changizi

Maybe. But eye mass scales up very slowly with body mass, and so retinal surface area even more slowly. One wouldn't expect a 3/4 power on this basis. For noses, I don't know how to think about its scaling relationship. Generally, these kinds of control and sensing arguments must have some validity, but it would *seem* one would then expect the sensorimotor parts of the brain to disproportionately increase with body mass, whereas the frontal parts of the brain need not. Instead, one finds the entire brain increasing in size. My guess is that these sensorimotor issues do end up, for some complicated reasons no one has yet quite pinned down, "forcing" the entire brain to get larger as it does. Some kind of computational principle might demand it. But we're largely in the dark as the nature of such a principle.

Right. That's the best "going theory". But it leaves one unsatisfied, because if it were true, one *might* expect that only the somatosensory and motor cortex should get larger in larger bodies. ...or at least expect that those two regions should disproportionately enlarge in larger bodies. But everything gets bigger, and somato-motor regions don't disproportionately enlarge. Even visual cortex keeps enlarging, which seems weird: the visual "surface area" around us would appear to not depend on body size.

Slightly talking past each other. Although blood perfusion modulates skin along a yellow-blue dimension and oxygenation modulates skin along a red-green dimension, the actual color one perceives on a spot of skin depends on the spectrum of the skin nearby. For example, veins appear blue-green to the eye, but are not "truly" blue-green in the sense that if you view veins through an aperture such that no surrounding skin is perceptually adjacent to it, it appears skin colored, not blue-green at all. I'm concentrating on the "dimensions" of color modulation, rather than the perceived color, because the perceived color is more context dependent. So, for example, you're right that red is a color signal humans show much more often than green; but this is because we have color-signaling mechanisms that shift one spot (like a cheek) to be relatively more oxygenated, and so the spot looks redder relative to the unchanged nearby skin. If, instead, the surrounding skin were to everywhere shift redder but leave that spot unchanged, then that spot would be appear green. But the latter doesn't appear to be something our skin does, so we don't typically see green. (Except over veins.) For the blue-yellow dimension, we do see both quite often. But blood perfusion does not only change the hue, but also simultaneously modulates the lightness. So as it yellows it also lightens, and as it blues it darkens. (Unlike for the oxygenation dimension, which is fairly equiluminant as it modulates.) Squeeze blood out from your palm, say, and it will become yellower. (...and lighten.) And more blood makes it bluer (and darker). E.g., your veins have more blood, and they are bluer (and also have a green shift because deoxygenated). Also, a brief tourniquet on your hand, say, will make it bluer (but this time with a red shift because it is relatively oxygenated as well, so purpler in all). (And note that the color difference between veins and the tourniquet case is a red-green one, because the main difference between them is one of oxygenation.)

Body mass correlates very well with brain mass. Sure, there is variation. But why the strong correlation? The Deaner paper concerns primates only. Deaner would not argue that there is not a strong body-mass trend across mammals generally. And the Schoenemann paper doesn't help the quandary. It concludes that, in a sense, brains actually increase even *more* quickly with body mass, once one rids of the fat out of the mass. The question would be, why do larger (non-fat) body mass animals have brains that scale up nearly as fast as the (non-fat) body mass? To make the enigma more concrete, a cow might have a brain of 450g, whereas a cat has a brain of 30g and a beagle-sized dog a brain mass of 70g. By any measure, cows are not "smarter" than cats and dogs. And certainly not ten times smarter! What is a cow *doing* with a brain that big? (This is also a nice problem, because it goes up against our modern-day intuition of thinking about the brain as a computer. Computers that are bigger can usually do more stuff. But not necessarily so for brains...)

I've spent some time studying why the brain changes as it gets bigger, from mouse to whale. But one big mystery no one in neuroscience has an answer to is why bigger animals have bigger brains in the first place. Brain mass tends to increase as the 3/4 power of body mass. And this seems to be unrelated to behavioral complexity; that is, a "mere" increase in body mass seems to be enough to lead to the brain mass increase. Why all this extra brain for no extra smarts? That's the enigma.

Point taken. ...but... would you hook yourself to the euphoria machine?

You're right that jaundice can make skin yellow. And, true, bruises can make it skin green (although bruises also lead to a sequence of colors as it heals). But normal healthy skin can modulate along the two opponent dimensions of color (red-green, and yellow-blue).

Oh. Sorry. I didn't notice how old it was...

True, and I agree with the sentiment in general. ...for sex, food, and loads of other things for which we were evolutionarily designed to have motivators, so that we were led to do what was needed to reproduce. But most of us *now* have goals in life that are quite abstract, and post-biological. I, for example, want to answer science questions, something evolution doesn't care about, and so I have no innate euphoria-motivations to do science. If offered the chance to enter a machine designed to give me the euphoria I'd get from satisfying my science aims, I'd refuse. (Not sure whether this latter interpretation is what the original poster was getting at anyhow.)

The original question included the following: My point is that there are computational complexity considerations that may bear on whether species with three sexes might have things more complex and difficult. (Whether they are valid considerations is another thing.)

While I agree with the second sentence you said in this quote, it does not imply that time is irrelevant. If something takes much more time, then the odds go down that it will still be the optimal solution. And if it takes exponential time, so that it require the age of the universe to get a feasible solution, then it potentially makes it impossible for it to be an optimal solution. (That said, most of these NP-hardness arguments are only if one wants exact solutions. Biology doesn't need exact solutions, which is why they're probably a bad way of reasoning about the three-sex question.)

I always reject this premise. Most people do not strive for euphoria. They strive to achieve specific things in life (whatever they may be), and satisfaction/euphoria comes via having those things achieved. To get the euphoria without the actual achievement of those goals would be missing the point.

Actually, human skin can become *any* color. Your skin can shift along a red-green dimension as the oxygenation of your blood shifts. And your skin can shift along a blue-yellow dimension when there is more or less blood in your skin. Every hue is thereby obtainable. That's all the hues on a *single* animal's skin. But perhaps your questions concerns the baseline skin color of a species. There are a heck of a lot of baseline skin colors across primates, although whether it is more or less than that of the eye colors, I'm not sure.

There is a well-known problem in computer science of this form. For two sexes, there is a fast algorithm that can take a bunch of males and a bunch of females, and sort them into pairs such that everyone is happy. But as soon as n>2 the problem switches into something disproportionately more computationally complex. It becomes something called "NP-hard", roughly meaning that you can't solve it fast. So, one *could* say that having greater than two sexes never happened because of the inherent computational complexity, relative to the comparative ease for two sexes. [Whether this computational argument is *really* the right one here is another question. For example, even if there were a magic oracle that would have helped some species handle this computational problem were it to evolve three sexes, it may be that no species ever would have had any selective benefit to having three sexes. But I dunno.]

Just joined SFN. My aim in life has always been to "try" to answer the "questions to the universe". I'm now a scientist, these days often focusing on the brain and evolution. If you ask a question in the forums about "why" (i.e., an evolutionary "why") and anything about the brain or body, my ears are likely to perk up, and perhaps I'll have something to suggest. (That's why I posted this in the "General Biology" section.) The scientific community ends up hyper-compartmentalizing everyone, and so most scientists end up losing their romantic (Carl-Sagan-esque) original interests in science. I'm interested in interacting with people who still have their "original" interests in science. ...both because it is inspiring, but also because the questions asked by non-specialists are often *great* questions -- maybe it will spur me toward a new fruitful research direction. Looking forward to potential interactions.