This is the fourth part on the evolution of human mating behavior, comparing evidence for promiscuity and pair-bonding in our species. Please see the introduction here.

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We left off with a list of eight traits in humans suggesting promiscuity in humans. Admittedly, the previous post was a little thick, as it dealt with imprinted genes and population genetics. The current one concerns human reproductive physiological and anatomical traits consistent with a multiple-partner mating structure, building on a couple of points addressed by Ryan and Jethá in their book. If you’re paying attention, that’s three posts concerning promiscuity and one (yet-to-be-written) on pair-bonding. Perhaps it seems I’m stacking the deck, but please reserve judgment. One reason more space is needed to make the case for the evolution of promiscuity is that the biology is less well known, and more effort is needed to bring it into the light. That single post on pair-bonding will be an important one, and quality matters just as much as quantity.

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Continuing on with our list of traits hinting at promiscuity…

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9. Sexual dimorphism in body size. This point remains somewhat contentious. In the majority of anthropoid species (monkeys, apes, and humans), males are the larger sex, with the degree of dimorphism ranging from slight to extreme (Plavcan 2001). This pattern correlates strongly with mating structure and male-male competition (Plavcan and van Schaik 1997). For monogamous species like gibbons, males and females tend to be roughly the same size. In species where females prefer larger males or where males compete for access to females, bigger males will leave behind more descendants. This is true for polygynous gorillas and dispersed, territorial orangutans, where males are physically about twice as large as females. A good non-primate example is elephant seals. On the other hand are horseshoe crabs, where smaller males cling to the backs of larger females and wait for the release of her eggs. This ‘reverse dimorphism’ is found in a few primates, but is slight and only in some prosimians such as lorises and lemurs.

Former sumo wrestler Konishiki and his wife Chie Iijima, an obviously cherry-picked example of extreme dimorphism. (From smh.com.au).

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