Measuring Male-to-Male Differences in Fertility or Effects of Semen Treatments.

Fertility is a convenient but meaningless term unless the outcome measure is stipulated and accounts for dependence of male fertility on the female population. We describe outcome measures and detail the impacts of the physiological status of each female and her external environment, as well as management imposed by humans. We explain the dominant role of the female reproductive tract as a series of hurdles for sperm seeking an ovum. Each spermatozoon in an ejaculate is unique, although usually most are morphologically similar. Semen seemingly contains three subpopulations of sperm, based on fate within a female and role in hampering the success of the ultimate winning spermatozoon; we define these subpopulations. The numerical size of each subpopulation placed into a female determines the shape of the dose-response curve leading to possible live young. Heterospermic artificial insemination provides far greater sensitivity to detect differences, partly because the female environment is identical for each comparison.