Potency of rat ejaculations varies with their order and with male age

In one session, a male rat may ejaculate 6 or 7 times, preceding each ejaculation with multiple intromissions. The potency of each successive ejaculation was investigated by examining the number of sperm later recovered from the genital tracts in one group of females and determining the number of embryos which were developing in another group of females. In nine older males (223-402 days), sperm and embryo number reached a peak at the third ejaculation; in fewer than 25% of mating sessions were these numbers maximal at the first ejaculation. In four younger males (103-172 days), two consistently showed maximal sperm output at the first ejaculation and two consistently peaked beyond it. However, despite these variations in sperm output, embryo counts in younger males were uniformly above 10 until the seventh ejaculation. Both age and ejaculation number had demonstrable effects on fertility.