In lizards and snakes, males possess paired hemipenes, each of which is usually grooved to allow sperm transport and spiny or rough at the tip to allow firm attachment to the female. To become erect, a hemipenis is evaginated (turned inside out) through muscle action and engorgement with blood. Only one is inserted into the female's cloaca at a time.
In some turtles, crocodiles, some birds, and mammals, males possess a penis centered along the midline of the body. During copulation it becomes erect due to engorgement with blood or lymph. When not in use, it is usually flaccid, and depending on the species, may be retracted into the body. The anatomy of the penis varies according to species.
Male ostriches have a conical shaped penis that is wider at the base. Male ducks have a penis that is coiled along the ventral wall of the cloaca when flaccid and which may have an elaborate spiral shape when erect.
All male mammals have a penis. Insectivores, bats, rodents, carnivorans, and most primates (but not humans) have a bone called the baculum or os penis that permanently stiffens the penis. During copulation, blood engorges the already-stiff penis resulting in a full erection.