Among mammals a larger proportion of females than males reproduce, the extent of the imbalance signalled by gender differences in size. Elephant seal males are three times as massive as females, while gibbons are characterised by physical equality. The former play winner-take-all, amassing huge harems. Exclusive possession requires violence to enforce, a reason for the shorter life-expectancy of elephant seal males. In contrast the gibbon is a monogamist, entering into a cooperative pair bond to defend shared territory and raise offspring.
Evolution's logic by which the future belongs to the fecund is operative in both cases, but there's more than one way to skin the cat. Obviously the size difference in our own species is modest, so some anthropologists may emphasise pair bonds while others argue for a more fluid serial monogamy, but in both cases the presumed evolutionary norm is not extreme polygamy.
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