In The Evolution of Beauty, Yale ornithologist Richard Prum elaborates on Darwins theory of the effect of sexual selection on evolution. Beyond survival of the fittest, the sexes have asymmetric interests. Males, with their cheap sperm, seek to sire as many offspring as possible. Females with their expensive eggs and limited lifetime reproductive opportunity, seek to pick the best mates. Males compete with one another for control of females. Females seek to avoid male control and to choose their mates freely. In many species, male competition results in bigger, stronger, and more weaponized males, as in huge sea lion males with long tusks. Prum focuses on female choice.

Female choice, given free rein, can lead to arbitrary standards of beauty and behavior in a species. Among neotropical manakins, females do all the work of raising chicks while males contribute only sperm. Males dance, sing, and flash their colors on communal display grounds known as leks; the females arrive, watch, pick a male for a quickie, and leave. The females favor only a few of the males; the rest may never get to mate. Blue manakins have even evolved a cooperative dance among a group of five or six males; females choose between groups of dancers, mating with the alpha male.

Prum moves from birds to humans. Humans, he points out, are far more cooperative than our African ape relatives, gorillas, chimpanzees and bonobos. Men and women dont differ as dramatically in size as male and female apes. Unlike apes, humans tend to monogamy, he says, because females need help raising the kids. Prum also cites surveys showing that women do not prefer big, square-jawed macho males; rather, they go for men with moderate physiques and gentle behavior. Prum goes on from here to many interesting observations on possible effects of female choice, such as why do men, unlike apes, have long, dangling penises?

Yet in offering a generalized account of human behavior, Prum misses a human society that supports the female choice theory especially well. That society is the Hadza, as described in Nicholas Blurton-Jones new book: Demography and Evolutionary Ecology of Hadza Hunter-Gatherers (2016).

The Hadza are an ancient hunter-gatherer tribe living in northern Tanzania near Lake Eyasi. Traces of their culture in the area date back at least 130,000 years. The area is too dry for agriculture and the tsetse fly makes it unsuitable for livestock. But theres an abundance of seeds, nuts, berries, honey, and especially, underground tubers. The Hadza live in small groups, moving every few weeks depending on seasonal availability of foods. While all other group-living animals, including apes, consist of close kin, Hadza groups are quite fluid, with unrelated individuals continually coming and going. Like all hunter-gatherers, the Hadza are extremely egalitarian and cooperative.

Hadza men spend their days hunting with poison arrows. But they dont hunt the small game they learned to capture as boys. Rather, they hunt for big game, like baboons, antelope, zebra, or buffalowhich they very rarely catch. Some men never catch anything. But when a man does nail a big animal, the meat is equally shared among the whole group, gaining him prestige. One anthropologist has called this a show-off strategy.

Hadza women do almost all the work, including caring for children and gathering and preparing food. They get little contribution from their husbandsmaybe an occasional piece of honeycomb or a small bird, which the men expect their wives to prepare. In compensation, however, its the women who chose their husbands (often for only a few years). What sort of men do Hadza women prefer? Successful huntersnot good providers!

When the men are not hunting, they sit around in the mens place chatting, smoking, eating tubers prepared by their wives, and fiddling with their bows and arrows. Theres almost no violence among the men. Disputes are resolved by long discussions, or at the worst, one of the men will leave and join another group. If you look at pictures of Hadza, both sexes are small, thin and wiryno great differences in size or appearance. Both sexes go for bead necklaces.

Like the blue manakins, the Hadza seem to fit Prums model of extreme female choice. The women dont depend on their husbands for much besides sperm. Theyre free to choose the show-off hunters, who sire more children, but may actually contribute less to their childrens nutrition. Judging by the peacefulness of the men, female choice seems to have tamed male-male competition.

While all hunter-gatherer societies are highly egalitarian, not all allow as much freedom to women. In the Amazon rain forest, Ache men supply some 80% of the food by hunting. These men may ritually sacrifice children over womens objections, and engage in lethal quarrels. Hadza women seem to derive their independence from the terrain, where it takes no more than a sharp digging stick and knife, a leather sling and water gourd, plus long hours working in the hot sun, for women to fully provision themselves and their childrenand grandchildren. Another unrelated African hunter-gatherer society, the !Kung, lead a very similar life.

The latest evidence from Africa shows hominids manufactured flint tools as long as 3.3 million years ago. Once there were stone knives, female hominids must have used slings to carry themalong with food and infants. A Hadza life style could date back millions of years. Anthropologist Sarah Blaffer Hrdy, in Mothers and Others: The Evolutionary Origins of Mutual Understanding (2009), attributes human cooperativeness to womens shared mothering of childrena trait quite absent in apes. She draws examples from the Hadza. Blaffer Hrdys female cooperativeness together with Prums female preference for cooperative males might explain the evolution of the most cooperative species on earth: humans.

In Aristophanes comedy, Lysistrata (411 BCE), Lysistrata persuades all the women of Athens and Sparta to withhold sex until their men agree to end the long-running Peloponnesian war. Was Aristophanes onto something?

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Part II Beauty, Cooperation, and the Hadza Hunter-Gatherers - HuffPost