Ridley points out that female polygamy is more common in species where males have larger testicles.

Does anyone have any ideas as to why this is the case?

Easy

Think about it. If you knew that your girlfriend was going to be humped by 20 other men that day, and your only consideration was that you wanted to get your genes into the next generation, the more semen you produce the better the chance that it will be one of your tiny little helpers that gets there first.

When a female chimp comes into season, just about every male in the community will try his luck. That isn't to say she won't favour males she likes, or that some males won't try to lure her away for themselves. Chimp testicles are huge in relation to their size. Gorilla testicles are tiny. In their groups the only male with access to females is the silverback.

Interestingly humans come somewhere in between.

The stuff I've learned researching this book . No wonder my daughter says I'm an oddball

Edited by: PeterDF at: 11/10/03 11:56 am

Yes, but there are alternatives, aren't there?

Perhaps females are humping 20 males in these species because it leads to less conflict within the groups. If males have larger testicles then they probably have higher sex drives. If all members of the group are satisfied, then there is less conflict.

I'm just wondering which is the cause and which is the effect. Its all to easy to confuse the two.

As an alternative, it is possible that males have bigger testicles because in such species, there are more chances to reproduce.

I think the correlation between testicle size and promiscuity is probably due to the simple "selfish gene" explanation, but I agree that sex can play an important part in social cohesion. This is especially important in bonobos of course. (Where humans might shake hands - bonobos have sex.)

The reason that chimp behaviour is so different to gorilla behaviour is probably because of the food they eat. Gorillas eat large volumes of food of relatively low nutritional value like roots and tubers, so they can afford to be slow moving and they can live in small inoffensive family groups wandering around to find what food they can.

Chimps often share the same parts of the forest with gorillas but they eat small amounts of high value food like fruit, and they occasionally hunt meat, so they have to be quick and clever to make sure they get the best of the food. They live in bigger more flexible groups that are more competitive. The males have to hunt co-operatively so it follows that they live in mixed fission/fusion groups within larger communities - hence the promiscuity.

Bonobos live in areas where there are no gorillas so they can find both kinds of food. So, it is thought that, because they don't need to compete for food in such a competitive way, that this is the reason that they are much more gentle and inoffensive than chimps.

This seems to be the theory. Makes sense to me anyway.