Is Low Male Fertility Adaptive? Implications for Male Infertility Research

BACKGROUND: Fertile human males produce significantly fewer sperm per gram of testes than most other animals, including primates. Human's low sperm output is associated with a lower pregnancy rate per episode of intercourse and a longer time to pregnancy than in most other species. This, coupled with the concealed nature of ovulation in women, means that human couples frequently require numerous acts of intercourse to achieve a pregnancy. The reason for the low fecundability of humans is unknown. METHODS: I performed a literature review examining the efficiency of spermatogenesis and time to pregnancy in humans to estimate the period of time and average number of episodes of sexual intercourse required for pregnancy. These estimates were corroborated with mathematical models. Hypotheses are suggested to explain the low fecundity of humans. RESULTS: The average, fertile human couple has sexual intercourse 17-35 times over 2-6 months to conceive, with some fertile couples requiring longer periods of time. These numbers and time period are greater than for most other species. Among the theories that could be advanced to explain the relatively low fecundability of humans compared with other species, two will be explored here: (i) a reduction in the risk of pre-eclampsia with repeated sexual intercourse with a single partner over several months and/or (ii) improved pair-bonding from repeated intercourse over time leading to improved outcomes for children. CONCLUSIONS: Reduced sperm production in men may be evolutionarily adaptive. Lower fertility and longer time to conception is associated with a reduced risk of pre-eclampsia and improved outcomes for offspring. Greater understanding of the mechanism(s) involved in the relatively low sperm output in men might provide clues to explain some types of male infertility. Additional research into the reduced efficiency of sperm output in men is warranted.