Scientists figure that when hominins began to walk on two legs, there wasn’t room in their upright pelvises for giant genital displays.[*11] These flaps shrank, but even now a woman’s labia may swell just a bit when she’s ovulating. The inner labia—nearly diaphanous flaps that nestle around the clitoris and its hood—can turn a bit darker with blood whenever we’re particularly turned on, and in a more pronounced way around ovulation. As women age, the inner labia tend to stay darker, a leftover from lifelong cycles of fertility.[*12]
