One possible problem is that runaway sexual selection demands polygyny—a mating pattern in which some males mate with two or more females. For runaway to work, some males must prove so attractive that they can copulate with several females to produce several sets of offspring. The least attractive males, as a rule, must be left single, heartbroken, and childless. Sexual competition must be almost a winner-take-all contest. In elephant seals, for example, one dominant male may account for over 80 percent of all copulations with females on a particular beach, and almost as high a proportion of all offspring. (Polygyny does not mean that every male gets to father the offspring of many females-
that would be a mathematical impossibility, given an equal sex ratio. It means rather that a few males mate often and produce many offspring, and most males mate rarely, producing very few offspring.)
If our ancestors were perfectly monogamous, runaway sexual selection could not have favored large brains, or creative intelligence, or anything else. Runaway would never have started. A crucial question is how polygynous our ancestors were. The more polygynous they were, the more potent runaway sexual selection could have been. The modern understanding of human evolution suggests that our ancestors were moderately polygynous—neither as polygynous as elephant seals, gorillas, or peacocks, nor as perfectly monogamous as albatrosses. The evidence comes from many sources, but I shall mention just two: body size differences and anthropological records. Across primates, species where males are much larger than females tend to be highly polygynous. This is because males compete more intensely and violently in more polygynous species where the stakes are higher, and this competition drives up their relative size and strength. Generally, the larger the sex difference in body size, the more polygynous the species. In humans, the average male is about 10 percent taller, 20 percent heavier, 50 percent stronger in the upper body muscles, and 100 percent stronger in the hand's grip strength than the average female. By primate standards, that is a moderate sex difference in body size, implying a moderate degree of polygyny.
Other evidence of polygyny comes from anthropological studies of human cultures and human history. Most human cultures have been overtly polygynous. In hunter-gatherer cultures the men who are the most charming, the most respected, the most intelligent, and the best hunters tend to attract more than their fair share of female sexual attention. They may have two or three times as many offspring as their less attractive competitors. In pastoral cultures the men who have the largest herds of animals attract the most women. In agricultural societies the men who have the most land, wealth, and military power attract the most women. Before the middle ages, in urban civilizations with high population densities, the men at the top of the hierarchy almost always had harems of hundreds of women producing hundreds of babies. The first emperor of China reputedly had a harem of five thousand. King Moulay Ismail of Morocco reputedly produced over six hundred sons by his harem. In European Christian societies from the medieval era onwards, monogamous marriage became the religious and legal norm, though powerful men still tended to attract many mistresses and to re-marry more quickly if their first wife died. For example, anthropologist Laura Betzig showed that throughout American history, presidents tended to mate more polygynously than men of lower political status. (This may be little consolation to politicians of mediocre musical ability, since popular male musicians such as Bob Marley and Mick Jagger allegedly behaved even more polygynously than presidents.)
Those of us brought up in European-derived cultures tend to think of humans as monogamous, but in fact mating in our species has almost always been moderately polygynous. For millions of years, there was enough variation in male reproductive success to potentially drive runaway sexual selection during human evolution.
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