what I do for fun

last night I was up until 1 am writing an essay for a professor.
Don’t worry, it wasn’t a required essay. It was just for fun, stemming from a discussion in class. Yes, this is telling, I had plenty of homework, NaNoWriMo, chores…and I went of on a wild tangent. Educational as it was, it was not required. The only benefit was from me learning stuff. I thought I was cool & smart & stuff…

Anyway, I’m curious, how can I make it better?

The professor’s response is in the below. Sources in extended entry

Why do humans have so little hair?
The other day in class our instructor brought up the idea that humans may have lost most of their body hair due to female choice. I asked, “Historically speaking, how much choice did females have?” From the class reaction, for a moment a felt a bit like Larry Summers, (who infamously suggested that it might be innate ability or innate preference that leads to more men than women in science).
Our professor quickly pointed out that in many bird species it is the female that chooses the showy male bird. I countered with elk, and the big bull gets the harem. We quickly agreed to limit the discussion of female choice to primates.
The professor said that it seems to be in primates, the more closely the males and females are in size the less likely it is for the male to have a harem. Judson agrees, pointing out that “In monogamous species, males and females tend to be roughly the same size…human males tend to be only slightly bigger than females, and there is considerable overlap, with some women bigger than some men” (Judson, pg. 162). Next our professor, and also Judson, pointed out the correlation between relative testicle size and monogamy, “

Next, consider physical differences between human males and other male apes. As you know, testicle size is usually associated with the risk of sperm competition. Males that are at low risk of sperm competition – either because they are good at defending their harem or because they are paired with a faithful female – generally have testicles that are small in relation to their body size. Males that are at high risk of sperm competition-either because they pursue a strategy of seducing the partners of other males or because most females mate promiscuously-generally have enormous testicles in relation to their body size… [gorillas having small testes, while chimpanzees having large testes]…human males have medium-sized testicles, suggesting a low to moderate risk of sperm competition. This fact, plus the relatively small size difference between men and women, is exactly what would be predicted for a mostly monogamous species.” (Judson pg. 163)

In an attempt to clarify my question, and the reasons for asking it, I present the following.

Through out human history there have been wars, and many times the women were treated as simply spoils of war. Using the Bible a historical social commentary, when the Isrealites attacked a city, if it did not surrender, when it fell “the women, the children, the livestock and everything else in the city, you may take these as plunder for yourselves” (Deut 20:10 – 14). This tradition was continued in medieval Europe, and “the women of a besieged town taken by storm might be raped” (Morrow, pg 378).
The Japanese soldiers raped thousands of Chinese women during the occupation of Nanking during World War Two (Morrow, pg 378)
This happened in the early 1990s in Europe, during the wars following the breakup of Yugoslavia. “Moslem and Croat women from many parts of Bosnia were herded into schools and warehouses and raped repeatedly….kept in these camps for several weeks….Those able to bear children were almost all impregnated” (Glenny, pg 209). It is currently happening in Dafur (the Economist, 13 Oct 2005, 9 Feb 06).
Anthropologist Napoleon Changnon, who was one of the first to study the Yąnomamö people of the Amazon noted that they “regard fights over women as the primary causes of their wars…Although few raids are initiated solely with the intention of capturing women, this is always a desired side benefit” (Changon pg 123).

The above examples show that through out history, especially during the case of a fight between two groups of men, the winners get to keep the females. Which shows why I used the analogy of the bull elk and his harem. Although, the above examples show that a group of men can capture a group of women, possibly for procreation. When captured during a fight between two groups of men, which happens more than we like to admit, we can assume that women do not have the option of choosing the less hairy rapist.

Is it humans that are the only species that fights for females? Doubtful, because for “males in many species, females are the limiting resource for reproductive success” (Wilson, pg 377). Bands of chimpanzees have been shown to attack other groups of chimpanzees and the “results indicate that males are largely successful in defending their females from outside males” (Wilson, pg 377).

