The phrase "battle of the sexes" applies pretty literally to the Assam macaques of southeastern Asia. During mating season, males become extremely aggressive, and females rely on various tactics to defend themselves... including all having sex at the same time.
In many species, males want to both maximize the amount of children they have and ensure that their mates' children are really theirs. This sets up a couple of completing impulses - to have as many children as possible, the males need to mate with as many females as possible. But if all the males are taking on multiple partners, then it becomes extremely difficult for any male to know which child is theirs.
That's the basic situation for Assam macaques, which live in populations of roughly three dozen monkeys, of which there are a dozen each adult males and females. The mating season lasts from October to January, and males tend to get more and more aggressive as the season progresses. Part of that might well be reproductive frustration - because females don't show any outward signs of fertility, and they mate at all points of their menstrual cycle, there's never any guarantee that sex will result in conception.
This leads to some serious confusion among the males as to who fathered which baby, and that has some serious benefits for the females and the population as a whole. Unlike in species where every father knows for certain which child is his, male macaques don't go around killing other males' offspring. What's more, males actually take on the bulk of the child-rearing responsibilities. For all their mating season aggression, male Assam macaques actually turn out to be pretty good dads, if somewhat inadvertently.
But this all only works if all the males remain uncertain about the paternity of their young. That's where the dominant alpha males complicate things - they should be able to bully and fight off the other males so that they can monopolize certain females, which in turn means they should know for certain which baby is theirs.
Researchers at Germany's University of Göttingen set out to solve this mystery, and they found a truly awesome answer. By studing a macaque troupe in Thailand, they discovered that females tended to mate all at the same time. On any given day, the females were either all ready to have sex or none of them were. The alpha male could of course only choose one mate, leaving all the other females free to find other partners.
It's a fascinating bit of cooperative behavior, and head researcher Ines Fürtbauer believes the females developed this strategy expressly to ensure they could all take on multiple partners without risking the wrath of the alpha males, not to mention giving males incentive to look after all the kids. Basically, crazy group sex builds social harmony... at least among macaques.