Fecundability

 Humans seem to be more difficult than non-human primates to fertilize. The chance of getting chemically pregnant (that is, the presence of chorionic gonadotropin a hormone released following implantation) with a single copulatory event at the peak of the ovulatory cycle is ~7%, assuming normal fertility and cycles (Wilcox et al 2001). Data this precise on great apes is much harder to come by, despite the wealth of studies on primate sexual behavior, though the consensus is that non-human primates are much more readily fertilized. Chimpanzee fertility seems to peak immediately prior to ovulation, with a chemical pregnancy probability of 64% on that day (Deschner et al 2003). Overall, a fertility probability of 1 (eg that an ovulating female, if mated with a fertile male, will produce a chemical pregnancy) seems to be widely assumed for most nonhuman primates over the duration of their ovulatory cycle (Deschner et al 2003); the actual number is presumably more around 90-95%, which seems to hold true for other primates as well. This sharply contrasts with the roughly 30% overall chance of a healthy 20-35 year old human female getting chemically pregnant over the course of an ovulatory cycle (Wilcox et al 1995).

There is human-specific difficulty in fecundability both before and after implantation. 30% of potential pregnancies end in a failure to implant, not even getting to the stage of chemical pregnancy (Chard 1991), a problem not observed in non human primates. There also seems to be differences in of carrying the pregnancy to term. A 5 year study of baboons found that there is an 86% chance of a chemical pregnancy being carried to term (Beehner et al 2006). This contrasts to the roughly 43% chance of a human chemical pregnancy being carried to term, with 43% lost in early pregnancy and another 14% in miscarriage (Maclon 2002). This means that non human primates, in addition to being more than 3 times more likely to get chemically pregnant in the first place, are also twice as likely to have offspring from that pregnancy.

It’s unknown as to what causes these profound differences in fecundability. Perhaps polygamy, multiple males mating with the female both well before and during ovulation, that accounts for this increased fertility. While studies have found mixed results vis a vis the relationship between polygamy and increased fertility in other systems (Hosken et al 2009), no one has looked at primate fertility in relation to monogamy. It could be that having the variety of sperm from multiple mates allows the most suitable, biologically compatible male sperm to fertilize the egg, resulting in a pregnancy that is more likely to go to term.

Over the course of a year or longer, these differences in fecundability narrow: both human and nonhuman primate females are likely to get pregnant. With fairly frequent ovulation, as well as no seasonal dependence on mating, humans can try to make a baby year-round, and have a good chance of succeeding. Further, the amount of resources needed to raise the child, combined with a long gestation, lessens the importance of getting pregnant quickly, the impact of fecundability. Also, human babies have a better chance of surviving past childhood, making the ability to quickly produce another child (to replace a loss) less important.