One of many exciting discoveries made in tetrapod biology in recent decades is that UV-sensitive vision is not just a thing that exists, but a thing that’s widespread.
Caption: would a live dinosaur - like this heterodontosaur - look utterly different if its tissues were photoluminescent? Brian Engh explored this possibility in this excellent piece of art, included in Woodruff et al. (2020). Image: Brian Engh.
We’ve known since the early 1980s that at least some birds can detect UV wavelengths, and research published more recently has demonstrated its presence in lizards of disparate lineages, in turtles, rodents and, most recently, amphibians. Some of these animals use their UV-sensitive vision to find food (like pollen-rich flowers) and perhaps even to navigate their environments (UV-sensitive vision in certain forest-dwelling birds might enhance their ability to see certain kinds of leaves, for example).
That’s great, but what’s even more surprising – though maybe it shouldn’t be – is that markings and tissue types in some of these animals are visible to other animals with UV-sensitive vision. Furthermore, some tissue types are able to absorb UV and re-emit it within part of the spectrum visible to we humans. It’s this aspect of the UV story – the possibility that UV is absorbed and emitted as visible light (typically blue light) – that we’re talking about hereon, not UV-sensitive vision. Note that the terms used for this phenomenon are slightly contentious among relevant experts. Most agree that the right term is fluorescence whereas others (including my colleague Jamie Dunning) argue that we should use the more specific photoluminescence. I have no proverbial dog in this fight but am going to stick with photoluminescence here seeing as it’s the one we used in the relevant paper.
Caption: in 2018, Jamie Dunning and colleagues reported the discovery of photoluminescence in puffins. Image: (c) Jamie Dunning.
The discovery of photoluminescence in animals is evidently of broad general interest, and I can make this assertion because several recent studies reporting its occurrence have received an unusual amount of public interest. Dunning et al.’s (2018) report on its occurrence in the brightly coloured bill plates of puffins, for example, proved a really popular discovery (Wikinson et al. (2019) followed up with a subsequent study on the keratinous horns of rhinoceros auklets), as did Prötzel et al.’s (2018) discovery of photoluminescent bones in chameleons. Remarkably, Prötzel et al. (2018) were able to show that the ‘glowing’ bones of these lizards are visible through the skin. At the time of writing, a study reporting widespread photoluminescence in living amphibians has just appeared, and it too has received a fair amount of general interest.
Here it’s worth making a critical point on the popularity of these studies in the popular media. There’s no doubt that this stuff is interesting, and certainly of relevance to biologists at large (for one thing, knowing about the distribution of fluorescence/photoluminescence could have all kinds of implications for surveying and collecting). But there’s concern that the studies are being framed in the wrong way, and that more thorough vetting is needed, in places. Also worth noting is that what role photoluminescence actually has to the animals that emit it is controversial, since some workers argue (a) that its visual signalling role hasn’t been sufficiently tested for, and (b) it may simply be too subtle to be of much use to the animals in which it’s present. Keep this in mind when reading the following!
Caption: Prötzel et al.’s (2018) bone-glow research on chameleons shows that the photoluminescing bones of these lizards were actually visible through the skin. Image: David Prötzel.
These caveats notwithstanding, if UV-themed visual displays are widespread in tetrapods, those of us interested in fossil animals are presented with an interesting set of possibilities. We already think that the many extravagant structures of non-bird dinosaurs and pterosaurs – they include cranial horns, crests and casques as well as spikes, spines, sails, bony plates and so on – functioned predominantly in visual display. Could they also have been photoluminescent, and could this have then been used to enhance the display function of the structures in question?
Caption: dinosaurs and pterosaurs are of course notable for their remarkable variety of what I term extravagant structures, a selection of which are depicted here. (a) Parasaurolophus, a hadrosaurid ornithopod. (b) Pachycephalosaurus. (c) Triceratops, a ceratopsid ceratopsian. (d) Dilophosaurus, a theropod. (e) Pteranodon and (f) Tupandactylus the pterodactyloid pterosaurs. (g) Miragaia the stegosaur. (h) Edmontonia the nodosaurid ankylosaur. From Woodruff et al. (2020), images by Darren Naish.
