Saturday, March 31, 2012

Dark coloration and male aggressiveness: Is there a link?


Red-winged blackbird (source). Is dark coloration directly linked to male aggressiveness?

For the past thirty years, psychologist Philippe Rushton has been using life history theory to explain human differences in many areas: IQ, sexual development, parental investment, mating system, time orientation, etc. Initially, he saw skin color as being incidental. In recent years, however, he has come around to the view that differences in skin color directly mediate these differences in life history:

The hypothesis that skin color is a genetic correlate of IQ was endorsed by Jensen (2006) who suggested that pleiotropy (genes having more than one effect) may underlie the relationship. Skin color became of greater theoretical interest after Ducrest, Keller, and Roulin (2008) reviewed the literature and reported that in 20 wild vertebrate species, darker individuals were more aggressive, sexually active, and resistant to stress than lighter individuals. Studied were three mammal species (African lion, soay sheep, and white-tailed deer), four fish species (mosquito fish, guppy, green swordtail, and Arctic char), four reptile species (asp viper, adder, fence lizard, and spiny lizard), one amphibian species (spadefoot toad) and 36 bird species. Darker individuals also tended to have a larger body mass and greater energy and physical activity such as grooming. Ducrest et al. (2008) confirmed the naturalistic observations using experimental studies such as the administration of melanocortins and the fostering of infants to non-biological parents. For example, the fostering studies found darker maned male lions are more aggressive and sexually more active, and darker barn owls mount stronger immune responses when their biological parents are darker, even though they had been raised by lighter foster parents. (Templer & Rushton, 2011)

This hypothesis is not a perfect fit for humans, as Rushton himself noted in an earlier study on human populations and behavioral differences: “We found the relationship between crime and IQ held (r=−0.35; Pb0.01), although the one between crime and skin color did not (ns). […] the East Asian countries had very low rates of crime but not the lightest skin color” (Rushton & Templer, 2009).

In humans, many things vary with latitude, and not just skin color. One of them is the means of sustenance, which in turn impacts parental investment, mating system, and male-male rivalry. In the tropics, for example, we see the following cascade of effects:

1. Year-round tropical agriculture enables women to provide for themselves and their children with little male assistance.

2. This greater female reproductive autonomy lowers the cost of polygyny for men. More men can afford to have second wives.

3. Because more men are competing for fewer available women, there is stronger selection for males with higher testosterone levels, more robust body build, and greater ability to fight off rivals.

It is hard to see how this cascade of effects could apply to nonhumans. Guppies, for instance, have not discovered farming. Nor is their coloration a climatic adaptation. It serves as a visual signal and not as a UV shield, as is the case with most of the other animals that Templer and Rushton cite.

Clearly, a cross-species correlation does exist between darker coloration and male aggressiveness. But there is an alternate explanation. Whatever the species, individuals are usually born with little or no pigment. Lighter coloration thus becomes associated with vulnerability and a need for parental care and protection. In contradistinction to this sign stimulus, adult males tend to evolve a darker coloration, especially in a context of intense male-male rivalry. This tendency was noted by Guthrie (1970):

Light skin seems to be more paedomorphic, since individuals of all races tend to darken with age. Even in the gorilla, the most heavily pigmented of the hominoids, the young are born with very little pigment. […] Thus, a lighter colored individual may present a less threatening, more juvenile image.

Our own species likewise shows analogous age and sex differences in skin pigmentation, as seen in a strong cross-cultural trend to associate darker skin with men and lighter skin with infants and women (Frost, 2011).

This mental association may influence human mate choice, especially under conditions of intense mate competition. Given that mate competition varies latitudinally among human populations, as described above, could it be that latitudinal differences in skin color result not only from selection for UV protection but also from sexual selection?

