Evolutionary advantage of monogamy

[shunyadragon]

[klaus54]

Isn't it true that monogamy is more common with a lower degree of sexual dimorphism?

K54

[phank]

Although I'm too lazy to look for it, I remember reading a study of sperm among various species, not just primates. Now as it happens, relatively few sperm attempt to swim to reach the egg. The majority are built differently - they have enormously long tails, not very good for swimming but quite good for interlocking with other sperm of the same variety, to create a form of blockage. This plug then inhibits subsequent mates' sperm from even getting started.

What's interesting is, there's a strong correlation between the percentage of blocking sperm, and the propensity for monogamy in the species. Fully mate-for-life monogamous species have few to none of these blocking sperm, while fully promiscuous species produce almost 100% blockers, with only a few intended for fertilization. In other words, in polygamous species defense is where the resources are spent. Humans fell about midway in terms of percentage of blockers, indicating a moderate amount of monogamy. And in fact that's what we see in practice. Kind of fascinating that this degree of monogamy has evolved into us to the point where we can examine sperm tails and discover our propensities reflected at a very low biological level.

[phank]

Isn't it true that monogamy is more common with a lower degree of sexual dimorphism?

K54

This is a proposal from the field of sexual selection. The idea is, in polygamous species there is much more intense competition among the males for access to females, and the winning males in many species get the whole harem (except for the backdoor individuals, but that's a different discussion). This competition might take the form of physical combat, where larger and stronger males win the females, which means the males eventually become much larger than the females. Or it might take the form of more elaborate display, where the males become much more ornate and colorful than the females. But the implication is that sexual dimorphism arises from polygamous competition.

I'm personally not convinced. There's too much "just-so story" to this approach, and I'd prefer an actual field study examining the tendency to monogamy compared with sexual dimorphism, according to some operational definition of each. I'm somewhat familiar with birds, and I don't think there's any more polymorphism among blue jays (for example) where the sexes look identical, and redwing blackbirds (for example) where the sexes look completely different.

[Epoetker]

Why doesn't the thread title read "Evolutionary advantage of monogamy enforcement"?

[phank]

I think the thread title should read "advantages of monogamy in the evolution of human societies". They don't seem to be talking about biological evolution.