I can't comment on the statistics because I haven't read the actual study. That was just an article based off of the study - it would have been nice if they gave some more specific statistics. What do you mean about this fitting with primate behaviour in general?
AFAIK, many non-human primate communities include a male that mates with several females, whereas the females typically mate with only the dominant male. I'm sure that's a simplification, but my general impression is that primate males are more promiscuous by nature. As a result, it would only make sense that human males are relatively more inclined to seek sex when it's a possibility.What do you mean about this fitting with primate behaviour in general?
Actually, there really is no order-specific generalization you can make regarding primate social-sexual systems. The social-sexual system you described would most closely resemble gorilla behaviour. Male gorillas compete for harems of females. One or two gorillas usually outcompete all other males for the complete control of several females for as long as they can. However, the variation among all primates is truly overwhelming. For example, chimpanzees organize themselves in multi-male, multi-female systems. Although alpha males or top ranking males will generally have more mating opportunity and generally sire more offspring, females have sex with all the males to increase paternity confusion (decreasing the chance of infanticide). Bonobos on the other hand are famously matriarchal. Females control sex and use it to dominate males. As a result every female will usually have sex with most other males and even many other females. Two more interesting examples: Gibbons are lesser apes and they are almost completely monogamous. They find one partner and usually mate for life. There is some evidence of external copulations - 'cheating' - but they are rare and the other partner is generally unaware of these external copulations. Ring-tailed lemurs are also female dominant. They have also have very strict breeding seasons. During a very brief 2 day window all the males will compete for access to females - but it is the female who will make the final choice of who to mate with and for how long. An interesting evolutionary rule to make sense of all this sexual primate mayhem is the general principle that social-sexual system can be dictated by sexual dimorphism. Sexual dismorphism is the size difference between the sexes. In species with extreme dimorphism (like gorillas) a few males usually dominate and control the reproduction of all females (this is true for other mammalian orders as well). However, when there is no sexual dimorphism (like gibbons) males and females tend to be monogamous. When there is sexual dimorphism - but it is not pronounced (like chimpanzees and bonobos) there is some other interesting variant of the two extremes. However, it is still unknown how ring-tailed lemurs and bonobo evolved female dominant social-sexual systems.
Thanks for that. I learned a lot in that comment. This is one of reasons why I created Hubski. Had I just read the article by myself, I would have walked away with the impression that the findings reinforced my misconceptions of primate behavior. Here, a specialist in the field not only sets me straight, but better educates me on the topic. It doesn't get much better than that. I had never heard of the sexual dimorphism principle. Now I am wondering about mass ratios and monogamy in humans.
No problem! One of the reasons I became interested in primatology was because studying our closest living relatives can tell us a lot about our own behaviour. If we are going to take the sexual dimorphism rule that seems to apply throughout the mammalian world, it would appear as though we have become more sexually monomorphic and less dimorphic over the past few hundred thousand years. Many palaeoanthropologists believe this is because monogamy has been increasingly selected for. From a purely functional perspective it would make sense that monogamy would be incredible beneficial for a species that has such a prolonged maturation period. For example, it takes chimpanzees and bonobos 6-7 years to become independent and sexually active. It takes humans considerably longer - in between 13-20 depending on culture and social system and a range of other important variables. It takes us so long to mature because our brains are so much larger (3 times larger than chimpanzee and bonobo brains). Anyway, as we evolved, and our brains started to expand (in between 2 million and 100,000 years ago) we would have needed more and more bi-parental care (mother and father). It is during this same time that our human ancestors (e.g., Homo habilis, Homo erectus, etc.) became less and less dimorphic (i.e., the male and female sex ratio reduced considerably). Obviously we are not completely monogamous, but our sexual patterns more closely resemble that of gibbons than chimpanzees. We are still more promiscuous overall than gibbons but if our social-sexual system resembled the chimpanzees there would be no such thing as an exclusive long-term relationship (i.e. marriage).
I was wondering what sort of behavior is favored when sexual dimorphism leans towards physically dominant females. Is that what happens with bonobos? What is the degree of sexual dimorphism in chimps? Their behavior is very interesting.
To be honest, there are no extreme examples of females being sexually dimorphic over males. However, hyenas and ring-tailed lemurs are both female dominant and they are slightly larger than their male counterparts. I think as a general rule, most evolutionary scientists would suspect female dominance to be connected with body size in most cases -- evolution just hasn't selected for this trait often. As for bonobos, they seem to be more monomorphic. I would personally say that male bonobos seem to be a little larger. However, female bonobos do not dominate male bonobos physically, they control them sexually. As for chimps, males are certainly sexually dimorphic to females, and it shows within their hierarchy. Males have their own dominance hierarchy and females have their own dominance hierarchy, but the top ranking males are always dominant to the top ranking females. Females have clever ways to combat this, however many times they cannot avoid being coerced to mate or relinquish food in certain situations.
So it's just that the smaller the amount by which males are sexually dimorphic over females, the more monogamous they are? Are there any completely monogamous primate species where the male still has the sexual dimorphism advantage over the female?