Factors that influence the proportion of offspring sired by a male following heterospermic insemination

Heterospermic insemination (HI) is the situation when sperm from two or more males are present in the female before fertilization. There are various means of distinguishing paternity after HI, extending from identifying the fertilizing sperm microscopically to examining the phenotype of offspring. Invariably, sperm from one male fertilizes a majority of the eggs. The advantage one male may have can be due to an innate difference between the males, an unequal ratio of sperm in the mixture or a more appropriate interval from insemination to ovulation when inseminations are not simultaneous. The relationship between the proportion of females that conceive from a homospermic insemination by a particular male and the proportion of eggs fertilized after HI by that male is positive. The ranking of quality of semen by morphological, motility or biochemical tests corresponds in nearly every case with the ranking resulting from an index derived from HI. The effect of treatment of the male on his fertility can be detected readily by a change in the proportion of offspring he sires from HI. Sperm suspended in a less than ideal extender, exposed to less than ideal handling procedures or stored for a prolonged period, will fertilize a smaller proportion of offspring when competing with sperm exposed to more suitable conditions. The advantage a sperm may have in HI may be associated with the speed of attachment to and penetration of the egg. HI provides a means of more accurate assessment of the relative fertilizing ability of sperm with fewer observations than with most other techniques.