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.
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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.
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