samples from seven of these Wet Tropics mammal species to extract and identify the luminophores contributing to photoluminescence. I used high-performance liquid chromatography and liquid chromatography/electrospray ionisation mass spectrometry to identify these luminophores. For two species of bandicoot (the long-nosed bandicoot (Perameles nasuta) and the northern brown bandicoot (Isoodon macrourus)), the northern quoll (Dasyurus hallucatus) and the platypus (Ornithorhynchus anatinus), the work presented here is the first attempt to isolate luminophores from the fur in these genera. I found evidence that supported the presence of coproporphyrin and protoporphyrin, and molecules matching the monoisotopic masses of uroporphyrin and heptacarboxylporphyrin, in the species studied here. These porphyrins had already been identified in the pelage of other mammal species, and exist in a range of organisms from bacteria to birds. Several other photoluminescent molecules extracted from the fur remain to be identified.
Thirdly, I investigated the lability of pink fur photoluminescence in response to light exposure, to ascertain whether observed intraspecies differences can be taken at face value, or whether they may be confounded by environmental conditions. I also tested the effects of wet preservation on both pink and blue fur photoluminescence. I conducted photobleaching experiments using northern brown bandicoot and long-nosed bandicoot pelts and found that pink photoluminescence noticeably fades in as little as two minutes of full sun exposure. These experiments have important implications for researchers working with porphyrin-based photoluminescence. Wet preservation in ethanol nearly extinguished the photoluminescence of both laboratory (Norway) rat (Rattus norvegicus) and bandicoot fur, but initial fixation in formalin partially preserved photoluminescence at a low level. These findings flag the probability of false negatives in studies based solely on museum specimens.
Finally, I investigated the plausibility of a visual function for fur photoluminescence by placing photoluminescent and non-photoluminescent models in the field and assessing the behavioural responses of wild animals to these models over a six-month period. I used remote cameras to observe behaviour under both full moon and new moon cycles to determine whether photoluminescence could be triggered by natural nocturnal lighting conditions. I found that wild nocturnal animals did not show a preference for either model, suggesting either that natural moonlight was not sufficient to stimulate photoluminescence, that wild nocturnal vertebrates were unable to detect photoluminescence in natural conditions, or that these animals do not use this visual property of fur when making behavioural decisions.