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Polar bears are white in color and white color is not a good absorbant of heat. Why then didn't they evolve a black color to absorb more heat?

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    $\begingroup$ This is thought to be why they have black skin, and they have white fur because it is camouflage in the snow where they hunt, a black bear wouldn't be much good at sneaking up on a seal in the snow! $\endgroup$ – rg255 Dec 29 '14 at 9:31
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    $\begingroup$ The white ones survived better and developed fur that would channel sunlight to the dark skin. $\endgroup$ – phresnel Dec 29 '14 at 15:13
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    $\begingroup$ This question seems to assume that living beings have reached a pinnacle of evolution and that a pinnacle of evolution entails perfect adaptation, neither of which are true. $\endgroup$ – Lightness Races in Orbit Dec 29 '14 at 17:00
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    $\begingroup$ Additional nit: "white...not a good absorber of heat" is untrue. "White" implies uniform reflectivity in the visible spectrum (more or less), and there's plenty of energy to be had in other portions of the solar irradiance. $\endgroup$ – Carl Witthoft Dec 29 '14 at 23:16
  • $\begingroup$ No seals ever surfaced near the ones with dark fur waiting for them at their air holes, so those bears died out, and only the “white” bears survived, since they were harder to see. $\endgroup$ – tchrist Dec 30 '14 at 23:17
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Polar bears do not have white fur - they don't have pigments in their fur hairs which makes them clear and translucent. The outer fur consists of hollow fibres, which shatter a part of the incoming light (the visible part) which makes them look white. This gives a very good camouflage in a white environment.

It was initially thought (see reference 1) that the hollow fibers work in the same way as fiber optic cables and direct the UV part of the light (the visible was thought to be shattered) towards the black skin of the bears where it generates heat, but this hypothesis has been disproved. The UV part of the light is simply absorbed by the bears outer fur and doesn't reach the skin (see reference 2 for details).

The hollow hair helps with the insulation of the skin and can change the color due to environmental influences. The tend to get yellow after feeding on seals due to the fatty substances from the animals. There have also been reports from zoos that algae lived inside the hollow part of the fur making the animals more grey and sometimes even green (see reference 3).

Reference:

  1. Light collection and solar sensing through the polar bear pelt
  2. Is polar bear hair fiber optic?
  3. The greening of polar bears in zoos
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    $\begingroup$ Wouldn't IR, rather than UV, be the relevant part of the spectrum for being warmed? $\endgroup$ – Superbest Dec 29 '14 at 16:38
  • $\begingroup$ UV has more energy. And how much IR is present in the arctis? $\endgroup$ – Chris Dec 29 '14 at 17:09
  • $\begingroup$ @Chris - The amount of energy in UV is utterly negligible compared to the bear's metabolism. See for instance en.wikipedia.org/wiki/Ultraviolet_index (where you get numbers of under a watt per square meter even at noon in the tropics). It might be useful for vitamin D production (if bears need it), but not for warmth. $\endgroup$ – Rex Kerr Dec 29 '14 at 20:51
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    $\begingroup$ @RexKerr That's probably why it plays no role. $\endgroup$ – Chris Dec 29 '14 at 21:26
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Bears initially were dark. But in snow it is really poor camouflage, their victims will see them to early. So they evolved to white color.
They have a lot of fat and good fur to keep the temperature, they don't really need more sun energy. They need more food.
In the same way - in the forest all bears are dark - for camouflage.
Read here
http://www.polarbearsinternational.org/about-polar-bears/essentials/fur-and-skin

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  • $\begingroup$ "in the forest all bears are dark" - What about panda bears? $\endgroup$ – nnnnnn Dec 31 '14 at 3:54
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    $\begingroup$ Panda bears are the perfect color for sneaking up on bamboo. $\endgroup$ – par Dec 31 '14 at 8:10
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How much energy could an Arctic-dwelling creature be expected to capture from sunlight? I think not very much. How much can it gain from having coloration that allows it to capture prey more easily? Quite a bit. Also, given the size of the polar bear, they will probably be generating more than enough waste heat from metabolic activity. Indeed, if you do a little research, you might discover that the polar bear's problem is, indeed, getting rid of heat:

In bears, the basal metabolic rate varies with climate and season; Polar Bear has highest BMR. Overheat easily when running; can't travel any great distance at speeds more than a walk. Low surface-area to volume ratio favors heat retention. Tendency to overheat enhanced by layer of fat 11 cm (4.5in) thick. Young cubs up to 6 months old, with little body fat, are susceptible to cold. Use more than twice the predicted energy for moving at a given speed, perhaps due to bulky body. Fat and fur both insulate. As long as bear isn't exposed to wind, body temperature and metabolic rate remain normal at -37 degrees C (-35 degrees F).

