Humans are oddballs in the animal kingdom for many reasons, but one of the quirkiest things is how we reproduce. Or, more to the point, how our bodies behave when we aren’t reproducing. Menstruation and menopause are both very rare. Of all mammals, only us, our primate relatives and elephant shrews menstruate. Menopause is even harder to come by — killer whales are our only sisters in hot-flushes, apparently.