Monday, April 28, 2008

Why is human head hair so long?

Head hair is much longer than hair elsewhere on the body. This lengthening has involved several evolutionary changes: faster rate of growth, longer growing phase, increased density, and greater resistance to physical damage (Khumalo, 2005; Loussouarn et al., 2005). The multiplicity of these changes is consistent with sexual selection: the selective pressure seems to have acted on an overall visual effect rather than on one incidental factor. In some non-human primates, head hair has lengthened for apparently similar reasons, perhaps because visual attention tends to focus, as in humans, on the face and its surrounding frame (Darwin, 1936 [1888], p. 906).

Head hair has lengthened only in those human populations that have lived in the temperate and Arctic zones, including some that have back-migrated to the tropical zone, e.g., Austronesians in Southeast Asia and Oceania, Amerindians in the tropical New World. Darwin noted "the extraordinary difference in the length of the hair in the different races; in the negro the hair forms a mere curly mat; with us it is of great length, and with the American natives it not rarely reaches to the ground" (Darwin, 1936 [1888], p. 906).

This point seems to be lost on those who think that hominids acquired long head hair at a very early date. Advocates of the 'Aquatic Ape Hypothesis', for example, believe that head hair lengthened during a putative aquatic phase of human evolution when an infant would have to hang on to its mother's hair while in the water "and if the hair floated around her for a yard or so on the surface [the infant] wouldn't have to make so accurate a beeline in swimming towards her" (Morgan, 1972, p. 36). Yet such lengthening could not have occurred until modern humans had begun spreading out of Africa—some 50,000 years ago at the earliest. This same point is also lost on those who argue that long head hair improved mating success among ancestral humans because it signified health and hence mate quality (Hinsz et al., 2001; Mesko and Bereczkei, 2004). Fine. But does health matter less in the Tropics?

In the literature, this lengthening of head hair outside the tropical zone is often ascribed to relaxation of natural selection: short frizzy hair helps dissipate body heat and thus loses its adaptive value in colder climates. But why would a colder climate cause head hair—and only head hair—to lengthen to such an extent and over such a short span of evolutionary time? If we turn to the sexual selection model, we see that non-tropical climates do alter selective pressures, but indirectly so—by altering human demography. When early modern humans left the Tropics, they entered an environment that reduced the supply of mateable men, thus making them a limited resource. This new environment intensified sexual selection of women and would have favored physical traits that retain visual attention, particularly from men.

Why would sexual selection favor longer-haired women? We know that most societies consider a greater amount of head hair to be an appropriate female characteristic (Synnott, 1987). Even in sub-Saharan Africa, where people are naturally shorthaired, women have traditionally lengthened their head hair with vegetable fiber, sinew, or hair from relatives, apparently to enhance their beauty (Bernolles, 1966; Sieber and Herreman, 2000). Head hair is identified with femininity partly because men begin to go bald as early as their 20s, but also because the scalp hairs of women seem to have a higher mean diameter and hence more volume, even in the naturally shorthaired New Guineans (Walsh and Chapman, 1966). It may be that men perceived a greater amount of head hair as more feminine from an early date, and this perceptual bias could have influenced male mate-choice wherever female-female rivalry for mates had become sufficiently intense.

Reference

Bernolles, J. (1966). Permanence de la parure et du masque africains. Paris : G.P. Maisonneuve et Larose.

Darwin, C. (1936) [1888]. The Descent of Man and Selection in relation to Sex. reprint of 2nd ed., The Modern Library, New York: Random House.

Hinsz, V.B., Matz, D.C., and Patience, R.A. (2001). Does women's hair signal reproductive potential? Journal of Experimental Social Psychology, 37, 166-172.

Khumalo, N.P. (2005). African hair morphology: macrostructure to ultrastructure. International Journal of Dermatology, 44(Suppl. 1), 10-12.

Loussouarn, G., El Rawadi, C., and Genain, G. (2005). Diversity of hair growth profiles. International Journal of Dermatology, 44(Suppl. 1), 6-9.

Mesko, N., and Bereczkei, T. (2004). Hairstyle as an adaptive means of displaying phenotypic quality. Human Nature, 15, 251-270.

Morgan, E. (1972). The Descent of Woman. London: Souvenir Press.

Sieber, R., and Herreman, F. (2000). Hair in African art and culture. African Arts, 33, 54-69.

Synnott, A. (1987). Shame and glory: a sociology of hair. The British Journal of Sociology, 38, 381-413.

Walsh, R.J., and Chapman, R.E. (1966). A study of the quantitative measurement of human head hair fibres. Man, new series, 1, 226-232.

11 comments:

anthrosciguy said...

Couple points:

When you mention sexual selection only in terms of longer haired females you seem to be forgetting that it's longer in both sexes. It may be that very long hair was selected against in the tropics due to either pests or possibly the greater ill effects of dampness during the rainy season. (In the tropics during the rainy season being too covered up tends to lead to rashes and/or poor healing of blisters and other small wounds.) This is speculation though, but I think it's sensible speculation.

Peter Frost said...

I've seen statements that hair can grow longer in women than in men.

