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Context and Variation

Context and Variation

Human behavior, evolutionary medicine… and ladybusiness.

Ladybusiness Anthropologist Throws Up Hands, Concedes Men Are the Reason for Everything Interesting in Human Evolution

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Like most modern anthropologists, I have challenged the idea that human evolution is entirely motivated by men’s desires, interests, behaviors and strategies. But feelings of doubt have nagged at me for years – impostor syndrome, internalized sexism, and just a general feeling of inferiority and small-brainedness. Then, PLoS Computational Biology published a piece by Morton et al (2013) suggesting that men’s preferences for younger women are what drove the evolution of menopause.

Despite citing the overwhelming literature on the topic, these authors situate human menopause as an “evolutionary puzzle.” They then point out that a model that recognizes male mating preferences for younger women, with a splash of infertility-causing mutations, produces the evolution of menopause. What this suggests is that if men choose to mate with younger women, female-specific, infertility-causing mutations that affect later reproduction could build up, since those women aren’t reproducing. Ergo, the uselessness and undesirability of older women – we all know how that is – leads to their becoming infertile and menopausal.

I can’t believe my feminism blinded me to such a raw, important truth. I now realize that, throughout the hominin lineage, the women were just sitting silently to the side, quivering, reproducing when commanded but otherwise riding the coattails of man’s evolution.

In order to understand this revelation, and the contribution Morton et al. are making to reinforce cultural bias the reality of the patriarchy, we need to walk through existing hypotheses of human longevity, menopause, and post-reproductive life. Even if, of course, we then discard them as frivolous lady ideas.

My understanding of menopause, before I realized my grave error

Morton et al. are basing their work, in part, on models that came before them. When trying to understand why humans have menopause, a related question is why we live so long and have these post-reproductive lives at all. Hamilton was the first (1966), and he created a model of human longevity that only used females, since they’re the ones who make the babies and thus set the pace of fertility (what a closet feminazi!). Tuljapurkar et al. (2007) took Hamilton’s old model, and added men to it. They found that the fact that men were able to continue to increase their reproductive success in later life was evidence that older male fertility was at least a partial driver of why we live so long. However, this is not the same thing as saying that men prefer younger women, nor that they drove the evolution of menopause, and the authors are careful to make those distinctions and be clear about social variables that affect mating preferences.

Wimps.

I used to think that part of the reason humans are menopausal may not be because menopause serves a particular purpose, but because we have extended lifespans. We may have extended lifespans not because longevity is selected for, but because longer periods of childhood and social learning were selected for. The thinking goes, if you stretch human lifespan out just at one end, you end up lengthening the whole thing, like silly putty. That said, there is at least one paper that shows practice doesn’t necessarily make people better at skills necessary for survival (Jones and Marlowe 2002).

Female reproduction is also functionally constrained. In order to select really nice eggs that make really nice babies in whom we will want to invest, we have to make all our eggs at once, at about five months gestation. This is a time that we’re protected from all sorts of environmental factors that could make our eggs wonky, and it gives us time to have a few massive culling events so we use our very best eggs for ovulation. The problem is, if you make all your eggs at once, then they are all going to expire at about the same time, putting a limit on how long women are fertile.

Before my revelation that the men are all who matter, I would have also favored the grandmother hypothesis. This hypothesis isn’t mutually exclusive with the others. Originally the grandmother hypothesis contended that post-reproductive life evolved because grandmothers are important to the reproductive success of their offspring (Hawkes 2003; Hawkes et al. 1998; Hawkes et al. 1997). Much of the first research into the grandmother hypothesis happened among the Hadza, where grandmothers do a lot of provisioning and helping. Subsequent work among other modern and historical populations have shown that grandmother presence can have a more variable influence on infant mortality and child health (Jamison et al. 2002; Ragsdale 2004; Sear et al. 2000). In a few studies, paternal grandmothers – so a mother’s mother-in-law – have a negative effect on their grandchildren (Strassmann et al. 2006; Voland and Beise 2002).

