Symposium on evolutionary origin of feathers: Panel discussion

Symposium on Evolutionary Origin of Feathers: Panel Discussion)

MADERSON: As Chairman, let me welcome you to the last portion of the symposium. My coorganizer Dominique (Homberger), and I had hoped that we could get this into an orderly fashion, but of course very few people cooperated! I will be working on an ad hoc basis from the very few written questions that have been submitted, but I am sure that topics will lead to one another.

All of the speakers have their own microphones, and they may come down to the projector and scribble on overheads to make any point they wish. Will they please unplug their microphones before they move. If anyone from the audience, at any time, wishes to make comments, would they please move over to the central aisle and take up positions behind one of the three microphones. I will do my best to recognize them in an appropriate order relevant to the question that is being addressed by the panel speaker.

I will start by reading a general question which is directed to Dominique (Homberger), Blair (Wolf) and Warren (Porter). Peter (Stettenheim) asks:"Downy texture traps air, but is it any value for thermal insulation if there is not an outer, relatively impervious layer to keep the warm air in, or control its outflow? The analogy is that of a windshield over a warm sweater. Does this not suggest that early feathers must have had pennaceous vanes at least distally, if the downy bases, or down feathers, were to be effective for insulation? How does raising or lowering the feathers control heat loss or retention? Could this have been an additional role for depressing the feathers?"

It sounds as if much of that question deals with controlled maneuverability (Maderson and Homberger, 2000), and perhaps Dominique (Homberger) would like to respond.

HOMBERGER: I totally agree with Peter [Stettenheim]'s point that down feathers, as we know them in nestling birds without a cover of contour feathers, actually create something like a "fur", and obviously it is very insulating. Even if wind were to ruffle it, it would still maintain the integrity of the insulating layer. However, what we know is that probably down feathers were not the original feathers. I think it has been argued in several papers, especially in the early Swiss and German literature (e.g., Becker, 1959; Lucas and Stettenheim, 1972), that the original feather may be more generalized like a semiplume or a semi-bristle. It makes much more sense to imagine a feather to be at least somewhat waned because a downy feather does not need the complex hydraulic skeleto-muscular system that contour feathers have. I think that the original feather had multiple functions, though I still maintain that it's primary function must have been an aerodynamic one. But, at the same time, as a side effect, it probably also started very soon to have some function in thermoregulation, for which, however, the complex hydraulic skeleto-muscular system is not necessary.

MADERSON: Would any of the other persons to whom that question was addressed like to respond?

PORTER: I certainly agree with Dominique (Homberger). Down comes in many different forms and certainly, if it is dense enough, you can get a lot of protection. In fact, if it got really, really dense, you would probably lose some of the insulation and probably get a lot more abrasion protection for the skin, or water repellency, or exclusion of dirt from the down. So there is a whole range of physical functions that are not necessarily mutually exclusive. Are you going to go on to the second part of the question about raising and lowering feathers?

MADERSON: By all means, Warren (Porter), you go on with that.

PORTER: Okay. Maybe I should not hog the microphone. Blair (Wolff), do you want to say something?

WOLF: You mean about raising and lowering the feathers and how that would affect heat transfer?

PORTER: [encourages Blair (Wolff) to speak].

WOLF: I think there are data in the literature (Walsberg et al., 1978) showing that raising and lowering the feathers can affect plumage conductivity by 25 to 30%. Although down feathers are deep in the plumage, I do not know whether they would have that kind of functional significance in creating a greater layer of unstirred air in the system itself. I think that downy feathers, being closer to the skin, may retard free convection somewhat. As Warren (Porter) has said, if they are really dense, greater conductivity through the feathers could actually reduce the insulatory effect.

MADERSON: Do you wish to respond to that Warren (Porter)?

PORTER: I would just like to elaborate a little bit on what Blair (Wolff) said. If you have elements that are lying fairly flat, one of the things that the raches do is to raise the insulation above the skin so that you have the potential for a great deal of convective flow at the skin surface. Furthermore, because feather tracts on birds are limited to relatively small areas of the skin, you have a lot of bare surface area. As long as the bird is resting and those feathers are sealed off, you do not have substantial free convection and easy convective movement-it is all conduction and radiation processes within the feathers. However, the minute the bird begins to open those feathers, or in flight, as the feathers are working like bellows, you could get a fabulous, huge transport of heat by both evaporation and convection processes. Also, if the feather elements are opened by feather erection, you can also significantly increase the radiant transfer. Normally, in a depressed feather condition, most of your heat transfer is going to be by conduction, maybe up to 80% or more. But when you start opening that up by erection, the infrared radiation transfer from the skin, which is typically much warmer than the environment, now occurs directly between the skin and environment rather than by short distance radiant exchange between elements in the feathers. This substantially increases the radiant exchange and yields a greater total heat transport. So there are really substantial advantages to having those piloerection capabilities.