Tiger-striped human hair - dream, or mission?
March 23, 2011 10:26 PM   Subscribe

Why does cat fur grow in multiple color bands, while human hair doesn't?

My tabby cat's fur is banded in different colors. If you look at a single hair, it has different colors in it in a repetitive pattern. Mine isn't like that - there is a single transition from brown to grey - and I would certainly notice if there were a tabby, or ticked, human's hair. Why do cats have this and people don't?
posted by jet_silver to Pets & Animals (6 answers total) 8 users marked this as a favorite
Well, cats, and most other mammals that have tabby fur, owe it to something called an agouti gene, which signals if it is to have the ticked fur or not. I would guess if humans have such a gene, something else is preventing it from expressing in our hair, or that it is unexpressed in all modern humans if it ever existed.
I would note, however, that I do have some hairs that will band from dark red to grey and back again, over their length, repeatedly. So perhaps other humans may have color bands that aren't quite so obvious as cats, dogs, horses, mice, etc.
posted by strixus at 10:41 PM on March 23, 2011 [2 favorites]

Well, there is another sort of stripe phenomenon in human hair, though not on a single strand, but in "patches" like cats often have, and it's called sectoral heterochromia. It's incredibly difficult to find information on it; Wikipedia's article on it used to mention other types than just eyes... I have it in my hair: a blond streak right on the top of my head, through otherwise dark brown hair. My paternal aunt has a patch of blond hair in front (now white), the rest of her hair is black; apparently my twice-paternal great-grandmother (mother of my father's father) also had a swathe of blond hair like I do, as well as central heterochromal eyes like I have, fwiw. The hair colors are hard to photograph, it stands out a lot more in reality. When I was a kid and no one dyed their hair, people thought it was cool; now that hair dyeing is a big thing, people think I got a bad dye job, sigh.
posted by fraula at 1:57 AM on March 24, 2011 [1 favorite]

My father has stripes of colour in his beard
posted by fire&wings at 7:34 AM on March 24, 2011

Best answer: I'm not sure these address the "why people don't" part of your question, but here are three articles by biologist Jerry Coyne on the evolution and genetics of cat coloration:

More on Cat Genetics
How It Got Its Coat
The Evolution of Cat Coat Patterns
posted by lholladay at 7:35 AM on March 24, 2011 [4 favorites]

Response by poster: Since my understanding of physiology is not good I was unhappy with how I asked the question but it was as close as I knew how to get. Evidently people -can- have ticking or repetitive color rings in their hair but it isn't very widespread. Therefore I think my question is answered: nothing keeps people from having ticking in their hair so human hair follicles can do that stuff but generally they don't; cats have an evolutionary reason to express it so they do.
posted by jet_silver at 8:11 PM on March 24, 2011

Marvelous question.

In some animals (deer mice, foxes, and rats at a minimum) the agouti pattern is known to be associated with more fearful behavior, more aggressive behavior, and larger adrenal glands in the case of foxes:

A relationship between nonagouti (black) coloration and docility has been shown in foxes, rats, and deermice. In foxes, Keeler found that the most fearful foxes were the wild-type reds, which carry the dominant agouti allele. The less fearful silvers, pearls, and ambers were all homozygous for nonagouti (Keeler 1970). The size of the adrenal glands of these different-colored foxes correlated with their fearfulness (Keeler et al. 1968).

In rats, Keeler (1942) and Cottle and Price (1987) compared the behavior of black and agouti rats that were descended from recent wild ancestors. The rats used by Keeler (1942) were third-generation descendants of a cross between a wild agouti male and domestic albino females. Cottle and Price's rats were the sixth generation descendants of wild rats who were heterozygous for agouti and nonagouti. All the rats within a study therefore had a similar genetic background and had been raised in similar conditions.

Both studies found that the black rats were more docile than the agoutis. More agouti rats than black rats bit, attacked and squealed during handling tests, and the agoutis' responses were more intense than those of black rats. Black rats were more catchable, strokeable, and handleable than agouti rats.

The same results were obtained in agouti and black deermice that had been bred in the laboratory for over 20 generations (Hayssen 1997). In handling tests, more agouti deermice bit and attacked the experimenter than black deermice. Black deermice were also easier to catch, stroke and handle than agouti deermice.

When I Goggled this just now, I found references to ongoing studies of cortisol levels (a good indicator of adrenal function) in agouti and non-agouti dogs, but no accounts of the conclusions of these studies. One article about the domestication of foxes I read in The Smithsonian a while back claimed, as I recall, that loss of the agouti pattern was associated with domestication of a number of animal species.

That human beings don't display the agouti pattern could be added to a long list of characteristics and traits which seem to suggest we are an auto-domesticated species.

I find a possible association of the agouti pattern with larger adrenal glands intriguing because higher levels of adrenalin have been nominated as one of the important reasons chimps are so much stronger than we are (by a factor of five in some accounts!).

So if you do come across a human with true striped hair it might be wiser not to rile him or her up too much.

Grey tabbies, by the way, are in my observational experience the toughest of all tomcats pound for pound.
posted by jamjam at 7:39 PM on March 25, 2011

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