Fur glowing under UV: a widespread consequence of porphyrin accumulation in mammals

Abstract

Spectacular photoluminescence (PL) phenomena have been increasingly reported in various organisms from diverse ecosystems. However, the chemical basis of this PL remains poorly defined, and its potential ecological function is still blurry, especially in mammals. Here we used state-of-the-art spectroscopy and multispectral imaging techniques to document new cases of mammalian ultraviolet-induced PL (UV-PL) and to identify free-base porphyrins and natural derivatives as the organic compounds responsible for the reddish luminescence observed in the hairs and spines of distantly related species. We argue that pink to red UV-PL is predominantly observed in crepuscular and nocturnal mammals because porphyrins are photodegradable, and that this phenomenon might not have a specific function in intra- or interspecific communication but consists of a byproduct of a widespread physiological condition, overlooked in mammals.Co-first authors: Séverine Toussaint and Jasper Ponstein

Using mass spectrometry to investigate fluorescent compounds in squirrel fur

“…Our findings are consistent with suggestion of Toussaint et al (2021) [40], who argued that fluorescence in mammals is widespread and due to porphyrins. Porphyrin-based fluorescence can result from porphyrins from the heme pathway accumulating in tissues.…”

“…Porphyrin-based fluorescence can result from porphyrins from the heme pathway accumulating in tissues. Interestingly, Toussaint et al (2021) [40] suggested that nocturnal animals are more likely to exhibit fluorescence because porphyrin compounds are broken down by exposure to UV, and so would tend to accumulate more in nocturnal species. While the over-accumulation of porphyrins in mammals might be expected to have adverse effects on an individual's physiology, this appears to be a widespread phenomenon and may not lead to any fitness consequences.…”

“…While there is some evidence to suggest that some nocturnal mammals may be capable of detecting fluorescent pigmentation it remains unknown whether fluorescence has an ecologically significant role [38,39]. Most recently, Toussaint et al (2021) [40] have suggested that fluorescence in mammals is widespread and caused by an accumulation of porphyrins. They further suggested that porphyrins break down through exposure to UV, and consequently, we might expect fluorescence to be more common among nocturnal species.…”

“…While porphyrins appear to cause fluorescence in the bones, teeth, and pelage of certain mammals [32,40], other fluorophores are known to cause fluorescence in mammal fur, including tryptophan and tryptophan derivatives [13]. Tryptophan is one of three aromatic amino acids [24], while tryptophan derivatives, including 3-hydroxlanthanalic acid, kynurenine, kynurenic acid, and xanthurenic acid are all conjugated molecular systems that contain an aromatic ring.…”

“…We currently do not understand the cause of fluorescence in flying squirrels, or whether this is an adaptive trait. Several potential adaptive benefits of fluorescence in flying squirrels may include anti-predator defense mechanisms and social communication; however, fluorescence may also be an artefact [1,40]. We consider establishing biochemical pathways a good starting point to better our understanding of the evolutionary processes leading to this trait.…”