Generally, it is unusual for males within a species to be the choosy sex. There are many reasons for this. In humans, following sexual reproduction, the female is obliged to endure a nine-month pregnancy and childbirth. This means that females naturally provide a greater parental investment to offspring, than males. Human males have a larger quantity of gametes than females, which are replenished at a rate of approximately 12 million per hour. Conversely, female humans are born with a fixed amount of egg cells which are not restocked over the lifespan. This provides males with a greater window of opportunity to mate and reproduce than females, hence females are usually more choosy.