Concealed ovulation

Concealed ovulation or hidden estrus is the lack of any perceptible change (e.g., a change in appearance or scent) when an adult female is fertile and near ovulation. Some examples of perceptible changes are swelling and redness of the vulva in baboons and bonobos, and pheromone release in the feline family. In contrast, the females of humans and a few other species[1] that undergo hidden estrus have few external signs of fecundity, making it difficult for a mate to consciously deduce, by means of external signs only, whether or not a female is near ovulation.

Sexual swelling in a female hamadryas baboon indicates she is ovulating. Unlike other primates, female humans show no obvious outward signs of ovulation.
  1. ^ Sandy J. Andelman (June 1987). "Evolution of Concealed Ovulation in Vervet Monkeys (Cercopithecus aethiops)". The American Naturalist. 129 (6): 785–799. doi:10.1086/284675. S2CID 83522515.