Thursday, January 4, 2018

Red-haired women are special




The Damsel of the Sanct Grael – Dante Gabriel Rossetti (1828-1882)



It's known that red-haired women, but not red-haired men, are more sensitive to pain. Red hair is also associated with a higher risk of developing endometriosis, Parkinson's disease, and decreased platelet function.

A study in the latest issue of PLoS One has confirmed that red hair, especially in women, is linked to certain health issues. According to a survey of over seven thousand participants, red-haired women do worse than other women in ten health categories and better in only three, being especially prone to colorectal, cervical, uterine, and ovarian cancer. Red-haired men seem to be as healthy as other men, doing better in three categories and worse in three. Reproductive success, i.e., number of children, is the only category where redheads of both sexes do better than other participants.

This study has also confirmed that red hair is naturally more frequent in women than in men. To a lesser degree, the same is true for blond hair and green eyes. These bright colors seem to result from a selection pressure that mainly targeted women, i.e., sexual selection. In other words, among early Europeans there were too many women and not enough men; hence, competition between women for mates favored those who could better catch the attention of men, such as through a palette of bright hair and eye colors.

Because women are overrepresented among redheads, it may be that estrogen promotes synthesis of red pigments by hair follicles, particularly during fetal development. Thus, if a baby is born red-haired and female, estrogenization of its body tissues should be, on average, near the top end of the normal range. It will therefore be more at risk of developing certain health issues.

Another hypothesis can be put forward. If red hair was the last hair color to evolve, the underlying alleles may not have finished adapting to the rest of the genome, and vice versa. This reciprocal adaptation is all the more necessary because one of the five alleles for red hair seems to be of Neanderthal origin. The hypothesis of incomplete adaptation does not exclude the hypothesis of high estrogenization. In fact, there may be interaction between the two factors. Although it's likely that sexual selection did produce new hair and eye colors, we must still explain why, in this palette of colors, red hair seems to show the greatest difference between men and women both in population frequency and in associated health effects. 


Reference

Frost P, Kleisner K, Flegr J (2017) Health status by gender, hair color, and eye color: Red-haired women are the most divergent. PLoS ONE 12(12): e0190238. 


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La rousse est particulière


On sait que les rousses, mais pas les roux, sont plus sensibles que les autres à la douleur. La rousseur est également associée à un plus grand risque de développer l'endométriose, la maladie de Parkinson, ainsi que des troubles de l'agrégation plaquettaire.

Une étude publiée dans le dernier numéro de la revue PLoS ONE confirme que la rousseur, surtout chez la femme, est reliée à certains problèmes de santé. Selon une enquête menée auprès de plus de sept mille participants, les rousses se classent pire que les autres femmes dans dix catégories de santé et mieux dans seulement trois, étant surtout susceptibles de développer des cancers du gros intestin, du col utérin, de l'utérus ou des ovaires. Quant aux roux, leur état de santé ressemble à celui des autres hommes : mieux dans trois catégories et pire dans trois. Le succès reproducteur, soit le nombre d'enfants, est la seule catégorie où les têtes rouges des deux sexes font mieux que les autres participants.

Cette étude confirme également que la rousseur est naturellement plus fréquente chez la femme que chez l'homme. Dans une moindre mesure,  c'est le même constat avec la blondeur et les yeux verts. Ces couleurs vives semblent être le résultat d'une pression de sélection visant surtout la femme, soit la sélection sexuelle. Autrement dit, il y aurait eu trop de femmes et pas assez d'hommes chez les premiers Européens, avec pour résultat une concurrence entre les femmes favorisant celles qui attiraient mieux les regards des hommes, comme par exemple par une palette de couleurs vives décorant les cheveux et les yeux.

Les femmes étant surreprésentées parmi les têtes rouges, on peut émettre l'hypothèse que l'œstrogène favorise la synthèse de pigments rouges dans les follicules pileux, particulièrement au cours du développement fœtal. Alors, si un enfant nait à la fois roux et de sexe féminin, l'œstrogénisation de ses tissus organiques doit être, en moyenne, vers la limite supérieure de la normale. La rousse sera donc plus à risque de connaitre certains problèmes de santé.

Une autre hypothèse est possible. Si la rousseur a été la dernière couleur de cheveux à paraître, il se peut que les allèles sous-jacents n’aient pas encore fini de s'adapter au reste du génome et vice versa. Cette adaptation réciproque est d'autant plus nécessaire parce que l'un des cinq allèles pour la rousseur semble être d'origine néandertalienne. Notons que l'hypothèse d'adaptation incomplète n'exclut pas celle de forte œstrogénisation. En fait, il pourrait y avoir une interaction entre les deux facteurs. S'il est vraisemblable que la sélection sexuelle ait produit de nouvelles couleurs des cheveux et des yeux, il faudra toujours expliquer pourquoi, dans cette palette de couleurs, la rousseur semble montrer la plus grande différence entre les hommes et les femmes, autant en termes de fréquence dans la population que sur le plan de la santé. 

5 comments:

Sean said...

Though not originating in Europe the European variant SLC24A5 is nearly fixed there, and the only effect we know for it was white skin. Assuming white skin was selected for, and knowing that red hair also had a big effect on skin lightening, one wonders whether the selection of red hair alleles was not just outright sexual selection but almost equally for the whitest possible skin to aid survival of the women and their children. The eliciting of provisioning of women and their children in a patriarchal Indo European society during the Indo European invasion of Europe may have been the reason that lighter skin was at a premium. The Indo Europeans certainly were not red-headed or even what would today pass for fully north European typical white when they arrived in Europe, but as their descendants expanded in a rolling wave of conquest, they certainly got more red headed and paler skinned the further they went (Ireland for instance).
The early Indo European era in north Europe was not necessarily the first time there was selection for lighter eyes and and hair by sexual selection, the Motala remains had it to some extent happened in a seperate population during the stone ager. What do you think of the idea there were parallel selection pressures in the Stone age and the early Bronze age with sexual selection for hair mand eye colours plus feminine craniofacial form and also selection for provisioning /care eliciting white skin both being particularly strong in the Bronze age?

Peter Frost said...

We found an association between red hair and reproductive success in both sexes. One can only speculate on the reason, but I suspect that red hair, as the rarest hair color, benefits the most from the novelty effect on the mate market.

We don't know where SLC24A5 originated. Yes, it shows up in early European farmers, but it could have introgressed from northern Mesolithic hunters, like SLC42A5. We are still working with patchy data.

Sean said...

'Secret' of youthful looks in ginger gene
BBC News website
.

Maybe there is a rare colour advantage for red haired mean and women, but I would think in the highly patriarchal society in which 60% of Europeans descended from 3 Bronze age men, the ones who reaped the rarity advantage from having red hair were women.

Anonymous said...

I´m not convinced by the sexual selection hypothesis, in Egypt don't forget gingers were put to death and its also a sign of witchiness in the Caucasus area. Can something stigmatised, also be sexually desired without a cultural impetus?

Sean said...

Helen Gloag
How Orgasm Could Dull Pain (in women).