Males may want to spread their semen as much as possible to ensure a large number of descendants; theoretically, their offspring number is unlimited, but the number of offspring a female can produce is counted. Thus, they want to be sure that the strongest hunk will give his best for her progeny.

Researchers investigating a chimp population in Mahale Mountains National Park in Tanzania have even found a way through which female chimpanzees ensure they mate only with "best quality big boys", whose important social and physical traits could be genetically delivered to their offspring.

Akiko Matsumoto-Oda from the Department of Welfare and Culture at Okinawa University in Japan found that chimp females avoid synchronizing their reproductive activities in order to eject from the mating system lousy males which otherwise could coerce them into sex.

This finding contradicts most previous studies of primates pointing out the fact that females synchronize their reproductive activities, even if this is quite tricky to prove analytically.

Matsumoto-Oda's team has imagined a new statistic tool, the estrus synchrony index, expressing the variance in the percentage of females with maximal swellings to cycling females observed daily.

ESI has high values when females synchronize their estrous cycles and low when they do not synchronize.

Ovulating females were detected by the size of their buttock pads, which is an indicator of the estrogen hormone level. The maximal swelling of the buttock pads coincides with ovulation and almost all copulations occurred during this period. Analyzing the nine years observation data, the researchers found a low ovulation synchronization and "avoiding synchronizing estrous cycles may be a female strategy to reduce male sexual coercion."

In chimpanzees' society, males can be very aggressive in requiring sexual favors from females, and being bigger, forced copulation, harassment and intimidation for sex is a common behavior. But when females do not synchronize their ovulation, they reduce the chance of sex for "lower quality males", as for the fewer disposable females there is a fiercer competition among males. The increased male rivalry ensures the fittest father for the next generation.