Even if human longevity and ovarian expiration dates are the reason for post-reproductive life, still, what you do with that post-reproductive life can have consequences for reproductive success and thus the gene frequencies of future generations. In some populations grandparents may influence the timing of birth: the presence of paternal grandparents increase the likelihood that the mother will give birth, and the presence of maternal grandparents can decrease it (Sear et al. 2003). This could be due to support in the form of provisioning or cooperative breeding… or it could be pressure from in-laws to make babies.

Finally, it might not be that one of these hypotheses is right, but that all of them make a contribution. You need to combine the major hypotheses in order for menopause models to work, and this has been done with theoretical modeling as well as using Sear and Mace’s empirical data from the Gambia (Shanley et al. 2007).

The proponents of these hypotheses would have you believe they are the most consistent with the physiological, primatological, and fossil evidence, but we can dispense with that. I’d rather go with my biases gut, and favor the Playboy Mansion Model.

Putting “men” back in “menopause:” the Playboy Mansion Model

Hugh Hefner and some of his earlier feminine companions. Being a proper male, he has of course traded them in for younger ones.

Hugh Hefner and some of his earlier feminine companions. Being a proper male, he has of course traded them in for younger ones. (Public domain, via Wikimedia Commons.)

There’s “men” in the name, and yet precious few menopause hypotheses put men front and center (well, except for (Marlowe 2000)). Thankfully, Morton et al. are helping to right this wrong.

Like I said before, Morton et al. created a model with strong male mating preferences for younger women, and added in some infertility-causing mutations. What this means is that, as they ran the model, the selection for younger women as mates caused the older women to be less and less important to natural selection. So the infertility-causing mutations could accumulate in older women, eventually leading to menopause. It seems like this would really only work if the model was fashioned after Hugh Hefner, since men would have to be continually trading in for a younger model for the older women to be selected for only rarely. And it would only work if the way something like menopause could evolve is through infertility-causing mutations, as opposed to issues of functional constraint and expiration dates.

Now, I know my silly little mind trails behind the dudely movers and shakers of human evolution, but I am having a bit of trouble with this. The model requires that fertility into old age is part of our ancestral history if menopause is to eventually evolve, yes? Then probably our closest living relatives, like say chimpanzees, don’t have menopause, unless it independently evolved more than once of course.

Wait, you mean there is controversy here, and some papers provide evidence to suggest chimpanzees have menopause, just not the long post-reproductive life spans (e.g., Hawkes et al. 2009)? Well, then surely chimpanzee males prefer young females, just like human males, which is why it evolved in them as well.

Wait, chimpanzee males prefer older females, the ones with established fertility (Muller et al. 2006)? (And this is not uncommon among many other species?) Well, crap.

So either menopause evolved a few times in our lineage, or it was already there near the start (because of the whole physiological inevitability thing). But if it evolved independently in chimps and humans, the reasoning must be totally different for each species, since male chimps don’t prefer young females and so don’t fit the model.

In evolutionary theory, we have this thing we tend to look for, called parsimony. What fits the data best? And it might be my feeble female mind, but I have a hard time reconciling the evidence suggesting chimpanzees have menopause with chimpanzee preferences for older females.

If men like Hugh Hefner did not drive the evolution of menopause, there are still many opportunities for men to insert themselves in human evolution. Here, I’ve thought of a few, to soothe the wounds of those who wanted an evolutionary Playboy Mansion to be a thing.

The complete irrelevance of women to human evolution

Maybe men provisioned women, which allowed them to stay at home – er, at the campsite – so they could care for offspring. That’s a way more important component of cooperative breeding than care among peers or by grandmothers, right? So men are why we were able to have altricial, big brained infants.

Maybe man the hunter is what got humans up on two legs, jogging around in their fancy barefoot sneakers, I mean, their bare feet. They’re the ones who sexily heft a spear to demonstrate our interesting shoulder girdle and capacity to throw. And of course, they must be the only ones who made stone tools.

