Melanocortins and behavior
In many vertebrates, there is an association between pigmentation and behavior. One potential reason for this is that genes influencing pigmentation also have pleiotropic effects on other traits, including behavior. A recent paper in Trends in Ecology and Evolution lays out this hypothesis:
In vertebrates, melanin-based coloration is often associated with variation in physiological and behavioural traits. We propose that this association stems from pleiotropic effects of the genes regulating the synthesis of brown to black eumelanin. The most important regulators are the melanocortin 1 receptor and its ligands, the melanocortin agonists and the agouti-signalling protein antagonist. On the basis of the physiological and behavioural functions of the melanocortins, we predict five categories of traits correlated with melanin-based coloration. A review of the literature indeed reveals that, as predicted, darker wild vertebrates are more aggressive, sexually active and resistant to stress than lighter individuals. Pleiotropic effects of the melanocortins might thus account for the widespread covariance between melanin-based coloration and other phenotypic traits in vertebrates.
This is clearly far from gospel truth; the authors are laying out the plausibility of this hypothesis and a framework for further exploration. The hypothesis is that higher levels of the molecules that bind the melanocortin receptors (the melanocortins and agouti proteins) lead to both darker pigmentation as well as pleiotropic effects in other tissues (I’ve mentioned before some of the effects of messing with these receptors in sexual behavior and metabolism). Analysis of the way pigmentation and various other traits vary in mouse models leads to results consistent with this hypothesis.
A corollary of this argument is that in vertebrates where pigmentation is controlled downstream of the melanocortins (ie. at, or further downstream of, MC1R), this correlation between pigmentation and other traits should not be consistently true. For this reason, the authors argue that humans should be exempt. However, they may be unaware that some difference in pigmentation both between and within populations is controlled by ASIP, a protein that binds MC1R, acting as an antagonsist for melanocortin binding. Humans, then, could be an ideal test case for the hypothesis–do phenotypes like aggression map to ASIP like pigmentation does? However, the unpalatable nature of this question makes it rather unlikely to be pursued in humans.
Labels: Genetics, Pigmentation
Even on a more direct level, what about the effects of neuromelanin on the substantia nigra region of the brain? http://en.wikipedia.org/wiki/Substantia_nigra
Is it affected by the same genes? I’ve noticed redheads are very high strung, while on the other end dark pigmented people are calm.
check siblings then? ASIP is pretty polymorphic in african americans.
However, the unpalatable nature of this question makes it rather unlikely to be pursued in humans.
right up until the chinese gov. starts to consider the project to produce the prototype super-soldiers ;-)
” However, the unpalatable nature of this question makes it rather unlikely to be pursued in humans.”
Ye shall not know the truth and not knowing the truth shall make you employable.
However, the unpalatable nature of this question makes it rather unlikely to be pursued in humans.
Just disguise it as a study of the blessing of diversity. You can’t study violence or anything bad, obviously. But set up one of those speed-dating studies and see if people with one allele at ASIP are less talkative, give fewer “yes” answers to “would you like to see X again?”, etc. This is palatable because it makes blond, blue-eyed, pale people look like contemptible buzzkills.
Apres ca, le deluge.
Humans would be a very poor test case for the hypothesis because there are too many uncontrolled and uncontrollable variables, both genetic and environmental. Much easier to test it in experimental animals. I think John Maynard Smith, with his usual good sense, once said that humans were the worst possible test case for anything in evolutionary theory.
There are also pigmentation mutations in flies that have behavioral affects. I think either the ebony or tan mutations (or both) have marked effects on the behavior of Drosophila. Not surprisingly, some of the Sean Carrol-ites are looking into this.
1: What DavidB said.
2: The exact discussion in the original article is as follows (emphasis added):
“These predictions hold only when variation in melanin-based coloration is mediated by variation in the level of the agonists at MC1R and the levels of melanocortins are coordinated across tissues (Box 1). By contrast, there should be no consistent association between melanin-based coloration and other phenotypic traits when variation in coloration is due to mutations at effectors of melanogenesis such as MC1R, which is the case in half of the studied species (Supplementary Table S3). In this respect, it is important to note that variation in melanin-based coloration between human populations is primarily due to mutations at, for example, MC1R, TYR, MATP and SLC24A5 [29] and [30] and that human populations are therefore not expected to consistently exhibit the associations between melanin-based coloration and the physiological and behavioural traits reported in our study.”
