Markus J RantalaUniversity of Turku

Finland

Naked skin increases the chances of heat loss, but at the same time it also increases heat gain and risks damage from the sun’s rays. Thus, it increases perspiration, leading to dehydration; this in turn may be detrimental in a dry savannah environment.

Man adapted to northern latitudes by changing the colour of the skin, not by regaining hair!

The parasite argument was first presented by Belt (1874). He suggested that a naked primate would be less liable to harbour ticks and other noxious parasites, which, in the tropics, may constitute a serious danger to health.

According to Darwin (1888), the weakness of Belt’s hypothesis is that it does not explain why the human species in particular lost its hair: ectoparasites are a problem for all primates, not for humans alone.

According to Darwin (1888), the weakness of Belt’s hypothesis is that it does not explain why the human species in particular lost its hair: ectoparasites are a problem for all primates, not for humans alone.

Ectoparasites were forgotten for a century.

As a consequence of change in the structure of human society towards a group-hunting primate, humans started to occupy the fixed ‘home bases’.

The earliest evidence of a home base dates to 1.8 million years ago.

Fleas can complete their life cycle only if their host lives in a permanently inhabited den or lair.

The use of home bases caused the explosion of the ectoparasite burden!

The most famous ones:

The Black Death (Yersina pestis)

Spotted fever

During the military expedition to Russia (1812) Napoleon lost about 500 000 soldier for spotted fewer.

Low level of hygiene and hairpieces provided great oppurtunity for lice!

The American Civil War (1861–1865):

Union forces lost 93 443 men in combats.

186 216 died because of spotted fewer spread by lices!

The easiest and fastest way to get rid of the head and pubic lice is shaving!

Short haired and naked individuals harbour less ectoparasites!

It is easier to remove ectoparasites from the naked skin.

As the ectoparasite burden increased, having fewer parasites may have become more important for survival than a warm fur coat.

Rogers et al. (2004) calculated on the basis of the number of silent mutations in African versions of the MC1R gene (which produces darker skin) that humans became naked about 1.2 million years ago in Africa.

This is consistent with the hypothesis that denudation took place during the period when humans started to occupy the fixed ‘home bases’.

Man, or rather primarily woman became divested of hair for ornamental purposes and that women subsequently transmitted the sexual advantage of nakedness almost equally to their offspring of both sexes.

Rantala et al 2010. Behavioral Ecology 21: 419–423.

Pre-Menopausal Post-Menopausal

% p

refe

renc

e fo

r h

airin

ess

20

30

40

50

60

70

Pre-Menopausal Post-Menopausal

% p

refe

renc

e fo

r h

airin

ess

20

30

40

50

60

70

Women’s preference on body hair correlated strongly with the age of the women (r = 0.424, N = 299, p < 0.001)

Low Fertile High Fertile Post-Menopausal Pregnant

% p

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renc

e fo

r ha

irine

ss

20

30

40

50

60

Estimated marginal means corrected by age

In fertile phase of the cycle women prefer:Facial and body masculinityVocal masculinityDominant behaviorTaller menSmell of masculine males

In fertile phase of the cycle women prefer:Facial and body masculinityVocal masculityDominant behaviorTaller menSmell of masculine males

Are our results consistent with these previous studies?

Chest hair growth has not been found to be correlated with levels of circulating testosterone (e.g. Burton et al., 1979; Lookingbill et al. 1988, 1991; Knussman et al 1992)

Chest hair growth has not been found to be correlated with levels of circulating testosterone (e.g. Burton et al., 1979; Lookingbill et al. 1988, 1991; Knussman et al 1992).

Instead, males with more body hair have more estradiol in their blood (Winkler & Christiansen 1993).

Low Fertile High Fertile Post-Menopausal Pregnant

% p

refe

renc

e fo

r ha

irine

ss

20

30

40

50

60

Estimated marginal means corrected by age

Women’s preference correlated strongly with the hairiness of her current partner even when women’s age was used as covariate (r = 0.456, P < 0.001).

Women’s preference correlated strongly with the hairiness of her current partner even when women’s age was used as covariate (r = 0.456, P < 0.001).

Thus, it seems that body hair might have effect on real mate choice in human!

The hairiness of women’s current partner did not correlate with his age (r = 0.116, N = 196, P = 0.106,), suggesting that in Finnish men hairiness of men body does change much after the puberty.

Hairiness of women’s father correlated with hairiness of women´s current partner (r = 0.368, p < 0.001) and also with women’s preference on the body hair (r = 0.132, p = 0.024).

Hairiness of women’s father correlated with hairiness of women´s current partner (r = 0.368, p < 0.001) and also with women preference on the body hairs (r = 0.132, p = 0.024).

Thus, there seems to be sexual imprinting on body hair or the preference is heritable.

It seems that body hair have effect on mate choice in humans!

There is individual variation in women’s preference on body hair that might explain why all men have not lost their body hair.

At least in Finland, shaving of body hair would increase the attractiveness of men.