We have a weekly discussion group here at the IVPP for the various students as ‘hosted’ by myself, Musings regular Corwin Sullivan and Xu Xing. Last week we ended up in an entrenched debate over the various hypotheses for flight origins in birds (you know, the usual ‘ground-up’, ‘tree down’, ‘WAIR’ and so on – and yes I will say more about these and others one day) but one offshoot was a discussion on the origin of feathers. Perhaps understandably the debate has been largely fixed on the insulation / display aspects of putative uses of feathers, but this is a far from complete list of feather or protofeather functions that may have led to their initial evolution and development. While I would agree that these are likely to have been the two most important factors the others should at least be considered and not left out of the discussion.
Not all of these are especially likely or even ever served this function in dinosaurs, but once protofeathers of some kind or other had appeared they certainly had the potential to be co-opted to a number of other functions, both those listed below and perhaps others we have not even considered. We will probably never know, or be able to resolve what initially prompted their origin but all of these functions are worthy of consideration either in terms of origin or use or either protofeathers or true feathers.
1. Insulation (internal) – in short keeping the animal warm. The debate still rages over whether or not any dinosaurs at all were endothermic (generating their own body heat like mammals and birds do) or what kind of homeothermy (maintaining a stable body temperature, which is not the same thing – look at tuna for example) some might have had. In either case, feathers could have helped keep in the body’s heat.
2. Insulation (external) – feathers could also be used to keep other things warm, or at least maintain a stable temperature, most obviously for eggs and hatchlings.
3. Display – this term covers a multitude of signals that is primarily used only ion the context of sexual signalling or at least intraspecific communication. However, we can also include threat or warning signals to other species (dinosaurs or otherwise) and camouflage is itself a form of signal (or perhaps rather an anti-signal) and should not be excluded.
4. Flight – while flight was obviously one long-term result that feathers enabled, we also see likely gliders in Microraptor and one would imagine that there were parachuters before this. Certainly this is a key function of feathers though one would hardly argue that this was likely to be their original function when they were short and thick.
5. Buoyancy – not something often (if ever?) seriously suggested, but certainly feathers can trap air to help float birds and could have achieved the same effect in at least some theropods I am sure and protofeathers could have perhaps done something similar if less effectively.
6. Defense – this has recently come to the fore thanks to the especially long and rather pointy filaments seen on the new Beipiaosaurus specimen (LINK?). Personally I doubt these were used for defence (theirizinosaurs had other aspects of their anatomy to keep any intrigued carnivores at bay LINK, and few other reasons besides) but that does not mean that others did not. The basic structure of the protofeather ‘rachis’ is one of a hollow tube of keratin which makes it not much different to a porcupine quill so certainly the possibility of defensive feathers one day appearing on a dinosaur is there.
There are then a fair few more issues at stake than just sexual display and homoethermy going on here and as I mention there may well be other out there, either very different or tangentially related to those here (extreme speculation alert). Might some have lined their nests with plucked protofeathers, or used them to shield the sun when fishing as some birds do, brushed dirt off scavenged food, who knows what? We’ll probably never get any evidence for behavious like these (or others) and while mad and unjustified speculation in (like mine just there) is unhelpful in the scientific literature, that doesn’t mean that we should focus exclusively on just one or two areas of feather function at the expense of plausible, or even likely alternatives.
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