Primates, for example, evolved trichromacy to better spot young leaves and ripe fruits. And once they added red to their Umwelt, they began evolving patches of bare skin that could convey messages by flushing with blood. The red faces of rhesus macaques, the red rumps of mandrills, and the comically red and bald heads of uakaris are all sexual signals made possible by trichromatic vision.