From http://library.sandiegozoo.org/factsheets/polar_bear/polar.htm

Of course, even if the polar bear was a perfect absorber, it could get at most ~1kW/m^2 of energy from the sun, and that only on a clear day in the summer. And that heat would be on the outside of its insulating fur & fat.

So there's the answer: a strong selective pressure towards white, little or none towards black.

Re waste heat: What is meant here is that when the polar bear (or any animal) consumes food, part of it goes to producing heat as an unavoidable byproduct of metabolism. Larger animals usually have to get rid of excess heat, thanks to the surface/volume relationship mentioned in the comments.

For a reference, try thinking about this: http://en.wikipedia.org/wiki/Thermoregulation And remember that not all answers are written down in a book (or web site, these days). Sometimes you have to actually think and reason out your own answers :-)


Added: I think the problem with this question is the assumption that polar bears (or indeed, any boreal mammal) need to absorb heat. Maybe that's down to anthropocentrism, since humans evolved from ancestors that lived in tropical climes, and so have adaptations (like sweating & nearly hairless skin) for cooling. But if you read the links, you discover that more often the problem is getting rid of excess metabolic heat, and most boreal mammals seem quite comfortable in what we primates consider cold. (I write this after coming in from playing with my two dogs, who love to run around in the fresh snow.)

You might also think about why almost all (non-domesticated) mammals have evolved protective or camouflaging coloration. (The only exception I can think of offhand is the skunk.) Indeed, a number of boreal mammals have evolved seasonal color changes (e.g. http://srel.uga.edu/outreach/ecoviews/ecoview130106.htm ), having darker summer colors (like their more southerly relatives), but turning white in winter. That suggests that the benefits of camouflage strongly outweigh any possible benefit from solar heat gain.

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    $\begingroup$ The point of energy waste isn't true. Bigger animals have a smaller surface in relationship to their volume. $\endgroup$ – Chris Dec 29 '14 at 19:27
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    $\begingroup$ Oh, and can you please add references to you answer? $\endgroup$ – Chris Dec 29 '14 at 21:54
  • $\begingroup$ This is a false answer. $\endgroup$ – Carl Witthoft Dec 29 '14 at 23:17
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    $\begingroup$ False how, exactly? $\endgroup$ – jamesqf Dec 30 '14 at 3:26
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In addition to camouflage, consider that during the colder parts of the year, when they need the heat the most, the nights are longer than the day.

Darker fur would radiate more heat away from the body than the lighter fur.

So it's possible that they did evolve the white fur for heat - but in their environment, the coldest part of the year heat retention is more critical than heat gathering, and this would bias towards white fur, not black.

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  • $\begingroup$ Why should darker fur radiate more heat? This is a question of the insulation through the deeper fur layers and the fat. $\endgroup$ – Chris Dec 29 '14 at 21:48
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    $\begingroup$ @Chris, this is basic physics: dark colors radiate heat more efficiently than light colors. $\endgroup$ – Mark Dec 30 '14 at 0:58
  • $\begingroup$ @Mark Only when there is heat to radiate. When the bear is insulated good enough (which they are) there is nothing which radiates of. In fact it is practically impossible to spot polar bears with a thermal camera. Additionally, dark fur would absorp light more easily than light fur. $\endgroup$ – Chris Dec 30 '14 at 8:47
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    $\begingroup$ @Chris I'm suggesting that one of the reasons there is so little emissions is due, in part, to the fur not being dark. It sounds like you are arguing that even if their fur were dark they would still not have heat emissions. Of course the fur only plays a part of the role in insulating the bear, but it still plays a part. $\endgroup$ – Adam Davis Dec 30 '14 at 13:32
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Although black is a better absorber of (heat) radiation, it is also a better emitter too. As Polar bears live in a cold environment, having black hair would cause them to radiate their body heat away more quickly. Their colour helps them to retain heat better - dark objects equilibrate to the temperature of their surrounding faster, lighter objects more slowly. If they were darker, they would require more food to survive.

