Conditional response….

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HUMAN SEXUAL DIMORPHISM IN SIZE MAY BE TRIGGERED BY ENVIRONMENTAL CUES:

Evolutionary biologists mostly assume that polygyny increases sexual dimorphism in size because, under polygyny, larger males monopolize mating opportunities and pass on their ‘large male’ genes to their sons. Available data on parent–child correlations in height among humans (Homo sapiens) do not support the crucial assumption that height is transmitted along sex lines. This paper instead suggests that human sexual dimorphism in size emerged, not because men got taller, but because women got shorter by undergoing early menarche in response to polygyny. It further speculates that, rather than genetically transmitted, the sexual dimorphism may emerge anew in each generation in response to the degree of polygyny in society. The analysis of comparative data supports the prediction that polygyny reduces women’s height, but has no effect on men’s, and is consistent with the speculation that the origin of human sexual dimorphism in size may be cultural, not genetic.

The authors present evidence that women who enter menarche earlier reach a shorter adult height while in societies where polygyny (defined as male reproductive variance:female reproductive variance) is more pronounced also tend to exhibit more sexual dimorphism. At the end of the paper the authors hypothesize that a biological-proximate process that might affect menarche are pheromones produced by the father which interact with his daughter’s physiology. Evidence which shows later menarche for girls who spent a greater time with their fathers is brought forward, the logic being that polygynous fathers can provide their daughters far less individual attention (ergo, pheromones).

Related: Father absence and reproductive strategy: an evolutionary perspective.

8 Comments

  1. Very interesting. The paternal-pheromone idea would also be consistent with statistics that show earlier sexual activity for girls raised by single mothers.  
    The more obvious idea is that dad discourages horny teenage boys from pursuing his daughter, and I’m sure that plays into it, but maybe this biological phenomenon also accounts for a lot of it…

  2. Well, is there greater sex dimorphism in African populations with massive polygyny than in more monogamous populations?

  3. It seems to me that the implication here is that we have several stable strategies that can evolve socially (like father sticks around to raise kids vs. father is just a sperm donor), and we’ve evolved mechanisms to determine which one we’re in and act appropriately, at least in some broad statistical sense.  
     
    This makes me wonder what other evolved-in “modes” of physical development and behavior might exist. For example, I think kids raised in really brutal conditions have some common physical and mental changes. I wonder if this is somehow adaptive, helping you live to reproduce in societies where the biggest, strongest guy brutally runs things.

  4. well, there’s another data point in favor of the social phenomenon that easy girls tend to come from families with absent or unloving fathers. the joke amongst my friends is that nothing promises an easy lay on a first date like divorce and arm hair.

  5. Well, is there greater sex dimorphism in African populations with massive polygyny than in more monogamous populations? 
     
    yes. at least that’s what their regression coefficient implies. same with asia. 
     
    they say they got their data from eveleth and tanner the UN, so should be easy to check up….

  6. Well, I don’t suppose the argument applies to gorillas, or any number of other species with that kind of dimorphism. Why suppose it is any different with humans?

  7. Well, I don’t suppose the argument applies to gorillas, or any number of other species with that kind of dimorphism. Why suppose it is any different with humans? 
     
    gorillas have enormous levels of sexual dimorphism. humans different in size by only 1.07. and gorillas don’t really have high levels of cultural variation in polygyny from what i gather. 
     
    one musn’t assume that all variables have equal impact across the animal kingdom-or even among primates. in any case, the paper points to data in variation on sexual dimoprhism between human populations, but such sexual dimorphism is usually an exceedingly slow trait to evolve (for obvious reasons that selection on one sex tends to effect the other).

  8. I have noticed a number of papers and people suggest that individuals are making a conditional response to the environment.  
     
    More than that, they are suggesting that the brains of the individuals are noticing something (unconsciously) and switching on/off genes that affect development. 
     
    It seems to me however, that what could be happening is that there are several phenotypes in a population that have different reproductive success in the different environments they are likely to encounter, but no one phenotype is driven out of the population because none of the others are driven to fixation. 
     
    It seems that it will be hard to distinguish between the two hypotheses. 
     
    On the other hand, primates have been social for a very long time (50 million years?), so it seems possible that conditional responses could have arisen, as they have in other species. 
     
    With respect to male pheromones driving menarche in female offspring, ummm, in the west, many fathers spend a great deal of time with their daughters, and yet, menarche seems to be more related to childhood nutrition.

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