Pigmentation, environment, and evolution

Factual Questions
1

I was thinking about the basic misunderstanding of natural selection exhibited by the OP in this thread and remembered the way this question is usually framed: “If evolution is real, why aren’t rabbits green?” My usual answer to that is, “Why don’t they have wings? or venomous fangs? Because they don’t need them; they’re doing very nicely as they are.”

But then I began thinking about animals and their colors. Birds and fish seem to have an infinite palette available to them, while mammals are all within the white-brown-black range.

Couple thoughts on this: [ul][li]As a rule, birds and fish have excellent eyesight and many (if not most) can distinguish colors; therefore (goes my thinking) they can use colors to communicate sexual signals. Amphibians don’t see so well, but poison arrow frogs are brightly colored because their intended audience–primarily birds–can see colors very well. So, leaving aside for now the whole “which came first, the chicken or the Easter egg” discussion, there’s the whole natural selection aspect of coloration, whereby color as nonverbal communication has developed in (and around) animals with good color eyesight.[/li]
[li]As a lifelong keeper of tropical fish, it occurred to me one day that the reason saltwater fish are so much more colorful than freshwater fish is probably due to chemical availability: salt water is extremely rich in the entire spectrum of minerals, but fresh water is (tautologically), much less so. Since fish tend to be, due to the principles of osmotic balance, chemically similar to their home water, saltwater fish have a much richer palette of colors (most of which have a mineral source) than do their freshwater counterparts.[/li]
[li]This leads us to mammals. Brown, brown, black, brownish black, white, brown, brownish white, brown, brown, brown. Is this because our metabolism is based on iron and iron oxide (i.e., rust; brown, brown, brown)? If the first life had begun as, say, a copper-based metabolism, would rabbits in fact be green?[/li]
[li]Taking this a step further, is the fact that the first successful forms of life (post blue-green algae, of course, and why didn’t we all evolve with that color palette?) are iron-based related to the ubiquitous brownness of the primordial, vegetation-free environment?[/ul][/li]In other words (and here at last is the General Question), is our hemoglobin iron based because rocks are brown?

2

[QUOTE]
*Originally posted by lissener *
[ul][li]This leads us to mammals. Brown, brown, black, brownish black, white, brown, brownish white, brown, brown, brown. Is this because our metabolism is based on iron and iron oxide (i.e., rust; brown, brown, brown)? If the first life had begun as, say, a copper-based metabolism, would rabbits in fact be green?[/ul][/li][/QUOTE]

I realize this wasn’t your official General Question, but to answer this one, I’d think not. Mammals are, for the most part, color-blind (or see limited ranges of colors). As such, the evolution of a colorful coat probably would not have proven sufficiently beneficial (and, in many cases, particularly with crepuscular or nocturnal species, may indeed prove detrimental) for the trait to be passed on.

In other words, I think mammal hair pigment has little to do with mineral intake (or composition), and more to do with a sort of evolutionary apathy (so to speak) towards color within the group.

3

Wouldn’t it be better to ask: Is our hemoglobin iron-based because iron is available? I’m not sure why the color would cause our hemoglobin to be iron-based.

Also, mammals have brown, red, and black colors because of melanin and its variants, not iron. Hemoglobin may play some role in coloration, like in the gills of fishes, but most coloration in animals is obtained through other pigments.

I’m not a biochemist, but I think melanin might not even incorporate any metals in most of its forms.

4

Just FYI - birds have no blue pigment.

5

[QUOTE]
*Originally posted by Darwin’s Finch *

I’m embarrassed that I gave the impression that I was dumb enough to need to have this explained to me. The only reason I didn’t specifically include it in my OP is because as far as I’m concerned it’s a given.

Again, this is a given. But camouflage coloration in the green range would be at least as beneficial as brown to some animals (some sloth species host mosses in their fur to achieve this effect), and evolutionary apathy wouldn’t favor one over the other, so there must be some other reason, likely chemical.

My question was prompted by the thought that, in all the billions of evolutionary mammal-years (think man-hours), no color outside of the brown range has ever successfully emerged. (Has it?) Mineral availability can’t be the only reason; after all, birds and insects developed blue coloration with refraction, not pigmentation. Colorblindness may account for it to some degree, but I’d argue the converse: that color vision might have developed in more mammals if it had been more useful to them; i.e., if they weren’t all various shades of the same color and therefore not in any need of color vision. That color vision developed in fish and birds because they were capable of displaying colors.

