main categories: mating investment and rearing investment. Mating investment consist of the sexual act and the sex cells invested. The rearing investment is the time and energy expended to raise the offspring after conception. In most species, the female's parental investment in both mating and rearing efforts greatly surpasses that of the male. In terms of sex cells (egg and sperms cells), the female's investment is typically a larger portion of both genetic material and overall verillity, while typically males produce thousands of sperm cells on a daily basis. Human parental investment Human women have a fixed supply of around 400 ova, while sperm cells in men are supplied at a rate of twelve million per hour. Also, fertilization and gestation occur in women, investments which outweigh the man's investment of a single effective perm cell. Furthermore, for women, one act of sexual intercourse could result in a nine-month commitment such as human gestation and subsequent commitments related to rearing such as breastfeeding. From Trivers' theory of parental investment, several implications follow. The first implication is that women are often but not always the more investing sex. The fact that they are often the more investing sex leads to the second implication that evolution favors females who are more selective of their mates to ensure that intercourse would not result in unnecessary or wasteful costs. The third implication is that because women invest more and are essential for the reproductive success of their offspring, they are a valuable resource for men; as a result, males often compete for sexual access to females. Males as the more investing sex For many species the only type of male investment received is that of sex cells. In those terms, the female investment greatly exceeds that of male investment as previously mentioned. However, there are other ways in which males invest in their offspring. For example, the male can find food as in the example of balloon flies. He may find a safe environment for the female to feed or lay her eggs as exemplified in many birds. He may also protect the young and provide them with opportunities to learn as young, as is the case with many wolves. Overall, the main role that males overtake is that of protection of the female and their young. That often can decrease the discrepancy of investment caused by the initial investment of sex cells. There