Success of fertilization in golden hamsters is a function of the relative gamete ratio

Gamete concentrations can be expressed as either absolute gamete ratios (sperm/egg), the concentration of the motile sperm (sperm/ml), or as a relative gamete ratio (sperm/egg x ml). We demonstrate that the success of hamster fertilization in vitro is a function of the relative gamete ratio and that any effect of the insemination medium geometry is minimal. Consistent fertilization occurs when the relative gamete ratio is above 10(3.5) sperm/egg x ml, but becomes variable above 10(5.0) sperm/egg x ml. At suboptimal relative gamete ratios, there is a strong sperm concentration effect on both the consistency and level of fertilization, whereas the absolute gamete ratio only affects the overall level of fertilization. These effects are seen when the sperm concentration is below 10(3.5) sperm/ml and when the absolute gamete ratio is below 10(2.3) sperm/egg. These influences are probably due to reduced sperm survival at low sperm concentrations and due to a sampling error that occurs when small numbers of sperm are transferred. When the absolute gamete ratios found in vivo in the hamster [Cummins and Yanagimachi, 1982, Gamete Res 5, 239-256] are converted to relative gamete ratios, they are similar to our in vitro results. Thus relative gamete ratios allow, for the first time, comparison between in vitro and in vivo data in the hamster, and between other rodents.