For instance, on the Kent Anthropology website http://lucy.ukc.ac.uk/LOCAL-ONLY/FHC/FHCL1495-6.html), I came across the following statement:
"Hair length is sexually dimorphic, not just because of cultural rules - females really can grow longer hair than males in most cases. This has been measured for Europeans, where in males the maximum length for wavy hair is about shoulder length and the maximum for straight hair is about midback length. For female Europeans wavy hair can usually reach the waist, and straight hair can reach the buttocks or longer."

Unfortunately, I've never been able to track down a primary reference.

I might add that sexual dimorphism is only a long-term consequence of sexual selection. Most genes influencing hair length are probably not sex-linked, so selection for long hair in one sex will produce a lengthening of hair in both sexes.

Eventually, the slight counterbalancing selection against long hair in men (because of the unnecessary metabolic cost) will favor the retention of sex-linked alleles that produce long hair in women but not in men. But this would be a lengthy process and humans have inhabited Artic and sub-Arctic zones for less than 30,000 years.

Sampieru said...

Super post Pr Frost. I have a couple of questions for you:

Is straighter and longer hair of East Asians related to a stronger sexual selection ?

Or do living in cold and windy places like Siberia favour straight/long hair ? (I mean, in cold and windy days it is better to have flat hair than curly hair, the colder the flatter ?)

Sampieru said...

Sorry my first question should be read like this:

Is straighter and longer hair of East Asians compared to Europeans related to a stronger sexual selection ?

Peter Frost said...

Sampieru,

In response to your first question, I don't see how long head hair can help to retain body heat. Body heat is best retained by stable pockets of 'dead air'. This purpose is best served by stiff, shaggy hair that is not too long and stays in one place.

In response to your second question, I think the dynamics of sexual selection were different in ancestral Europeans and ancestral East Asians. During the last ice age, the steppe-tundra zone lay further to the north in Asia than in Europe. Male mortality was thus probably higher on the Asian steppe-tundra, with the result that sexual selection of women was correspondingly intensified.

On the other hand, because life was more precarious on the Asian steppe-tundra, it was also more difficult for humans to maintain a continuous presence there throughout the ice age. At the glacial maximum, northern Asia seems to have been a polar desert with no human life.

So, yes, sexual selection may have been more intense on the Asian steppe-tundra, but the populations undergoing this selection may have repeatedly gone extinct, especially at the glacial maximum. This may be why we see hair and eye color polymorphisms in Europeans but not in East Asians. The evolution of such traits may require not only intense sexual selection but also an extended period of time and a large gene pool.

doctorsumitseth said...

Hello Dear

I find your blog interesting and educational being a forensic pathologist I am interested in Forensic Anthropology.

could you please help me answering this question with explanation

Q. Fragmentation of medulla of scalp hair is a feature of all except:

a) negroes
b) mongolians
c) caucasions
d) aryans


My email id is doctorsumitseth@gmail.com


Warm regards
Sumit

kamagra said...

This is something really curious, also the head hair continue growing after you died, that's something interesting, I say it because I could see my grandmother's reminds and I saw her head hair very long.m10m

bobmehling said...

In comparison to other primates, the disparity between head and body hair in humans is extreme. In chimps and bonobos the head hair is roughly the same length as body hair. I'm unsatisfied with current explanations about why this evolved. Short, thin body hair in Homo sapiens can be explained by adaption to climate and wearing clothing, but why did our head hair grow so long? Sexual selection and sexual dimorphism do not adequately explain why this evolved. On average, uncut hair in both men and women grows to the waist or longer, including races from the tropics (reference Rastafarians). The longest hair on record currently belongs to a man (http://www.thaifocus.com/news/stories/longhair.htm) at over 14 feet. Since most modern humans cut their hair, there is no statistical evidence to claim one sex has longer hair than another. Sexual dimprphism implies differences between the sexes (reference the male lion's mane or the peacock's tail) In humans sexual dimorphism can be found in the beards and larger upper torsos of males but not in length of head hair. If long hair was the product of sexual selection, both sexes seem to be selecting it. Haircuts and hairstyles evolved culturally, not biologically, and the percieved preference for long female hair may be simply cultural bias. If only we could know from fossil evidence if this disparity began to evolve in Homo erectus- but hair does not readily fossilize.

Anonymous said...

I think it may have to do with hormones. It seems that men with more testosterine have trouble with balding, thinning hair. Perhaps men began to select for women with thicker longer head hair because it was an indicator of less testosterine and more estrogen/progesterone, which would translate to higher fertility? Also, some women do go bald when they get older, usually past menopause when those female hormones are decreasing. A younger woman with more head hair would be more attractive for those reasons.

Anonymous said...

Could long hair perhaps been favored because it could be used as string..?

Dorian Kiss said...

Hi.

My primary theory is that human head hair evolved to be longer to increase the surface area available for vitamin D synthesis. Moving into colder regions, humans had to wear clothing to keep warm, and this decreased the surface area available for vitamin D synthesis. Vitamin D is synthesised in the oils on the surface of the skin and hair, and then absorbed either through diffusion across the hair and through the skin or through licking by the tongue. Long hair could flow over the top of clothing, allowing humans to be clothed whilst still being able to synthesise vitamin D over the same areas. Eskimo people are able to survive with their hair covered because they get a lot of vitamin D through their diet rich in seafood.

What do you think of this theory? I have not been able to find anyone else write of this theory anywhere on the internet…

Thanks for your time,

Dorian Kiss