The Sociobiology of Sperm

Here's what my esteemed Professor (E.T. Rolls) has to say about that.

Monogamous primates well spread out over territory have small testes, for example gibbons and some tarsiers. Polygamous primates living in groups with several males in the group have large testes and frequent copulation, e.g. chimpanzees and monkeys. The reason for this appears to be sperm warfare - in order to pass his genes on to the next population, a male in the polygamous society with competition between males needs to increase the probability that he will fertilize a female, and the best way to do this is to copulate often, and swap the female with sperm, so that his sperm have a greater probability of getting to the egg to fertilize it.

Therefore in polygamous groups with more than one male, males should have large testes, to produce large numbers of sperm and large quantities of seminal fluid. The largest testis size in relation to body weight is found in the chimpanzee, who live in multimale groups, are highly promiscuous, and have on average 13 partners per birth.

What about humans? Despite being apparently mainly monogamous, they are intermediate in testis and penis size - bigger than expected for a monogamous species....

So, take the hint.