Darwin’s idea of sexual selection is a bold one. Males fight; females choose. To most people, it may account for the most easily understood idea in all of evolution since we humans are, after all, hypersexual organisms. In almost every species that reproduces sexually, it is the male who solely engages in brutal battles just to win his lover’s heart. Mating is so critical to our survival, and, of course, to our diversity as a species that we often take it for granted. In this respect, male competition would not be as fierce if it weren’t for sperm competition.

Unlike females, who are born with a finite number of eggs, males don’t develop sperm until puberty. When it comes to mating, because sperm compete numerically, males producing more sperm have a higher chance of fertilization success. Therefore, when there is a high sperm competition, males need not mate as often as they would with low competition. When, however, sperm competition is low, males are better off maximizing the number of mating events—a smart and economic approach evolutionary biologists call evolutionary stable strategy, also known as ESS.

When a male faces no competition from rival males, he is more likely to release all of his sperm. As the number of competing males increases, the amount of sperm released by that male decreases. This is to avoid uncertainty of fitness values in the female’s eyes. The correlation between male competition at high density and sperm release is, therefore, dependent on the probability of the fertilization of the egg. It is based on this model that we can get a pretty good idea of not only how male competition acts upon fertilizing the egg, but also how male competition can introduce variation to a population. Sounds complicated? Okay, here you go: the lower the population density, the less a male ESS releases sperm to fertilize a female’s egg. This means that males should produce greater numbers of sperm when competition is high.

Sperm competition is not always seen as a threat to males, though. Sometimes, a male can get around sperm competition by preventing a female from mating with rival males. Mate guarding has been observed in many species of insects such as Drosophila melanogaster. This rather fancy-looking fly, which is a popular organism in genetic analysis of sexual reproduction, secretes a seminal fluid protein that holds a female back from mating—at least for some days.

The classic evolutionary explanation for competition among males can be provided by studying male behavior and morphology. The rate of sperm production and the frequency of sperm being released by a male in accordance with the density of the population in which it lives can have various outcomes in regards to fertilization success. As far as anyone could tell, to carefully analyze and time mating intervals is what males are good at. In the end, though, it is females that make the final decision. There, I said it.

No, actually I haven't. But now that you mentioned it, I took a peek inside the book on the web. It does seem like a worthy read especially for someone who is fond of the biology of sexual reproduction. P.S. Thank you for taking the time to read my article

Incidentally, it may well be that female mammals can continue oogenesis after oocytogenesis.

Johnson J, Canning J, Kaneko T, Pru J, Tilly J (2004). "Germline stem cells and follicular renewal in the postnatal mammalian ovary". Nature 428.

Johnson J, Bagley J, Skaznik-Wikiel M, et al. (July 2005). "Oocyte generation in adult mammalian ovaries by putative germ cells in bone marrow and peripheral blood". Cell 122.