Tuesday, December 20, 2011

On crests and feathers

People have long wondered what exactly the quite weird and wonderful head crests of both dinosaurs and pterosaurs were doing there. Why did they evolve - what were they for? Today Lethaia published (online) a paper by David Hone, Darren Naish and Innes Cuthill entitled Does mutual sexual selection explain the evolution of head crests in pterosaurs and dinosaurs? In the paper, Hone et al propose that a potential key evolutionary factor has so far been largely overlooked - that of mutual sexual selection.

Citipati, an animal unusual in combining a large, bony head crest with feathers. While the display feathers here are speculative, similar structures are known from the oviraptorosaur Caudipteryx. Art by Niroot Puttapipat.























'Mutual sexual selection' is pretty self-explanatory. While 'sexual selection' is a one-sided process, with one sex selecting for highly dimorphic traits in the other (Hone himself gives the "endlessly repeated" example of peacocks), 'mutual sexual selection' is, well, mutual. As such, it is likely that both genders of the animal concerned will have ornamentation, or similar sexually selective traits. As is noted in the paper,
"An instructive example is the crested auklet, Aethia cristatella...in which both sexes bear feather plumes on their heads [and] both sexes prefer mates with longer crests" (p. 3)
The authors contend that palaeontologists are largely ignoring this idea and failing to realise its changing status in behavioural ecology. They propose that since "there are many circumstances under which male mating time and effort are limited", and given the varying quality of females, it makes sense that males should be selective rather than simply trying to copulate with everything in sight (pp. 11-12).

In formulating this hypothesis, the authors run through a number of others that have been proposed down the years. Some, such as the evolution of crests as weaponry or thermoregulatory devices, can obviously be ruled out for a lot of species. However, what's probably going to raise people's heckles is the authors' rejection of the 'species recognition' hypothesis, that is to say the idea that (some) crested dinosaurs and pterosaurs evolved their displays so that members of the same species could recognise one another. As the authors point out, this idea doesn't explain why "lambeosaurine hadrosaurs required large crests for species recognition, when...members of [the] closely related iguanodontian lineage did not" (p. 10). They also note that a lot of animals today don't require such obvious signals to be able to differentiate between even very similar species. For example:
"...tyrant flycatchers notorious for showing little to no morphological variation exhibit clear boundaries between species, despite sympatry" (p. 9)
Mutual sexual selection also neatly solves a problem as regards ceratopsians - that although sexual dimorphism has been proposed for certain species, "the proposed degree of sexual dimorphism is weak" (p. 5) with all mature individuals in a species seemingly being near-equally well-adorned with fancy head ornaments. The same has been found to be true of certain pterosaurs and theropods. As far as theropods go, the authors note the prevalence of crests in relatively basal clades (like the coelophysoids and ceratosaurs), but hypothesise that feathers might have replaced head crests as a sexual display in more advanced coelurosaurs and especially maniraptorans (the clade that includes dromaeosaurs, troodonts, oviraptorosaurs and birds). The known presence of display feathers on animals like Caudipteryx and Epidexipteryx would appear to back up this claim (pp. 12-13).

Of course, one problem with this is that some oviraptorosaurs have well-developed crests, but were presumably fully feathered. While acknowledging this as an "anomaly", the authors point out that the clade is very unusual among coelurosaurs in this respect. Furthermore, they also contend that modern birds that possess bony head crests - like cassowaries and certain hornbills - are also very unusual in having them (p. 13).

Noting that ornithodirans (dinosaurs and pterosaurs) likely relied heavily on vision - with a great deal of evidence backing this up - Hone et al also propose that
"...the evolution of the flight-capable feather and of flight itself may well have its roots in the evolution of ornithodiran sociosexual display." (p. 14)
It's an idea that's been proposed before, but here it's presented in the context of mutual sexual selection. Could it be that, in maniraptoran dinosaurs, it was a case of both sexes trying to impress each other that sped along the evolution of the flight feather?

Obviously, this is really just scratching the surface of what's in the paper and, knowing me, I've probably cocked up somewhere along the line (cf. some of the Planet Dinosaur reviews). I'd urge you to get hold of a copy of the paper for all the information - it's actually very accessible for laymen (I should know!).

14 comments:

  1. Thank you for this. Ideal primer for me before delving into the paper proper.

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  2. One thing we originally wrote for the paper but was eventually cut for both length and arguments-between-authors reasons (I thought it was worth noting, Darren didn't) is that we don't *actually* have any oviraptorosaurs with both feathers and crests. While I happily admit that we should expect things like Citipati to have feathers, we don't know for sure, and all three of the oviraptorosaurs that are preserved with feathers (Caudipteryx, Similicaudipteryx and Protarchaeopteryx) lack crests. It's unlikely, but not inconcievable, that these genera had feathers or crests but not both.

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  3. @Dave Surely it's far more likely that Citipati had feathers than didn't? Same goes for any maniraptor (although of course only more 'primitive' feathers are known in therizinosaurs).

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  4. Yes it is. That's why I said 'not inconcievable' and said I happily admit it's most likely they did have feathers. But I think it is a minor point worth making, it's not impossible that feathers and crests are totally mutually exclusive, if very unlikely.