Of course, I could have dispensed with part of my OP if I had done some minimal research (i.e., a Google search) on melanin, which it turns out, despite my assumption to the contrary contains no iron. (Does anyone know if melanin is the pigment responsible for pigmentation in all mammals?)

And upon further consideration I see my original hypothesis has another, even more fatal, flaw: what good would adaptive coloration have done the earliest life forms? If a tree falls in a forest . . .

But I’m still left wondering to what extent the availability of mineral raw materials has dictated the evolution of pigmentation in animals, if at all, and by extension the development of color vision.

6

IANAScientist, but I think I may detect a logic flaw in the initial question: there is a supposition that mammals have not evolved colors in part due to the fact that most mammals can only see a restricted range of colors.

My observation is this: even if that were true, don’t mammals co-exist with multi-colored birds and reptiles, many of them predatory? For example, don’t raptors and snakes eat rodents and other small mammals? Don’t constrictors eat monkeys?

My point is that there must be other forces at work that have led to the limited color palette of mammals compared to other animal families. One could imagine that there might be some cases where rabbits might be better off green, but for the most part, apparently natural selection has determined that the brown/gray/white palette suffices.

I am struck by the “moths in London” study where a once-white species of moth mutated to black due to natural selection during the sooty industrial revolution.

7

WordMan, I make no such supposition. In fact, I suggest the opposite: that mammals can see a restricted range of colors because they display a limited range of colors.

If you read it again, you’ll see that you and I ask very similar questions.

8

Hmmmm, lissener I suppose you’re right…but if so, it still feels like you’re question is a little turned around (again, my impression - I could be completely wrong here). Fish, Reptiles and birds are able to take advantage of the minerals or whatever around them to evolve colors - this has worked to their advantage vis a vis survivability (for those varieties that have color). Mammals, comparatively have not evolved nearly as many colors - I guess the essence of your question is: why?

Is it a lack of ability to take advantage of minerals or other chemicals that fish, birds and reptiles use? Seems doubtful…remember IANAScientist, so this is conjecture

Is it because mammals have simply not benefitted from a similar variety of color? Seems most likely.

From there, one would ask - why have they not benefitted? Is it because mammals are for the most part color blind (according to a post above) - could be; if colorfulness is more due to sexual selection than predatory selection then a non-color-perceiving mate won’t care what color you are…

Or maybe the other survival attributes a mammal has (e.g., warm-bloodedness, fur, speed, etc…) have enabled it to get by without color? Possible…

Let me know if I am breaking down the question in a reasonable manner and where your question lies along that logic tree and maybe I (and more importantly, others) will understand where you are trying to head better…

9

[QUOTE]
*Originally posted by lissener *
**

Well, if it makes you feel any better, I did not feel that you needed it explained to you, but since I had no idea what you were really getting at, I’d go with something I could (more or less) explain :slight_smile:

I think a lot would depend on the predators involved. Plants, for the most part, only really stand out as “green” when viewed in direct sunlight. In a darker environment (under the forest canopy, for example), browns and greys are probably just as effective as green might be as camouflage against a predator which doesn’t have a great deal of color differentiation ability.

**

I’d argue that the lack of showy pigment in mammals has a lot to do with who their neighbors were during the Mesozoic. It is widely thought that dinosaurs, like birds, had at least some degree of color vision, so a green mammal would make for a nice tasty, and visible, treat (remember that during the Triassic, when both mammals and dinosaurs were first appearing, there were not the lush landscapes we are familiar with today, so there was less “green” available at ground level). Mammals could get around this in two ways: 1) by not being an obtrusive color - stick to the non-descript browns and greys of the leaf litter and such, and 2) by not being active during the day, where contrast is greater. I think it can be successfully argued that during this time, bright colors weren’t useful to mammals, and as I alluded to earlier, may well have been detrimental.

By extension, since early mammals’ retinas were adapted for the night-life, it would take some major changes in both life-style and morphology before color vision became useful (or practical). After 160-odd million years, it may have been too late (in a genetic sense) to experiment wildly with color, and the ability to see it, once the dinosaurs were gone. Since, by then, the evolutionary status quo of nocturnal life had succeeded, there may simply not have been any further significant selective pressures which might make color beneficial.

Which is not to say that it hasn’t happened, of course. Primates are pretty good at color vision, of course (er…that doesn’t sound quite right, does it?). But even then, most primates (with few exceptions) still tend to be limited to the standard mammalian palette. So the ability to see color does not necessarily coincide with the ability to display color.