A physiological response in which tissue such as the penis or clitoris fills with blood and becomes swollen and firm, regulated by the autonomic nervous system.
Most birds lack a penis—both sexes mate by pressing a single opening called a "cloaca" together. Ducks are an exception: their corkscrew-shaped penises can reach 42.5 cm, rivaling their body length. Unlike mammals, they achieve erection via lymph fluid in about one second.