Their black skin is irrelevant as the light has already been scattered/reflected away by the white/colourless hairs - as they look white, it's the reflected light that you are seeing.

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    $\begingroup$ It would be great if you could add some references to support your response! $\endgroup$ – Bez Dec 30 '14 at 11:32
  • $\begingroup$ interesting answer! Please add references and get some well-deserved credits here $\endgroup$ – AliceD Dec 30 '14 at 11:41
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Why then didn't they evolve a black color to absorb more heat?

This assumes that evolution is an active process, working by organisms' will or need.

Do not forget that the theory of evolution works from generation to generation, by random (imperfect and chaotic) mutations, most of which are unfavorable.

The polar bears did not evolve a black color because they can't decide that. If you believe in evolution, and you believe that a black color would draw more heat, increasing the animal's chances of survival over its white peers (so much that the white peers would eventually die off), then we are simply waiting for the black color to come around. Evolution is never finished.

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This is a brief expansion on DrAlchemist's response; one that I intended to make myself, but saw that s/he had already covered the basics.

To see that black is a better emitter of heat we have to leave biology and go to physics, specifically quantum mechanics, even more specifically black body radiation. Like most things, absorption and radiation are equally balanced when it comes to a given material/color. A white object is slow to absorb heat and a black object is quick to absorb heat, as we have likely all seen demonstrated in elementary school. What my teacher (and likely yours) did not mention is that the white object cools slowly when taken out of the sun, but the black object will cool much more quickly. Black heats up quickly, and radiates just as quickly when the temperature differential inverts.

As a mental exercise, consider an idealized object that absorbs electromagnetic energy perfectly, called a black body. Turn a heat lamp on it, and it will absorb all the heat it receives. Turn the lamp off and, as in the argument above, it will become a perfect emitter of the radiation it absorbed.

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    $\begingroup$ This does not provide an answer to the question. $\endgroup$ – Chris Jan 1 '15 at 1:09
  • $\begingroup$ @Chris I think it does provide an answer by hypothesizing that because polar bears are white, they lose less heat due to radiation. However, this is probably not true: First of all polar bears lose heat by convection, not radiation, since they do not live in a vacuum. Second, if this was true you would expect the fur to be black and the skin to be white, since it is the skin that is warm and not the hairs. $\endgroup$ – Superbest Jan 1 '15 at 1:39
  • $\begingroup$ Minor quibble: radiation occurs whether in a vacuum or not. And all we care about is which color is on the outside; a white metal ball painted black is the same as a black metal ball. But I do agree that convection is by far the primary engine for heat loss in the bears' environment and absorption/radiation is negligible. Ultimately I think their color doesn't matter one bit when it comes to heat retention, particularity since they spend a fair amount of time swimming in the Arctic ocean, a far better conductor of heat than even the frigid arctic wind. And yet they do not die of hypothermia. $\endgroup$ – ylandrum Mar 10 '15 at 22:32
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I think an important point hasn't been mentioned yet.

Why then didn't they evolve a black color

The changes introduced by evolution initially are random. Only the persistence of those changes over generations can be thought of in a "smart" way.

Technically, evolution can lead to some things getting worse! For example, if mutations improve a creatures eye-sight but at the same time worsen it's hearing, both traits may end up surviving assuming the improved eye-sight makes more of a difference than the worsened hearing.

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Well, Polar bears do have black skin; But, living in the harsh arctic conditions they've adapted several traits such as thick layer of blubber, wide paws etc. But their most peculiar trait is having a white fur coat; unlike other bears, polar bears have evolved to have white fur with hollow hairs which easily allows their black skin to absorb the radiations from the sun. So there's no problem with the heat intake.

Now, the reason behind their white fur lies in their quest to food. The white fur enables them to easily hide/blanket in their surrounding (the white masses of snow) i.e. it helps them in camouflage.

And, I guess that's it! Thanks!

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  • $\begingroup$ Welcome to bioSE. Can you add some references to your answer? $\endgroup$ – WYSIWYG Dec 30 '14 at 8:25
  • $\begingroup$ The claim with the light absorption has eben dismissed. See the reference in my answer. $\endgroup$ – Chris Dec 30 '14 at 8:34

protected by AliceD Mar 23 '15 at 2:16

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