When the female capuchin comes on heat she acts like a drunk on Viagra. She pursues her target. She squeaks and she flirts. She teases, pulls fur, tickles, rubs her body against his. The male ignores her with implacable dignity. It withstands all her advances with statesman-like calmness.
On the fourth day it succumbs.
It is on the fourth day the female is at her most fertile and receptive. She doesn’t appear to appreciate this, the evolutionary drive to mate being a fairly blunt tool, as it were. But the male knows. The male is attuned to the universe.