In humans, the polygyny rate does in fact significantly correlate with darkness of skin color (Manning et al., 2004). This correlation seems to hold up even if we control for latitude. In sub-Saharan Africa, high-polygyny agriculturalists are visibly darker than low-polygyny hunter-gatherers, i.e., Khoisans, pygmies, although both are equally indigenous (Bourguignon & Greenbaum, 1973, pp. 171-175; Cavalli-Sforza, 1986a; Cavalli-Sforza, 1986b; Weiner et al, 1964). Because year-round agriculture makes women more self-sufficient and polygyny less costly, fewer women remain unmated and men are less able to translate their mate-choice criteria into actual mate choice. Such criteria include a preference, widely attested in the African ethnographic literature, for so-called 'red' or 'yellow' women (van den Berghe & Frost, 1986). Less mate choice means weaker sexual selection for light skin in women and, hence, less counterbalancing of natural selection for dark skin in either sex to protect against sunburn and skin cancer (Aoki, 2002; Frost, 1994; Frost, 2007; Frost, 2008).

A higher polygyny rate might also, correspondingly, lead to stronger sexual selection for darker-skinned men, either because women tend to prefer them, or defer to them, or because such men can more easily intimidate rivals in a context of intense mate competition.

References

Aoki, K. (2002). Sexual selection as a cause of human skin colour variation: Darwin’s hypothesis revisited. Annals of Human Biology, 29, 589-608.

Bourguignon, E. & L.S. Greenbaum. (1973). Diversity and Homogeneity in World Societies, HRAF Press.

Cavalli-Sforza, L.L. (1986a). Demographic data. In Cavalli-Sforza, L.L. (ed.). African Pygmies, pp. 23-44. Academic Press.

Cavalli-Sforza, L.L. (1986b). Anthropometric data. In Cavalli-Sforza, L.L. (ed.). African Pygmies, pp. 81-93. Academic Press.

Ducrest, A., Keller, L., & Roulin, A. (2008). Pleiotropy in the melanocortin system, coloration and behavioural syndromes. Trends in Ecology and Evolution, 23, 502-510.

Frost, P. (2011). Hue and luminosity of human skin: a visual cue for gender recognition and other mental tasks, Human Ethology Bulletin, 26(2), 25-34.
http://media.anthro.univie.ac.at/ISHE/index.php/bulletin/bulletin-contents

Frost, P. (2008). Sexual selection and human geographic variation, Special Issue: Proceedings of the 2nd Annual Meeting of the NorthEastern Evolutionary Psychology Society. Journal of Social, Evolutionary, and Cultural Psychology, 2(4),169-191.
http://137.140.1.71/jsec/articles/volume2/issue4/NEEPSfrost.pdf

Frost, P. (2007). Comment on Human skin-color sexual dimorphism: A test of the sexual selection hypothesis, American Journal of Physical Anthropology, 133, 779-781.

Frost, P. (1994). Geographic distribution of human skin colour: a selective compromise between natural selection and sexual selection? Human Evolution, 9, 141-153.

Guthrie, R.D. (1970). Evolution of human threat display organs, Evolutionary Biology, 4, 257-302

Jensen, A. R. (2006). Comments on correlation of IQ with skin color and geographic–demographic variables. Intelligence, 34, 128−131.

Manning, J.T., P.E. Bundred, & F.M. Mather. (2004). Second to fourth digit ratio, sexual selection, and skin colour. Evolution and Human Behavior, 25, 38-50.

Rushton, J. P., & Templer, D. I. (2009). National differences in intelligence, crime, income, and skin color. Intelligence, 37, 341-346.

Templer, D. I., & Rushton, J. P. (2011). IQ, skin color, crime, HIV/AIDS, and income in 50 U.S. states. Intelligence, 39(20), 437-442.

van den Berghe, P.L., & P. Frost. (1986). Skin color preference, sexual dimorphism and sexual selection: A case of gene-culture co-evolution? Ethnic and Racial Studies, 9, 87-113.