In a brand-new paper published this week in Historical Biology (or on its website, anyway), Cary Woodruff, Jamie Dunning and I set out to consider this very question (Woodruff et al. 2020). At the risk of spoiling the surprise I’ll say that we don’t provide a hard or definitive answer; our aim instead is to bring attention to the possibility that photoluminescence might have been present in some of these animals. We encourage the testing of this possibility and suggest some specific ways in which this testing might be performed. Of incidental interest is that our collaboration evolved from a Twitter discussion (which is currently findable here).
Caption: a palaeontologist ponders new papers on photoluminescence, and then gets talking to one of the relevant researchers. And I chimed in as well, sorry. The rest is history…
I should also add that our idea isn’t especially new. Ever since UV-sensitive vision was first reported in birds back in the 1980s, the idea that extinct dinosaurs might have made use of photoluminescence has been mooted (though, let me make the point again: you don’t need UV-sensitive vision to see photoluminescence). I’ve incorporated photoluminescence into more than one dinosaur-themed media project, most recently Dinosaurs in the Wild.
Caption: the idea that Mesozoic dinosaurs might have been exploiting photoluminescence isn’t altogether new. Here are rough sketches I produced depicting the concept of a photoluminescent Leptoceratops produced for the travelling visitor experience Dinosaurs in the Wild. Image: Darren Naish.
A few specific points are worthy of attention. Above, I mentioned Prötzel et al.’s (2018) chameleon-themed ‘bone glow’ study. Bone-based photoluminescence has also been reported in frogs, specifically in the Brachycephalus pumpkin toadlets (Gouette et al. 2019). Could those dinosaurs superficially similar to chameleons (namely ceratopsians: like some chameleons, they have bony frills and prominent horns) also possess bone-based photoluminescence and, if so, could they exploit it in chameleon-like fashion? Well, probably not, mostly because the much larger size of these dinosaurs means that their skin was too thick for this to work (Woodruff et al. 2020).
Caption: for fun, let’s use toy ceratopsians rather than the real things. Could these dinosaurs have had ‘glowing’ bones as modern chameleons do? No, almost certainly not. Image: Darren Naish.
One of the most unusual things about non-bird dinosaurs possessing extravagant structures is that males and females are extremely similar (albeit not necessarily identical) with respect to the form and proportional size of said structures. As regular TetZoo readers might recall from several articles published here within recent years (see links below), some workers interpret the extravagant structures of Mesozoic dinosaurs as functioning within a model of species recognition. According to this model, the structures function as banners used to signal membership of whatever the respective species is. I don’t think that this is valid for a bunch of reasons and in fact I don’t think that extravagant structures have an important role in species recognition at all (Hone & Naish 2013, Knell et al. 2013). An alternative model posits that extravagant structures mostly have an intraspecific function, work as sociosexual signals of reproductive quality, and evolved within the context of sexual selection. This is the model that I and my colleagues support (Hone et al. 2011, Knell et al. 2012, 2013, Hone & Naish 2013), and a lengthy debate that’s been thrashed out in the literature over the past decade pits species recognition and sexual selection as opposing schools of thought.
Caption: at left, mutual sexual selection at play in the Great crested grebe, as illustrated by Julian Huxley in 1914. At right, cover of the famous issue of TREE which includes Knell et al.’s (2012) seminal review.
But if this is so, why is it that ostensible males and females in the dinosaur species concerned are monomorphic: that is, they have similar extravagant structures? Back in 2011, Dave Hone, Innes Cuthill and I argued that these animals might have evolved their extravagant structures within the context of mutual sexual selection (Hone et al. 2011), this being the strategy where both males and females use their extravagant structures in sociosexual display. But while we know that extant monomorphic animals really are monomorphic, we’re not sure that this is (or was) the case for extinct ones: it could still be that their structures differed in hue, colour or some other visual property. If we’re speculating about the possible presence of photoluminescence in extinct archosaurs, the possibility exists that “monomorphic elaborate structures in pterosaurs and non-bird dinosaurs were not monomorphic in life” but differed in how they photoluminesced (Woodruff et al. 2020, p. 5). We were inspired by the sexually dimorphic photoluminescence of chameleons and Brachycephalus frogs.
