Supposedly, if women live together their hormonal cycles start to synchronise, thanks to a pheromone. If that were true it would mean that they all have their period simultaneously. Just think about it.
This “menstrual synchrony” argument was first reported in 1971 by psychologist Martha McClintock, who noticed signs of it in her own college dorm. But it may not really exist. Studies have had mixed results, often reporting no synchrony at all.
Assamese macaques, however, have evolved an unmistakable kind of synchrony: they all have sex at the same time.
Assamese macaques live in troupes of a few dozen, including about a dozen adults of each sex, plus offspring. Although there are strong social bonds within the troupes, they are dominated by the males, who compete vigorously to mate with the females.
The mating season runs from October to January, and the males become increasingly aggressive as it goes on. The males do show some solidarity. If a female attacks a male, other males will rally to his defence. But it is the females who form close friendships with each other, while males are only loosely allied with their fellows.
The females also have ways of resisting the males’ control of the troupes, says Ines Fürtbauer of the University of Göttingen in Germany. For one thing, like human females, they do not show external signs of fertility, so males have no way of knowing whether the female they are mating with is actually able to conceive. The females mate throughout their cycles, further confusing the issue. As a result, the dominant males can’t monopolise fertile females. Instead each female mostly mates with her preferred male, regardless of how high-ranking he is – although she will mate with every male at some point.
This suggests that the females are trying to keep all the males friendly. Not knowing who fathered which baby, the males ought to refrain from killing young. In fact, Fürtbauer says, the young spend most of their time being cared for by the males.
It’s easier for a female to keep the males onside if she mates with all of them, but the dominant males will try to monopolise her. To find out how the females get around this problem, Fürtbauer and colleagues monitored a troupe of wild Assamese macaques in Thailand over two mating seasons. As well as monitoring their behaviour, they took samples of their dung: the hormone levels in it told the researchers where each female was in her cycle.
There was no sign of the females synchronising their hormonal cycles, but they did synchronise their sexual receptiveness. On a given day, each female was more likely to mate if other females were mating. Spoilt for choice, the alpha male could only mate with some of them, ensuring that the other females could mate with someone else.
Fürtbauer thinks the females are working together to thwart the dominant males, ensuring that each female can sleep around without getting punished for it.