(Adapted from Do pigmentation and the melanocortin system modulate aggression and sexuality in humans as they do in other animals?, by J. Philippe Rushton and Donald I. Templer, Personality and Individual Differences, Volume 53, Issue 1, July 2012, Pages 4–8. [PDF].)
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Pigmentation—coloring—varies greatly across species. In 2008, Anne-Lyse Ducrest, Laurent Keller and Alexandre Roulin, three ecologists at Switzerland’s University of Lucerne published a review article in Trends in Ecology and Evolution [Pleiotropy in the melanocortin system, coloration and behavioral syndromes, September 2008] on the relations between pigmentation, sexuality, and aggression in 45 vertebrate species. They found that darker-colored individuals had higher levels of aggression and sexuality than lighter colored individuals across three species of mammals (African lion, soay sheep, and white-tailed deer), four species of fish (mosquito fish, guppy, green swordtail, and Arctic char), four species of reptiles (asp viper, adder, fence lizard, and spiny lizard), one amphibian species (spadefoot toad), and 36 species of birds.
Ducrest and her co-authors’ explanation: increased levels of melanocortin hormones, which determine coloring, are linked with increased testosterone and other steroids that stimulate aggression and sexuality, among other things.
To test this, Ducrest & Co. experimentally varied melanocortin dosage levels. They found concomitant increases (or decreases) in aggression and sexuality.
They also carried out cross-fostering studies, placing darker and lighter offspring with adoptive parents of the opposite pigmentation. Cross-fostering did not alter the offspring’s coloring or behavior. Male lions with darker manes remained more aggressive and sexually active than those with lighter manes. Darker feathered barn owls continued to have a stronger immune response to stress (another linked phenomenon) than lighter feathered barn owls.
It was the biological, not the adopting, parent that determined both the offspring’s coloration and its behavior.
This link between coloring and behavior has been confirmed in many other species—even tortoises. In Russia, a 40-year-study bred for tameness in silver foxes and found that lightness coincidentally emerged. After 40 years, the selected foxes were as tame and eager to please as domestic dogs—and the dark coat colors originally evolved as camouflage in the wild had been replaced by piebald. (Piebald coats are often seen among domestic animals—in dogs, cats, sheep, donkeys, horses, pigs, goats, mice, and cattle).
Similarly, selecting for tameness over 30 generations of Norway rats caused the proportion of piebald rats to increase rapidly until over 70% had white bellies and about 50% had white feet and ankles—“socks.”
But what about humans? Despite all the evidence
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