Weiner, J.S., G.A. Harrison, R. Singer, R. Harris, & W. Jopp. (1964). Skin colour in southern Africa. Human Biology, 36, 294-307.

14 comments:

Jesse Jackson said...

Science be raciss

UncleTomRuckusInGoodWhiteWorld said...

Does this make sense in terms of Southern India? I don't think so, and Tamils are pretty dark (on average).

Anonymous said...

Tamils are pretty dark (on average).

Yeah. Light by Andamanese standards though. Those people (gracile or at least tiny hunter gatherers) are seriously dark.

http://flickrhivemind.net/Tags/andamanese/Interesting

http://www.andamanese.net/image/AnvitaAbbiWithOnges.jpg

http://www.outlookindia.com/images/anvita_abbi_lang_20071119.jpg

They seem darker than Western Africans like Nigerians or Ghanaians.

Sean said...

Ducrest et al(2008) looked at individuals of vertebrate species. I think it holds true for comparisons of individuals from the same human ethnic group that the darker skinned young men tend to show more vigor and have traits useful in male-male competition for females. Fathering 4 children before the age of 30 tripled the chance of living to be 100 years old in a study. Many offspring & long life seems to be a reflection of a primordial 'biological quality' that it would pay to advertise. It could be an honest signal that women recognize and prefer. K.Lorenz says that the robin with the reddest breast is in every other respect the best.

I don't think the re productively useful suite of traits includes IQ. I've read it suggested that individuals who were sexual early developers rarely, if ever, develop into great intellects.

A section of FWDW that discusses the traditional Canadian farmers beliefs and the perception that swarthy men were aggressive - maybe they were also healthier.

Ben10 said...

@ Sean and THE_TRUTH

Regarding the last post on 'monkey people', do you know if chinese scientists gave confirmed or not the nature of the 'Yeti scalp' visible in one tibetan monsatery and in wiki at:
http://en.wikipedia.org/wiki/Yeti

It seems a forensic DNA analysis should not be too difficult, on the hairs AND the skull bones, even if the hairs are coming from a goat, it doesn't mean the skull bones are a fake too.
"...In 1960, Hillary mounted an expedition to collect and analyze physical evidence of the Yeti. He sent a supposed Yeti "scalp" from the Khumjung monastery to the West for testing, whose results indicated the scalp was manufactured from the skin of a serow, a goat-like Himalayan antelope. Anthropologist Myra Shackley disagreed with this conclusion on the grounds that the "hairs from the scalp look distinctly monkey-like and that it contains parasitic mites of a species different from that recovered from the serow."

Anonymous said...

Regarding intelligence and early development:

"http://www.krepublishers.com/02-Journals/T-Anth/Anth-11-0-000-09-Web/Anth-11-4-239-09-Abst-PDF/Anth-11-04-259-09-503-Deb-R/Anth-11-04-259-09-503-Deb-R-Tt.pdf

Similarly, the fact that menarche occurs later in the poorer social classes compared with the well to-do in developing countries has been ascribed to disparities in nutrition and health (Charzewska et al. 1976; Chowdhury et al. 1977; Eveleth and Tanner 1990; Foster et al.1986; Gandotra and Das 1982; Rana et al. 1986; Tan-Boom et al.1983; Uche and Okorafor 1979)."

http://onlinelibrary.wiley.com/doi/10.1002/ajpa.1330250509/abstract

"Recent studies of the effects of socioeconomic stratification on stature and age at menarche in Poland are reviewed.

three nationwide growth surveys in 1955, 1966, and 1978

Individuals from the larger urban centers and small families with college-educated fathers, on the average, tend to be the tallest and to mature earliest, while those from peasant (farmers) families are at the opposite extreme in size and maturity. Also, the data show marked secular trends towards increased stature and earlier maturation."

Early maturation seems to be linked to high class within population. This might be because of nutrition differences, or maybe not. It seems like if the correlation between class and IQ were high, you wouldn't really get a pattern of earlier maturing low IQ persons.

