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Originally Posted by Watser?
Ok, I have been wondering about this for some time.
A lot of amphibians and reptiles have green skin and a lot of birds have green feathers, yet I have never heard of a mammal with green fur. Are there any green furred animals and if not, why not? It seems like a good camouflage colour.
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Interestingly, I did some research regarding that question for my master's. As you've no-doubt noticed, many mammals are red-orange or reddish-brown in color, but very few, if any, have green fur. (In researching the topic for my master's work, I could find no examples of mammals with green fur.)
The "exception" is that sloths have green fur, as
ceptimus points out, but that's because of algae growing in their fur. Given how slowly sloths move, and given that they generally live in very humid environments, and given that they're not especially energetic or thorough when it comes to personal grooming, they can literally have algae growing in their fur.
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The most common pigment on the planet is green (chlorophyll), and green pigments are common in fishes, amphibians, reptiles, and all sorts of invertebrates. But there are virtually no birds with green-pigmented feathers, and there are probably no mammals with green fur. Why is that?
There are two things to consider. First, green pigments tend to be quite unstable, and second, neither fur nor feathers are living tissues.
Why do green pigments seem to be relatively unstable, compared to most other naturally-occurring pigments? I don't know that anyone has answered that question, but it may have to do with solar output. The Sun's energy output peaks in the green part of the spectrum, so any molecule that's of the right size to intercept and reflect green light is going to be especially vulnerable to being disrupted by the relatively large amounts of green light striking it.
Whether that's part of the explanation or not, naturally-occurring green pigments seem to be rather unstable. Consider chlorophyll-
a and chlorophyll-
b, the two main pigments that plants use for photosynthesis. Both pigments are rather unstable, and soon break down when exposed to sunlight. Plants must, therefore, devote considerable resources to repairing and replacing their chlorophyll, which is constantly being disrupted by solar radiation.
In the fall, when many plants
stop repairing and replacing their chlorophyll, it soon breaks down, revealing other pigments such as xanthophyll that were present all along, but masked by the chlorophyll. This is why leaves change color in the fall. It is
not because new pigments are being produced; it's because chlorophyll is breaking down, so that you can see the other, stabler pigments that had been present all along.
Green pigments in animals seem to be rather unstable as well. For example, any fisherman who has caught a green fish knows that the green color quickly fades after the fish dies. Similarly, a living Green Snake (genus
Opheodrys) is bright green, but the color quickly fades to dull blue when the snake dies.
But what about birds? After all, feathers, like fur, aren't living tissue, and so any pigments that are present cannot be replaced if they break down. Well, as it turns out, there are very few birds whose feathers are green because of pigments.
In most birds with green feathers, the green is not due to pigments, but to feather structure. If you examine the feathers under a sufficiently-powerful microscope, you can see that they have tiny ridges that are spaced at just the right distance to scatter and reflect light in the green portion of the spectrum.
This is why the feathers look green, not because they contain pigment molecules that preferentially reflect green light.
In other words, most birds have green feathers because of
diffraction, instead of pigmentation. That's also why the feathers often seem to shimmer and to change color, depending on the angle you're viewing them from. Blue feathers are often a result of diffraction instead of pigmentation as well.
You can see this easily with the feathers of a Blue Jay (
Cyanocitta cristata), and it's a trick I used to play with the kids when I was working as a naturalist at a Girl Scout camp. I'd hand them a Blue Jay feather and ask them what color it was. They'd say "blue," of course. I'd then tell them to hold it up against the sky, so that they were seeing light that passed through the feather, instead of light that was being reflected from it. To their surprise, the feather would then look brown. That's because the feather contains brown pigments, but not blue pigments; consequently, it looks blue only when you're seeing light that is being reflected from it.
So, the short answer to why mammals don't have green fur is that there's apparently no real way for it to work. Green pigments
could be deposited into a hair shaft as the hair was being formed, of course, but those pigments would quickly break down as the hair grew out of the follicle and was exposed to sunlight -- and there would then be no way to repair or replace the pigments. And the structure of fur, unlike that of feathers, isn't really suited to use of diffraction for coloration.
You may have noticed that many mammals are orange, red-orange, or red-brown in coloration. This probably functions as effective camouflage in grassy or leafy environments.
Say what?
Color vision in vertebrates depends upon specialized cells in the retina known as
cones. Cones function well only in relatively bright light, and give us the ability to distinguish different colors of light. The other type of retina photoreceptor cells,
rods, is very sensitive to low-level light, but does not distinguish between light of different wavelengths (colors). So, nocturnal animals tend to have lots of rods but relatively few cones.
The thing is, there's abundant evidence that most modern mammals are descended from nocturnal ancestors that lived during the time of the dinosaurs. As such, compared to most other vertebrates, most mammals have very poor color vision.
For comparison, most birds have four different kinds of cones and therefore have what is known as
tetrachromatic vision. Presumably, they're much better at distinguishing different colors than we are. Pigeons appear to have
five different kinds of cones, and so are apparently even
better at distinguishing different colors.
Like birds, most amphibians and reptiles see color quite well. But most mammals do not.
The great majority of mammals have only two types of cone cells, and so have
dichromatic vision. To such an animal, red and green apparently look the same. So, reddish or orange-ish fur is actually excellent camouflage, as far as most mammals are concerned. So an orange-and-black-striped tiger, for instance, is actually
very well camouflaged as it stalks through tall green grass -- as far as the antelope that it's hunting are concerned.
Primates (including humans, of course), almost uniquely among mammals, have three kinds of cones, and so have
trichromatic vision. This makes us much better at distinguishing colors than are most other mammals.
What was the selective advantage of trichromatic vision, considering that it comes at the cost of relatively poor night vision? It's thought that it was selectively-advantageous to our primate ancestors because it allows them to easily locate ripe fruit in a forest by its color.
Incidentally, there is one group of birds that is primarily nocturnal. Unsurprisingly, like most mammals, they have limited color vision. These are, of course, the owls.
Relatively small owl species, such as Screech Owls (genus
Megascops) face two main types of predators -- larger owl species and mammals such as Fishers (
Martes pennanti). Interestingly --just like mammals -- many small owl species are reddish-brown in color, but few (if any) are green. After all, most of the predators that might attack them are effectively color-blind, so reddish-brown feathers are just as effective for camouflage as would be green feathers.
Cheers,
Michael
Re: A Question For The Lone Ranger
I think that would be a slooooooow ride.
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