Absence really may make the heart grow fonder, according to a report on short term human mating. The more time couples spend apart, the more sperm the husbands inseminate in their wives, says the report in Personality and Individual Differences (2000;28:929-63).
When couples spend 100% of their time together, men inseminate 389 million sperm per ejaculate, but when only 5% of their time is in each other's company, the figure almost doubles to 712 million.
The report says that this so-called sperm competition is one of the clues that suggest that historically women have departed from a singular strategy of long term mating.
"This increase in sperm insemination increases the odds of crowding out and displacing a competing man's sperm, which is precisely what would be expected if humans had an ancestral history of some casual sex and marital infidelity," says the report.
"Women's non-monogamous mating is the hidden side of women's sexuality," it adds. "If ancestral women had never engaged in short term mating, men could not have evolved the powerful desire for sexual variety."
One reason short term mating may have evolved, it suggests, was as a fertility backup: "An ancestral woman married to a man uninterested in sex may have had a more difficult time getting pregnant. If this is correct, then fertility backup is a possible function of a woman's short term mating." The report continues: "Alternatively, a partner's lack of sexual interest may signal to the woman that he is channelling his sexual interest and perhaps commitment elsewhere, in which case the woman might benefit by doing likewise."
The report, which also points to male sexual jealousy as a response to an "adaptive problem of female infidelity," says that another clue to non-monogamous mating is the size of testes.
"Across primate species testes size is strongly correlated with the nature of the mating game. The greater the sperm competition, via promiscuous or polyandrous mating, the larger the testes size relative to body size," it says.
Male gorillas, for example, have relatively small testes—0.031% of body weight—and the female of the species is indeed highly monogamous. Chimpanzees, on the other hand, are highly promiscuous and have much larger testes—0.30% of body weight.
Human males are somewhere between chimps and gorillas, with 0.79% of body weight, suggesting, says the report, that ancestral humans were unlike promiscuous chimps, but not invariably monogamous.
