According to Trivers' (1972) evolutionairy theory of parental investment the difference between male and females in terms of parental investment and reproductive behaviour can be explained by the amount of time, effort and money invested by men and women in children. Put simply, men can opt out of parental investment in a way that women cannot.
Anisogamy refers to sex differences in the size of the sexual gametes- the egg and the sperm. The egg is more costly for the female than the sperm is for the male. Males produce sperm in millions at little physical cost. Females on the other hand, produce eggs, the gamete (eggs) she supplies is one hundred times larger than the sperm and each egg contains a store of food for the growing embryo, whereas the sperm does not being just a gamete nucleus and cytoplasm tail; she also has a limited supply of these (producing one ovum per month). Furthermore, her reproductive life is also shorter than the man's (30 years). Anisogamy clearly influences parental investment because it determines the number of children either gender can have, for example, the maximum number of potential offspringattributed to one man, a Moroccan Emperor is 888, whereas the highest number produced by one woman is 69. These are rather extreme numbers but they show the number of potential offspring is much lower for females than males and so she will invest in her offspring more because she can have less than the male. Even if a female was to have sex with lots of men in a year she will still only produce one single offspring. The female human's investment in each offspring is much greater. Therefore, her best chance of reproductive success is the survival of that one offspring.
Reproductive costs are also far higher for the female and so evolution predicts she will invest more than the male. Following conception she has to carry the foeus for 40 weeks (gestation). The female invests heavily during gestation as the nutrients are drawn from her body for the developing child, more will be needed as she increases in size, her kidneys have to work harder to eliminate waste, and then she must give birth which is a risky process. After birth the female also invests more in the baby, for example by breast feeding (lacation) which requires a diet high in sugars and fats to produce the nutrient rich milk and so these will be drawn from the mother's diet and if this is lacking will drain her resources. Compared to the male whose contribution to offspring can begin and end with the donation of sperm clearly the female bears the greater burden and so this predicts higher investment by the female.
According to TRIVERS the difference in parental investment will determine which sex will be the choosiest when selecting a partner to reproduce with. In humans it is the female because they invest more than men and the consequence of mating with an unsuitable…