Caption: could the in-situ, fully intact armour of ankylosaurs like that of the amazing holotype of Borealopelta, shown here, give insight into the potential of photoluminescence in these animals? Image: CC SA 4.0, original here.
Finally… speculating about the presence of photoluminescence is all very well and good, but can we test for it? In those cases where part of the integument is preserved, we can, by shining blacklights at the respective specimens. The problem, however, is that we might not be seeing the original light-emitting properties of the animal. Seemingly positive results might be a consequence of the fact that various tissues (bone included), minerals and preservatives fluoresce under UV (Woodruff et al. 2020).
As a preliminary test, we looked at the osteoderms of the spectacularly preserved ankylosaurs Borealopelta and Zuul under UV light… we did get results, but it’s difficult to know what, if anything, these results tell us about any condition present in life (Woodruff et al. 2020). I should add that people have been shining blacklights at fossils for a long time and seeing all kinds of interesting results (hat-tip to the pioneering work of Helmut Tischlinger); in no way are we implying that we’re anything like the first to do this.
Caption: people have been examining fossils with UV light for decades. These images show the Jurassic pterosaur Bellubrunnus roethgaengeri, illuminated via the use of UV. Image: Hone et al. 2012 (original here).
And that about wraps things up for now. As will be clear, our paper is not much more than a preliminary set of speculations and suggestions for further work, and isn’t intended to be an in-depth analysis of the proposal. But – as I see it – that’s ok: the scientific literature really shouldn’t be considered focused on results alone, since review, discussion and valid speculation are valuable and worthy too. I hope you agree.
UPDATE (adding 4th March 2020): this article has been somewhat modified relative to its original version, since a misunderstanding on my part meant that I was previously describing photoluminescence as a phenomenon especially relevant to animals with UV-sensitive vision. Substantial thanks to Michael Bok for his interest and assistance and for sending comments which enabled me to modify the article.
For previous TetZoo articles on the biology and life appearance of Mesozoic dinosaurs and pterosaurs, see (as usual now, linking to wayback machine versions due to vandalism and paywalling of ver 2 and 3)…
Zuniceratops and the early acquisition and alleged dimorphism of ceratopsian brow horns, April 2009
Necks for sex? No thank you, we’re sauropod dinosaurs, May 2011
Did dinosaurs and pterosaurs practise mutual sexual selection?, January 2012
Sexual selection in the fossil record, September 2012
Dinosaurs and their ‘exaggerated structures’: species recognition aids, or sexual display devices?, April 2013
Refs - -
Dunning, J., Diamond, A. W., Christmas, S. E., Cole, E. L., Holberton, R. L., Jackson, H. J., Kelly, K. G., Brown, D., Rojas Rivera, I. & Hanley, D. 2018. Photoluminescence in the bill of the Atlantic Puffin Fratercula arctica. Bird Study 65 (4), 1-4.
Goutte, S., Mason, M.J., Antoniazzi, M.M., Jared, C., Merle, D., Cazes, L., Toledo, L.F., el-Hafci, H., Pallu, S., Portier, H., Schramm, S., Gueriau, P. & Thoury, M. 2019. Intense bone fluorescence reveals hidden patterns in pumpkin toadlets. Scientific Reports 9, 5388.
Prötzel, D., Heß, M., Scherz, M. D., Schwager, M., van’t Padje, A. & Glaw, F. 2018. Widespread bone-based fluorescence in chameleons. Scientific Reports 8, 698.
Wilkinson BP, Johns ME, Warzybok P. 2019. Fluorescent ornamentation in the Rhinoceros auklet Cerorhinca monocerata. Ibis 161, 694-698.
