Bicolor/double-sided feathers as part of a thermoregulatory apparatus?

Question

The project I'm working on right now has a much longer day than ours, which would result also then in higher thermal swings. Looking at earth and humans, depending on the solar radiation an area gets, the native inhabitants are more likely to have lighter or darker skin tones, in order to absorb harmful solar radiation. But if that radiation is blocked one way or another (i.e. by feathers or underlying skin melanin), I can see the opposite colour gradient occurring to maintain a good heat balance. When less sun is available, you'd want to compensate by absorbing more solar radiation; when more is available, you'd want to do the opposite. My idea here being that a species has evolved with feathers that are, on one side, matte black, and on the other, a sort of shiny white. Basically maximizing the possible range of albedoes. In order to maintain a stabler temperature, these feathers are able to rotate to show one side or the other to the sun, thereby absorbing the desired amount of solar radiation - probably, this would entail a mixed array of black and white for the vast majority of the time. This is a binary planetary system, as well, which means that the second planet will provide a reasonably large source of reflected light at night, allowing this mechanism to function to some degree during both daytime and nighttime.

My question is, would this be a reasonable thermoregulatory strategy? Keep in mind that it can absolutely be supplemented by heat exchange membranes, internal heat generation, etc... I only want to know if this could provide enough useful temperature management to actually develop in the first place.

A couple (edited) extra bits of info that seem potentially important:

• These are flightless birdlike animals, living on a large island chain. They don't have any predators on land and don't ambush hunt.
• Importantly, this is mediation between extremes of day and night temperatures. There are no seasons, just very long days and nights that get much hotter and colder. This is why actually altering or shedding feathers doesn't work.
• Camouflage is not a concern
• This can serve other purposes (i.e. mating display)
• By "black" and "white", I do mean maximum absorption of light and minimum reflection of light, as far as is biologically realistic.
• I've so far understood active thermoregulation to be much more energy intensive than passive, i.e. controlling the amount of energy you absorb rather than generating or dissipating it. Using less energy means less food use, so more can exist in an area with limited food reasources; this is part of why I wanted a passive thermoregulatory mechanism.

Answer 1

I can't argue with @L.Dutch, there's no example of life on Earth using color-changing feathers (or skin, or scales, etc.) for temperature regulation. Darker human skin lowers the chance of skin cancer and slightly lowers the risk of burning, but it does nothing for thermal regulation.

But that doesn't mean your world can't have it. Let's answer the question from the perspective of the science-fiction tag.

1. Some birds change color through abrasion, meaning the tips of feathers are worn off, exposing a different color underneath. This method could be rationalized by your feathered creatures moving less often during one climatic period and moving a lot during another.

2. Some hummingbirds change color based on the position of the feather (giving credence to your idea of turning a feather over, although that's a pretty complicated action that's unlikely to evolve).

3. Most birds change color simply by molting (shedding feathers and growing new at the change of seasons).

Any of these could be used to rationalize the color change you're looking for and that thermally regulates their temperature because you declare it to be so in your world.

Be careful what you ask for: you tagged your question science-based

Once again supporting @L.Dutch's answer, it should be noted that the color of the feathers have almost nothing (possibly nothing at all) to do with thermal regulation. Feather density, down, and the ability to expand/constrict blood circulation in the skin would have much, much more to do with it.

Why? Because there is a great deal more light streaming down from a star than the visible spectrum, and color changing white-to-black would at best only affect visible spectrum light. It wouldn't affect the thermally efficient infrared spectrum or the usually-burns-your-skin ultraviolet spectrum (much less any of the other spectra). This is why you don't see color as a natural thermal regulation.

Besides, you might want to ask yourself why the Bedouin wore black robes and herded black goats in the desert. (Hint: it wasn't because they couldn't get white robes or white goats.)

Answer 2

If we look at our planet and at animals living in areas with extreme climates, like the polar areas or warm deserts, where thermoregulation is critical, we see that none of them has evolved such a feature.

Creatures switching from a dark to a white fur between seasons actually follow the opposite pattern: white fur in winter, brown fur in summer. They do so because their main concern is not thermoregulation but camouflage: a dark fur on a snowy background stands out, and there is not much point in saving few Joules of body heat when you have to spend as much if not even more to evade a predator.

On the other extreme, creatures living in hot deserts have to bear freezing temperatures at night and scorching hot ones during the day, and they again do so without changing color of their fur: they manage well with just a single tone.

Answer 3

Very interesting question. Now L. Dutch and JBH gave you why thermal regulation cannot be linked to color, however, I will try to give you a solution to what you need:

Thermal regulation can be helped with moving/reorienting feathers to improve or hinder air flow. You can have two colored feathers that will look one color when inflated (allowing air flow) and another color when tucked tightly like a blanket. If you use dark color for night and light color for day time, it can also help with camouflage a bit as darker colors blend better at low light conditions while bright blueish white could blend with day time sky.

Answer 4

I'm going to take the devil's advocate position, just for a little balance. Just because it hasn't happened doesn't mean it couldn't.

Right now, feathers have a mechanism similar to how human hairs stand up when you get chilly. The official term is horripilation. Instead of goose bumps, the feathers fluff up and produce an extra layer of insulating air trapped within the feathers. This is actually fantastic insulation, and mammals do it, too.

What you're looking for is rotational horripilation. Basically the same mechanism, but while fluffing up, it also flips the feathers over. For this to work, you either need pinfeathers, like penguins are covered with, or you need some mechanism to keep them sorted so they all flip over in unison.

An alternative might be that the animal has alternating stripes, and the muscles pull one color up and the other color down such that the preferred color of feather covers the other. If you can't imagine this, let me know and I'll draw a picture. Given the control that octopuses have over their chromatophors, I don't see this as too far fetched.

I think this is possible for flightless birds, but the horizontal orientation of feathers is important for wings to work properly. Maybe it could work out that only the upper body of the animal changes color?