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The evolution of nakedness. Markus J Rantala University of Turku Finland. The cooling device hypothesis. Nakedness is not benefical in savannah !.
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The evolution of nakedness Markus J Rantala University of Turku Finland
Nakedness is notbenefical in savannah! • 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.
Otherprimatesliving in savannahhavenotlosttheirfur, insteadtheyhavedevelopedhairiercoat!
Homo sapiensleftAfricaalready100 000 yearsago! • Man adapted to northern latitudes by changing the colour of the skin, not by regaining hair!
Problems of the hypothesis • The aquatic mammals that have lost their hair have large and fusiform bodies with small appendages; large animals possess low thermal conductances, while small appendages minimize the area of skin in contact with water. • Vascular adaptations to reduce heat loss.
Problems of the hypothesis • A naked mammal of the shape and size of the early hominids would have found maintaining a high body temperature in the sea energetically very expensive. • Even those species of comparable body mass that inhabit warm tropical freshwater bodies have retained a thick coat.
The ectoparasitehypothesis • 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.
The ectoparasitehypothesis • 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.
The ectoparasitehypothesis • 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. • Ectoparasiteswereforgotten for a century.
The modernectoparasiteshypothesisRantala 1999 • 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.
Humansbecameonlyprimatesthatharbourfleas! • Fleascancomplete their life cycle only if their host lives in a permanently inhabited den or lair. • The use of home basescaused the explosion of the ectoparasite burden!
Ectoparasitesarevectors for manylethaldiseases The mostfamousones: • The Black Death (Yersinapestis) • Spottedfever
The enemywitheightfeet • During the militaryexpedition to Russia (1812) Napoleon lostabout 500 000 soldier for spottedfewer. • Lowlevel of hygiene and hairpiecesprovidedgreatoppurtunity for lice!
The enemywitheightfeet The American Civil War (1861–1865): • Union forceslost 93 443 men in combats. • 186 216 diedbecause of spottedfewerspreadbylices!
How to getrid of ectoparasites? • The easiest and fastestway to getrid of the head and pubiclice is shaving!
The selectiontowardsnudity • Short haired and nakedindividualsharbourlessectoparasites! • It is easier to removeectoparasitesfrom the nakedskin. • As the ectoparasite burden increased, having fewer parasites may have become more important for survival than a warm fur coat.
Whendidhominidslosetheirfur? • 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’.
Darwin’s (1871)theoryabout the hairlessness • 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.
It is commonly thought that women’s preference on male body hair changes between years like fashion and is influenced by media.
We tested the role of body hair on mate choice and factors that influence on individual variation in preferences on body hairs. Rantala M.J., Pölkki M. & Rantala L.M. 2010. Behavioral Ecology 21: 419–423.
Results Women’s preference on body hair correlated strongly with the age of the women (r = 0.424, N = 299, p < 0.001)
Women’spreferences for maletraitschangeacross the menstrualcycle. • In fertilephase of the cyclewomenprefer: • Facial and bodymasculinity • Vocalmasculinity • Dominantbehavior • Tallermen • Smell of masculinemales
Womenpreferences for maletraitschangesacross the menstrualcycle. • In fertilephase of the cyclewomenprefer: • Facial and bodymasculinity • Vocalmasculity • Dominantbehavior • Tallermen • Smell of masculinemales • Areourresultsconsistentwiththesepreviousstudies?
Bodyhair and sexhormones 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)
Bodyhair and sexhormones 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).
Ourstudysupports the hypothesisthatwomenprefergreatermasculinity in malebodies at timeswhenfertility is the highest! Estimatedmarginalmeanscorrectedbyage
Results • 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).
Results • 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.
Results • 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).
Results • 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.
Conclusion • Itseemsthatbodyhairhaveeffect on matechoice in humans! • There is individualvariation in women’spreference on bodyhairthatmightexplainwhyallmenhavenotlosttheirbodyhair. • At least in Finland, shaving of bodyhairwouldincrease the attractiveness of men.
Correlations with hairyness r p Muscularity -0.23 0.057 Testosterone -0.246 0.042 BMI 0.285 0.017 WHR 0.367 0.002 Age 0.431 <0.001
Correlations after controlling the effect of age r p Muscularity -0.258 0.034 Testosterone -0.198 0.106 BMI 0.177 0.150 WHR 0.264 0.030 Attractiveness 0.341 0.004
Which traits predict body attractiveness -------------------------------------------------------------------------------------------------------------- Body attractiveness rating Bivariate correlations Partial correlations (all other variables controlled) Predictor rs p R2 p ------------------------------------------------------------------------------------------------------------------------ Muscularity 0.555 <0.001 -0.623 <0.001 Body mass index (BMI) -0.537 <0.001-0.377 0.002 WHR -0.666 <0.001 0.300 0.044 Age -0.393 <0.001 -0.227 0.074 Hairyness-0.454 <0.001 -0.160 0.209 Height 0.026 0.834 0.177 0.165 SHR 0.196 0.393 0.068 0.594 -----------------------------------------------------------------------------------------------------------------------