But are there times, perhaps when there is peace and plenty and the female has a choice between suitors? Certainly, it is one of the hallmarks of civilization. But I am more concerned with our evolutionary history, and that will have to deal with time before civilization. Perhaps there were no wars, because there were not enough people to form tribes.

Once again to the Bible there is the story of Ruth, who actually chose who she wanted to marry. The Bible does not mention if he was hairy, only that he was a land owner.

It is true that men and women “discriminate on the desirability of potential mates, partly on the basis of physical qualities” (Gangestad, pg 531). But evolutionary psychologist Geoffrey Miller says “Looks are important, but only for the first five minutes’. …Miller’s studies suggest that a man’s sense of humor, and his musical or artistic ability – traits that typically do not bring material advantages – seem to be valued by women as indicators of high intelligence and creativity.” (Anderson, pg 33).

A better indicator for mate preference than looks might be smell. In the classic smelly t-shirt experiment, men and women wore t-shirt to bed, and the next day people were asked to pick the t-shirt that smelled the best (Anderson pg 32). Consistently people “prefer the smells of those whose genes at [the MHC] complex are different from their own” (Judson, pg 52). The major histo-compatibility complex (MHC) is “most variable region of the human genome and plays an important role in immune function” (Anderson, pg 32). This area seems to give each person a unique odor, and “spontaneous abortions are more likely when couples match at particular MHC genes” (Judson, pg 52).

So why is there a lack of hair on humans compared to other primates?
“A recently mooted hypothesis that human hairlessness evolved late in human evolution as a result of the adoption of clothing and the need to reduce the load of external parasites finds no support in light of the overwhelming evidence of the importance of hairlessness in thermal sweating and whole-body cooling in maintaining stable core temperature and homeostasis.” (Jablonski, pg 599).

(The Plague showed that external parasites could prove harmful to clothed modern humans.)
Of the primates, humans have the highest ratio of eccrine sweat glands to apocrine sweat glands (Hanna, pg 272). The eccrine sweat glands are used for thermoregulation, secreting mostly water (Martini, pg 168). The apocrine sweat glands produce an oily, potentially odorous, secretion (Martini, pg 167). Maintaining a homeostatic body temperature is important for creatures that depend so much upon their brains. Less body hair helps thermal regulation in a hot environment “because it increases thermal conductance and permits additional heat loss through sweating” (Jablonski, pg 598-599). Studies have shown that “humans …the highest capacity for heat loss through evaporation of perspiration exceeding that of monkeys [macaques and baboons] by a significant amount…. on the basis of the anatomical distribution of eccrine glands, these primates [great apes] also seem inferior to humans in sweat capacity” (Hanna, pg 260.)

I do openly admit that many times it is the female that chooses the male, especially in our current society. But when society is lacking, since on average “it is undisputed that men have greater physical strength than women” (Quinn pg186), there are many cases when females do not have the option of picking the less hairy man. The better explanation is that humans are less hair so they can sweat to beat the heat.

But GBatT,

How come other apes and chimpanzees, which evolved in equally hot, humid environments have also not become somewhat hairless? Why haven’t ALL species in hot humid environments evolved hairlessness?????

Furthermore, you are not going back far enough in time to surmize a hypothesis. The examples you quote for human male bravado over female ‘weakness’ were from historical dates of maybe 2-3,000 years ago. Humans evolved from chimps and apes over 5 million years ago!!!! None of your references adequately address this discrepancy. Finally, I hate to be a stickler, but none of your references are experts in evolutionary biology…I am not swayed.