In short: The behavioural effects of melanocortins are not directly caused by their effect on colour-related genes (MC1R), but by their effects on other, non-colour-related genes. In humans, most of the variation in skin colour is caused precisely by mutations at these colour-related genes, so the discussed correlations would not apply.
However, p-ter says that one component of the melanocortin system (ASIP, frequently mentioned in the article) is actually involved in human skin colour variation, and may also have behavioural effects by affecting the other, non-colour-related genes.
What’s so unpalatable about it? Maybe if one actually framed it in a fair way, people may actually be more open to the truth.
So if you say, dark skinned people may be more likely to be sex crazed and violent, while light skinned people may be more likely to be crazy and impotent, I think most people would not react to this negatively. They would realize that we all have our strengths and weaknesses
However, what tends to happen is that the majority light-skinned scientific populace will likely obsess over the “aggression” while humanizing the “crazy” part by using phrases like “less resistant to stress”.
Much easier to test it in experimental animals
depends on what the question is. in this case, it’s known from knockouts and overexpression in mice that messing with the melanocortin system leads to pleiotropic effects outside pigmentation. but the question here is whether variation in pigmentation in natural populations influences other traits. to do this you need to study variation in natural populations (this entails dealing with all the other environmental and genetic variables present in nature, no matter the species). the vertebrate with the best developed resources for mapping in the case could be humans.
I’ve noticed redheads are very high strung
Wikipedia say “… people with red hair have differential sensitivity to pain compared to people with other hair colors”, citing:
Liem EB, Joiner TV, Tsueda K, Sessler DI (2005). “Increased sensitivity to thermal pain and reduced subcutaneous lidocaine efficacy in redheads”. Anesthesiology 102 (3): 509?14. doi:10.1097/00000542-200503000-00006. PMID 15731586.
(Disclaimer: I have red hair. Feel free to speculate that I’m a surviving neanderthal :-))
I question that this theory can be applied to humans.
I have spent time in Malaysia, which has a great number of Tamil Indians (from southern part of India). These people are very dark-skinned (darker than most African-Americans), yet, they do not act anything like stereotypical African or African-American behaviors. Indeed, they seem to have it together better socially than the Malays (who are usually lighter skinned). They might not have the work ethic of the Chinese, but of the three races of Malaysia, the Indians act the most like “white” Americans.
Perhaps Tamils act differently in India than in Malaysia. I have never been to India, so I have no clue.
Any comments?
as p-ter alluded to only a few of the loci would be appropriate targets for this hypothesis anyway. obviously it isn’t a slam dunk to say that japanese are more “aggressive, sexually active and resistant to stress” than northern europeans, though the latter is a lighter population. this would only be *one* of many independent variables if it is a variable at all. so best to look within populations.
These people are very dark-skinned (darker than most African-Americans), yet, they do not act anything like stereotypical African or African-American behaviors. Indeed, they seem to have it together better socially than the Malays (who are usually lighter skinned). They might not have the work ethic of the Chinese, but of the three races of Malaysia, the Indians act the most like “white” Americans.
I’m not sure about the genetic basis of this pigmentation difference (my guess would be it’s largely due to slc24a5?). another reason this would be good to test in humans–there are populations that differ in pigmentation due to loci like ASIP (which would be expected to follow the correlation) and populations that different due to other reasons (which wouldn’t).
(my guess would be it’s largely due to slc24a5?).
hm. probably wrong direction. see here, tamils in sri lanka have about 25% of the derived variant. and east asians don’t seem to have any of it. cambodians seem to have a bit of derived, but lot less than that (that 25% is the lowest frequency i’ve seen for an indian population, the neighboring sinhalas in the paper linked are 50% derived).
probably wrong direction.
yeah, you’re right. should have thought that through a little more.
Is the referenced paper concerned only with aggressive behavior in natural populations? I can’t access the full version, but it isn’t mentioned in the Abstract. I think it would be extraordinarily difficult to study this for any non-human species. Imagine, say, trying to establish whether black panthers in the wild are more aggressive than ordinary leopards. The mind boggles. For humans, there would be the glaring problem (not arising in other species) that differences in pigmentation are correlated with ethnic and social status, so that any findings would be moot.