Maybe the male gaze is the reason we ladies have breasts and butts that look the way we do. You know we wouldn’t carry all this around if we didn’t have to. And let’s not forget, female pleasure is really just a byproduct of the male drive to reproduce: the clitoris is just a woman’s adorably ineffective attempt at a penis.

So really, even if men had little to do with menopause, they get… well, they get pretty much everything else, so long as you whiz by several decades of egalitarian anthropology research. Which is fun, like roller skating!

And girls just want to have fun!

Acknowledgements

Thanks to Bastard Colleague from Hell – er, Charles Roseman – for reading an earlier draft of this post and providing comments.

References

Hamilton WD. 1966. The moulding of senescence by natural selection. J Theor Biol 12:12-45.

Hawkes K. 2003. Grandmothers and the evolution of human longevity. American Journal of Human Biology 15(3):380-400.

Hawkes K, O'Connell JF, Blurton-Jones NG, Alvarez H, and Charnov EL. 1998. Grandmothering, menopause, and the evolution of human life histories. Proceedings of the National Academy of Sciences, USA 95:1336-1339.

Hawkes K, O'Connell JF, and Blurton Jones NG. 1997. Hadza women's time allocation, offspring provisioning, and the evolution of long postmenopausal life spans. Current Anthropology 38(4):551-577.

Hawkes K, Smith KR, and Robson SL. 2009. Mortality and fertility rates in humans and chimpanzees: How within-species variation complicates cross-species comparisons. American Journal of Human Biology 21(4):578-586.

Jamison CS, Cornell LL, Jamison PL, and Nakazato H. 2002. Are all grandmothers equal? A review and a preliminary test of the "grandmother hypothesis" in Tokugawa Japan. American Journal of Physical Anthropology 119(1):67-76.

Jones NB, and Marlowe FW. 2002. Selection for delayed maturity - Does it take 20 years to learn to hunt and gather? Human Nature-an Interdisciplinary Biosocial Perspective 13(2):199-238.

Marlowe F. 2000. The patriarch hypothesis - An alternative explanation of menopause. Human Nature-an Interdisciplinary Biosocial Perspective 11(1):27-42.

Morton RA, Stone JR, and Singh RS. 2013. Mate Choice and the Origin of Menopause. PLoS Comput Biol 9(6):e1003092.

Muller MN, Thompson ME, and Wrangham RW. 2006. Male chimpanzees prefer mating with old females. Current Biology 16(22):2234-2238.

Ragsdale G. 2004. Grandmothering in Cambridgeshire, 1770-1861. Human Nature-an Interdisciplinary Biosocial Perspective 15(3):301-317.

Sear R, Mace R, and McGregor IA. 2000. Maternal grandmothers improve nutritional status and survival of children in rural Gambia. Proceedings of the Royal Society of London Series B-Biological Sciences 267(1453):1641-1647.

Sear R, Mace R, and McGregor IA. 2003. The effects of kin on female fertility in rural Gambia. Evolution and Human Behavior 24(1):25-42.

Shanley DP, Sear R, Mace R, and Kirkwood TBL. 2007. Testing evolutionary theories of menopause. Proceedings of the Royal Society B: Biological Sciences 274(1628):2943-2949.

Strassmann BI, Hug BF, and Welch K. 2006. A new twist on the grandmother hypothesis: adverse impact of paternal grandmothers on Dogon grandsons. American Journal of Human Biology 18(2):275-276.

Tuljapurkar SD, Puleston CO, and Gurven MD. 2007. Why Men Matter: Mating Patterns Drive Evolution of Human Lifespan. PLoS One 2(8):e785.

Voland E, and Beise J. 2002. Opposite effects of maternal and paternal grandmothers on infant survival in historical Krummhorn. Behavioral Ecology and Sociobiology 52(6):435-443.

The views expressed are those of the author and are not necessarily those of Scientific American.

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