Also the subject of a news article in National Geographic: Bigger Testes Can Offer a Competitive Edge
When competition for females is fierce, males of some species have evolved bigger testes to trounce their rivals, a new study has confirmed.
Specifically, the research shows that testicle size matters in highly competitive animal societies in which females mate with many males or in which females live in groups ruled by an alpha male that must constantly defend his harem.
But that doesn't mean that females seek out more endowed males. Rather, the rivalry occurs after mating, as sperm battle inside the female.
Not surprisingly, males with larger testes produce more sperm—giving the male, so to speak, more bang for his buck.Just how much all this applies to humans is always interesting to consider. Here's a curious paper with a lot more speculation than data: Semen Displacement as a Sperm Competition Strategy in Humans
We examine some of the implications of the possibility that the human penis may have evolved to compete with sperm from other males by displacing rival semen from the cervical end of the vagina prior to ejaculation. The semen displacement hypothesis integrates considerable information about genital morphology and human reproductive behavior, and can be used to generate a number of interesting predictions.