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Classifying origin and purpose of secondary sex characteristics in humans - Analysis and theory


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Many human secondary sex characteristics are seemingly clear and obvious in their benefits to the organism, e.g. greater body size in males would give an advantage in intra-male competition and the performance of physical labor, the major downside being greater caloric expenditure. However humans have many secondary sex characteristics that don't seem to increase the organism's chances of survival. I'll briefly list these "unnecessary" secondary sex characteristics that I've thought of, and hypothesize how those traits became prevalent in humans.

Neoteny in females: Men probably over-selected for neoteny since youth was associated with fertility. What is curious is that the male preference for neoteny seems to have beaten out the female preference for skull masculinization, since the modern human skull is more neotenous than the discovered skulls of our ancestors. This is consistent with the observation that males historically had more control over sexual selection due to their greater physical strength and possession of resources.

Larger than necessary breasts in females: Women's breasts are larger in comparison to their bodies than in other mammals. This is most likely due to sexual selection.

Hair patterning in males: There are a number of areas on the male body where hair is found in certain consistent patterns. I'll enumerate these areas below.

  1. Male pattern hair loss: Certainly not sexually selected for today, but it may have been in the past. May have conferred some advantage in competition with other males if male pattern hair loss was associated with seniority and correspondingly greater status.
  2. Facial hair: Growth patterns seem to have little variation, with growth on the bottom of the cheeks, below the chin, and above the lips. The consistency in the growth pattern suggests that sexual selection played in a major role in producing specific growth patterns, since selection through competition with other males would be unlikely to produce such consistency. May have conferred some advantage in competition with other males if facial hair was associated with greater seniority and status.
  3. Torso hair patterning: Another curious trait for its fairly consistent patterns across the world, with hair on the chest, a strip of hair leading from the center of the chest down towards and past the navel, connecting with hair in the pubic region. The consistency of the pattern, and the low likelihood of its relevance in intra-male competition, suggests that it is strongly sexually selected.

Skull masculinization in males: Certainly sexually selected for today, and was probably sexually selected for in the past as well. May have also conferred advantages in social competition with other males.

Voice deepening in males: Likely to have been both sexually selected and beneficial in competition with other males. Research has shown that men with deeper voices are preferred by women, and are perceived as more authoritative by other men.

Larger than necessary penis in males: Almost certainly due to sexual selection, it is easy to imagine that men with larger penises would have more repeated sexual encounters.

Forearm vascularity in males(?): Especially on the underside of the forearm males have many veins near the surface of the skin. The placement of these veins on the underside of the forearm where they face towards and are thus guarded by the torso is consistent with the observation that veins near the surface of the skin are at risk of laceration, and therefore they should not be placed in areas which are more vulnerable such as the outside of the forearm. Common knowledge dictates that some vascularity is attractive to women, and that veins would be safer if they were farther away from the surface of the skin. This suggests that forearm vascularity may have been sexually selected in males, but it is questionable.

There may be a greater number of these "unnecessary" secondary sex characteristics (USSC) than what I have listed above, but I could not think of them. The first question I had is why do males seem to have a greater number of USSC's than females? I answered this question by assuming that every female that is capable and wanting to reproduce does, and therefore there is no way that females who lack a certain USSC could be completely prevented from passing on their traits; meanwhile we must assume that not every male who is capable and willing to pass on his traits does.

My second question is why would females select for USSC's in the first place? I cannot believe that the behavior of any organism is random and purposeless. If we think hard enough we should be able to identify rational explanations for their behavior which increase the fitness of the species. A good place to start may be the case study of peacocks. Male peacocks have large colorful tails, a great example of a USSC. Ronald Fisher's sexy sons hypothesis already answers the question of why it is the best interest of a female to choose the male with the largest and most dazzling tail, but it doesn't explain why the females begin selecting for a USSC, or why they choose a specific trait.

Females have a difficult problem to solve. They must quickly assess the fitness of males using credentials that cannot easily be spoofed by the "lesser" males. I theorize that solving this difficult problem is the purpose of selecting males based on USSC's. The most attractive females ostensibly have the best genetics, and if they choose the males with the most prominently displayed USSC's then the expression of those USSC's will become genuinely correlated with the expression of the best genetics. Now the females have solved their problem; they can quickly assess the genetic quality of each male at a glance by assuming that males who express USSC's most prominently have the best genetics in general. The initial choice of the USSC trait that will be used as a marker for genetic quality may be due to a sort of fad among the females, so it would effectively be random, the only strict requirement being that the trait must not be excessively detrimental to the organism's chances of survival. From that point onward the sexy sons hypothesis can explain the rest.

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