Early maturers come from higher class background, so if higher class backgrounds have better IQ, we should expect great intellects to more frequently be precocious maturers.

Ben10 said...

I personnaly see 'evolutionary trends' in Evolution, even if this is anathema in Darwinism.
I also recognize 'hominization' as a particular evolutionary trend. 'hominization' being defined as "encephalization + pedomorphisation/neotenysation".
For example, when some of the australopithecine ancestors decide to leave the 'hominization trend', they comfortably keep their ancestral features but they become chimps and gorilla instead of australopithecine.

On the other hand pedomorphization implies change and risk: a lost of specialization and a lost of adult features, like a sort of 'simplification' of the adult form of the neotenic individuals. The 'change' comes from the fact that the neotenic individuals cannot keep the ancestral way of living. This must be very uncomfortable as these first individuals are 'unfit' in the old tribes and they must reinvent themselves. The 'risk' obviously, is to fail and disapear.
Double anathema now!, since here, the 'unfits' are those who drive the Evolution!

So yes, i see a link between dark coloration AND male aggressiveness: these are just adult features. AND this is also the definition of the alpha male!
Unfortunatly, as comfortable must be the position of the alpha male in the society, his testosterone-driven rule over harems of females can only lead down the ladder of hominization as it hapened to chimps and gorilla a few millions years ago. Maybe it's no coincidence that these failed-hominids retain a harem-like structure. The Silver back is an obstacle to Evolution.

Instead, the trend for 'encephalisation + neotenization' demands for adults 'less adults' in today's standard, that is, males less agressive and females more androgynous. But it's clear that such males can not stand the competition with modern silver backs. Soon, the 'white, nice guy' will be another word for 'unfit'. And once again, the unfit must reinvent himself to survive.

Sean said...

Rushton's theory doesn't seem to predict that white women would have relatively low rates of infertility, as is the case. For men dark pigmentation, and the suite of traits that go with it, is an aid to fitness and a benefit. For women dark pigmentation is not beneficial. It follows daughters and sons would be impacted differently, I wonder if dark couples are more likly to have sons. In a ethnically mixed population the white men and non white women would lose out I think.
About IQ I have read that Black women tend to have higher IQs than black men more often than is the case with whites, relatively speaking.

Anonymous said...

I wonder if dark couples are more likly to have sons.

Sex ratios in the tropics are slightly more female for what it's worth. Sex ratios are slow to evolve, but maybe that would reflect an environment where male children are a higher risk, higher reward proposition (i.e. go for winner takes all multi-mating with no provisioning rather than a single mate with provisioning)

Anonymous said...

there is a practical reason for females to be a shade lighter than their mates from their own ethnic from the same latitude, and that's because of babies, for women to have babies they have to get higher amount for calcium and vitamin d for their fetus. and this might be responsible for men picking women who are lighter given their latitudinal disposition. This is somehting that i dont see anyone pointing out on evo blogs.

Anonymous said...

Saw a couple of young African-American girls recently, with their parents. Relatively low white introgression. They were in the 5-7 YO range.

They were far more "vigorous" than white or Asian girls of that age would be.

Anonymous said...

for women to have babies they have to get higher amount for calcium and vitamin d for their fetus.

possibly more importantly they need to maintain folate levels and actually having darker skin would be better for that (the prevailing theory about why tropical people have dark skin being to prevent folate destruction in the skin).

since no one knows exactly how this dynamic cashes out, this isn't "somehting that (you would) see anyone pointing out on evo blogs."

UncleTomRuckusInGoodWhiteWorld said...

Anon 1:

the Andamanese you showed are probably a similar color to most Sahel people...true darker than most West Africans though, quite a bit darker.

Still my point is that many Southern Indians are the same color as many West or East Africans (south of the Sahel).

Susan Burns said...

I did a post about the relationship between domestication and pigmentation in my blog Yam Suph.

http://falashaleott.blogspot.com/