1. Anderson, Nick “The Reduction of Seduction” the Scientist Sept 2006, pg 28-33

2. Judson, Olivia Dr. Tatian’s Sex Advice to All Creation, 2002, Metropolitan Books, New York

3. Morrow, Lance “Rape” The Reader’s Companion to Military History Robert Crowley & Geoffrey Parker editors, 1996, Houghton Mifflin Company, New York

4. Glenny, Misha The Fall of Yugoslavia: the Third Balkan War, 1993, Penguin Books, New York

5. Chagnon, Napoleon A. Yąnomamö the Fierce People, 1968 Holt, Rinehart & Winston, New York

6. The Bible, New International Version,

7. The Economist “Darfur’s Despair” Oct 13, 2005 print edition

8. The Economist “Bring on the Blue Helmets” Feb 9, 2006 print ed

9. Quinn, N. “Anthropological Studies on Women’s Status” Annual Review of Anthropology 1977, 6:181-225

10. Wilson, ML, Wrangham, RW “Intergroup relations in chimapanzees” Annual Review of Anthropology Oct 2003, 32: 363-392

11. Jablonski, Nina G. “The Evolution of Human Skin and Skin Color” Annual Review of Anthropology Oct 2004, 33:585-623

12. Gangestad, SW & Scheyd, GJ “The Evolution of Human Physical Attractiveness” Annual Review of Anthropology, 2005, 34:523-548

13. Hanna JM, Brown DE “Human Heat Tolerance: An Anthropological Perspective” Annual Review of Anthropology 1983, 12:259-284

14. Martini, Frederic Fundamentals of Anatomy & Physiology, 7th edition, 2006, Pearson Benjamin Cummings, San Francisco


One Response to “what I do for fun”

  1. BJ Says:

    Ever hear of the Aquatic Ape Theory? 🙂 As I recall, it basically says our ancestors tried to pull a whale-move, literally tested the waters long enough to lose our hair, then decided that sucked & moved out to the savannas — sans hair. The first major problem with that theory (dismissed by all “serious” scientists) is that while actual whales have evolved away from hair, many other aquatic mammals have done just the opposite (e.g., beavers, sea otters, fur seals). Also, the hair loss route is associated with a major weight gain in the form of blubber, and streamlining (whales are basically fish/torpedo shaped) for a dedicated aquatic lifestyle. And none of the really fat people I’ve ever seen strike me as proficient swimmers. 🙂

    But I must say, dude, that’s a lot of work & research you did for a non-required essay. …Good job! 🙂

    1. Chagnon was my into to Anthro prof at UCSB — wish I’d paid more attention (but there were like 500? 700? students in each class session!)
    2. Thanks for reminding me that I have “Dr. Tatian’s Sex Advice to All Creation” — I should really read it eventually.

    OK, so seriously, this is an interesting question. It also begs the question, why did we keep the hair we did? (Which I’ll get to later.)
    I agree with your prof in that your “brutal male” examples are all too recent, though I understand why you used them. The flaw, which you acknowledged, is that humans certainly didn’t live in extensive, highly-organized, large-population societies when this trend towards hairlessness evolved. Even the Yanomamo are far too civilized & ritualized to help us make sense of this adaptation. And they, like most smaller, tribal societies, raid for women in order to mix up the gene pool — all that MHC stuff! It’s more like wife-swapping than warfare. But in our “modern” world, during lawless situations (true wars), this good adaptation is taken to the extreme & becomes overtly brutal. Evolution proceeds much more slowly than technology! And yet, since it appears to work in spreading your genes, perhaps war, pillaging & organized rape are here to stay — despite how we may feel about it during more sober times.