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  5. What a coup! Two posts on new research in a week netting comments from the researchers themselves. Thanks for ducking into our shabby little corner of the web, Dave.

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  6. @David Orr: Shabby indeed. I think you grossly underestimate the esteem in which your blog is held by so many in the online palaeo community. O, great Solomon... (Sorry, you'll tell me if that annoys you? *Sheepish*)

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  7. "Same goes for any maniraptor" *twitch* ;)

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  8. A cool paper, as far as I can make out from the blog posts (no access from home). Let me throw the cassowaries-use-crests-as-vegetation-splitters thing into the ring. When they run, cassowaries lower their heads and the crest takes the impact of branches - something you can see even in zoo animals by the scratched and often even dented appearance of the crests. Clearly, that may be the reason why they are so unusual in having crests AND feathers.

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  9. Dave: Feathers and crests where? Does one presume that the animals are crested and scaled, or crestless and fully-feathered? Presumably, they should have both by simple use of the phylogenetic bracket, especially as basal forms have feathers all over their bodies.

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  10. Please *do* read what I wrote. I said in my first comment they most likely did. i said in my clarifying comment that I'm not saying there didn't have feathers.

    I am NOT saying that they for sure had feathers OR crests. I am saying that it is not IMPOSSIBLE. Which it isn't. I've not said or even tried to imply otherwise.

    Yes I understand the EPB and yes I understand parsimony. I am merely trying to highlight a minor point that it's possible that feathers were a bit more plastic at the base of Maniraptora, especially in the hypothesised light of signalling being the dominant evolutionary pressure behind feather evolution.

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  11. Can we all just take a moment and tell Niroot that we've never seen a better Citipati? We haven't, you know.

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  12. Dave, I don't mean to be rude here, but I am trying to ask where the feathers would be exclusive of crests, not that you think they are; I did not say that you did not believe in the one to the exclusion of the other, merely asking about consideration for the effective separation of the two in the same region of the body:

    Are we talking bone-crested versus feather-crested when only dealing with heads? The example I would offer is the potential for tail-based display or species-recognition structures on the tail, where cranial crests would be less viable (perhaps?), while the same could be true in larger oviraptorosaurs. Because we presume that the three taxa of oviraptorosaur with known feather distribution are basal or basal to particular subgroups of Oviraptorosauria, we can presume that similar morphologies would be plausible later on; without direct evidence otherwise, we should presume these feathers to be identical, and thus they would cover the whole body and be fully pennaceous. That derived oviraptorosaurs, including the biggest-bodied ones show tail anatomy conforming to a "tail-wagging" behavior (at least in part), one can presume that size is not a constraint on the expression of where these feathers should go.

    I rather like the idea of that in derived oviraptorids, feathered crests may have replaced bony crests, as there is both a trend in smaller size as well as reduction in crests toward, say, the "Ingenia" yanshini end of things; this could be emphasized by the extreme brevity of so-called "ingeniine" oviraptorid arms compared to "citipatiine" arms.

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  13. Well I'd take slight issue with the idea that we should presume feathers to be identical further up the tree. Now I don't think it's unreasonable at all (let's get that clear!) but following what we do know of feather evolution, we'd probably expect cassowaries and kiwis and simialr things to have full coatings of fully pennaceous feathers and they don't. So there is at least the *capacity* for derived lineages to have more basal forms of plumage (even no having derived feathers at all for kiwis) or limited plumage (ostriches have more or less bare necks and thighs). Add in to the fact that we have no idea what ornithomimids have and the evidence for therizinosaurs is pretty limited and although the feathers we do have for ovriaptorsaurs are in pretty basal forms, I'd again say I don't think it impossible that derived things like oviraptor either weren't fully feathered, or had only primitive type plumes, or had a full body coverage but of only very small feathers. Any of these (or some weird combination of these) would then make a cranial crest more 'normal' in terms of our hypothesis.

    Once again, I'm not trying to champion 'Oviraptor was bald coz it had a crest', but say that there are all kinds of exceptions to classic rules. Yeah, all birds lay eggs, that's as solid as it can be. All oviraptorsaurs were feathered like Caudipteryx? Far, far less so. Still on the side of much more likely than not, yeah. But I'd say hardly 99.9% given what we know of feather distribution / evolution.

    Now as noted crests could be doing a number of things, and while we do argue that in many / most / nearly all cases there's a reasonable case to be made for sexual selection / social dominance, it could be something else. Indeed that part of the reason we flag up cassowaries in the paper. So yeah, the crests could be combbined with feathers and tails etc. to produce some hyper display, or maybe the arm feathers are relaly limited in derived forms and the crests are display only, or maybe the crests are for something else and the feathers on the arms are massive and displayed with. Hard to say.

    Ultimately I guess my original point in flagging up the putative discrpancy between crestless caudipterigid fossils and featherless oviraptorid fossils, was to try and avoid people assuming that the mere presence of crests in the latter screwed up our hypothesis. Looks like I failed badly! ;)

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