  The traditional theoretical perspective on sexual selection described above is an elegant model. However, it relies on several assumptions in order to be applicable to actual cases. Hrdy (1986) has described three broad categories of assumptions needed to successfully apply this model to real situations. The first assumption is that male investment in the production of offspring is small relative to female investment. Ruth Hubbard (1990) points out that the challenge here is determining the appropriate way to characterize and measure investment. Eggs are larger than sperm. So, if one simply considers gamete size, male investment is smaller. However, males do not use a microscopic eyedropper to dispense one sperm at a time. Hubbard questions whether investment ought to be measured at the level of the individual gamete. When one includes the total amount of energy and resources that are need to produce sperm and semen, the energy required to develop and maintain secondary sexual characteristics (differences between the sexes that are not directly linked to the reproductive system), the costs of male-male competition, the costs that males of many species invest in defending a territory, and the effort that the males of some species put into parental care, male costs may turn out to be higher than researchers have historically expected. These male costs may be further increased if one measures them over an individual's lifetime as opposed to a single reproductive event. It is important for researchers to justify their characterization of what counts as investment.