    However, I’m not very persuaded by the female choice argument either. To me, female choice tends to push males into looking or behaving in counter-adaptive ways (i.e., stupid show-offs), while the females remain safely inconspicuous. The females go for the freakiest male who, despite this obvious self-handicapping, is able to overcome it & still survive. (This is also a great argument for the emergence of true altruism.) An example might be peacocks whose display tails get so heavy, they dramatically increase the risk of being caught & eaten by a tiger, etc. Meanwhile, peahens stay safely drab & small-tailed. And when mating season is over, males drop their huge tail feathers for smaller, safer ones. But in every example I can think of, female choice always pushes males to do/be something “stupid” so as to prove his worthiness. Yet, in our human example, females have even *less* hair than males. Did your prof offer any reason why females would favor less-hairy males? Otherwise, I don’t buy it.
    In fact, I’m inclined to argue just the opposite. Maybe hairlessness is due to *male* choice! After all, us guys love looking at those titties! 🙂 The question of why women have full breasts, even when not breast-feeding is another (puzzling) oddity reserved solely for the human primate. I’d say these go hand-in-hand. I’m not a fan of Richard Leakey’s “breasts as a visual substitute for a rump we lost by standing up” theory, but it’s in the right direction. Breasts, as with facial hair & body hair, may just be an indicator of sexual maturity. So, this distinction would be even clearer in hairless females. Can you imagine trying to check-out the knockers on a Sasquatch?! Of course, breast size (at least today) is *highly* variable in individuals, but as a population, it’s a remarkably recognizable trait of sexually-capable females. That men have also lost much of their body hair may simply be a limitation of our biology — then again, body hair amounts can be very different between different races.

    This also brings us back to why we have kept so much hair on our head. As a natural hat? OK, probably — but then we might expect big hairy shoulder-pads, too. And more perplexing is why would it grow so extremely long? The hairs on my arms & legs are only an inch or so long. But, as both you & I personally know, humans can grow their head hair one or two-feet long or more! Why? I’d suggest that it’s like a lion’s mane (very interesting studies done with lions!) — it’s a true indicator of general health.
    Hair is protein: a very costly resource. So a full head of long, thick, rich hair indicates you’re meeting your basic nutritional requirements with lots of extra to spare (or “waste”) for hair growth. Same with full breasts in women, right? If you’re starving, I think breasts are the first thing you lose. Perky breasts say, “Look at me, I’ve proved my ability to get enough food, so I’m ready to meet the needs of any future offspring — you know you want me!” Same goes for head hair. In both men & women, it says, “I’m personally healthy & able to provide.” And baldness?, you ask. Well, like Huntington’s disease, it occurs too late in life (or at a rate too low) to dramatically affect reproductive fitness, so it remains in the population. [This “low-rate” argument is also used sometimes as an explanation of why true homosexuality lingers in our species, assuming a genetic basis. It’s clearly maladaptive, but it occurs so rarely that there’s insufficient pressure to eliminate to alleles, etc.]
    Bottom line: a hairless female body (torso) provides much more information regarding reproductive fitness (health & maturity).

    Of course, hairlessness could also be the result of a bottleneck. Perhaps there was a temporary, dramatic infestation of hair parasites & our ancestral population was cut down, leaving only the “naked apes” to reproduce. Afterwards, it has nothing to do with anything, it’s just what we got stuck with.
    Or perhaps it simply began as a subtle, but distinguishable feature during mate selection. A little genetic drift, a little random choice… There’s nothing really adaptive about it — it’s only use is as a mark to identify your group & therefore who to have sex with. And before you know what’s even happening, the population has diverged based on this feature and gets stuck in a feedback loop. So maybe a “less hair” directive just kept our ancestors from wasting their time trying to screw “chimpanzees.” 🙂

    One other thing regarding your strong, brutal male idea… You assume that men used their strength to dominate women (as seen in your modern examples), but I might argue that men originally used their strength to fend off other men. Whether he’s mate-guarding his harem or defending an attractive territory, a bull elk’s aggression is directed mostly towards other males. This is certainly the case with sea lions (Otariidae) — they defend the best part of the haul-out beach. The sexual dimorphism between females and the gigantically larger males isn’t so that the males can rape the females (you don’t need to be seven times bigger for that), it’s so males can intimidate & fight off other huge males. Neither elk nor sea lions are corralling or collecting reluctant females — the females choose to remain there. I could imagine the same thing with a band of proto-humans. Females could have lots of choice & power (could even be matriarchal), while men grow larger simply to protect & defend (not dominate) them.

    An excellent question! Glad you asked me. What do you think of my response & ideas?

    PS: Should I post this on your blog, too? 🙂

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