A pseudo-penis is any structure found on an animal that, while superficially appearing to be a penis, is derived from a different developmental path.
Mammals
In mammals, all intact developmentally typical males have a penis, but the clitoris in the females of the following species is sufficiently enlarged that it is usually termed a pseudo-penis: spotted hyena, juvenile fossa, binturong, lemur and spider monkey. The enlarged clitoris in the fossa is supported by an os clitoridis, a bone similar to the os penis found in most mammals. However, the fossa's os clitoridis and pseudo-penis shrink as the juvenile female grows, unlike that of other pseudo-penis species.
The mammalian pseudo-penis appears to be simply for display, though the spotted hyena is an exception: the female spotted hyena additionally uses her pseudo-penis for urination, sexual intercourse, and to give birth. In addition, this makes it difficult for males to mate without the full co-operation of females, which means that mating preferences of the female are predominant.
Spotted hyenas are a matriarchal society, where adult female hyenas dominate the adult male hyenas. Female spotted hyena are also more aggressive than the male spotted hyena. When a male hyena leaves its natal clan, it behaves submissively to all newly encountered hyenas; as a result, when a male hyena settles down with a new clan as a breeding male, it is submissive to all natal clan members. As a result of the submissive behavior in males, it was hypothesized that the male hyena erected its penis as a show of submissiveness. During greetings, hyenas would stand parallel to each other and sniff or lick the erect penis or anal scent gland.
There are severe reproductive costs as a result of the female spotted hyena's pseudo-penis. Nearly all female spotted hyena's first-born cubs are stillborn, as the placenta is not long enough for the extended penile birth canal. In addition, the first birthing process is time-consuming, as it requires the meatus of the pseudo-penis to tear, allowing the fetus to pass through; as a result, the first-born often die of hypoxia.
Female spider monkeys have a clitoris that is referred to as a pseudo-penis because it is especially developed and has a shallow perineal groove that retains and distributes urine droplets as the female moves around. The clitoris of female Geoffroy's spider monkeys is large and protrudes, looking like a penis. This organ, called a pendulous clitoris because of the way it dangles externally, is actually larger than the male flaccid penis. As a result, females are sometimes mistaken for males by human observers. The enlarged clitoris is believed to aid males in determining sexual receptiveness, allowing them to touch the clitoris and smell their fingers to pick up chemical or olfactory cues to the female's reproductive status.
Birds
Only 3% of avian species have a phallus. The most common genital among birds is the cloaca; a direct tract for elimination and reproduction in both of the sexes. Certain bird species, such as the ratites, screamers, waterfowl, and cracids (a family of arboreal galliformes) exhibit a phallus in males. A notable example of a bird with a pseudo-penis is the red-billed buffalo weaver, which do not use their pseudo-penis for direct insertion during copulation; however it does play a part in successful mating and stimulation. Similarly to the red-billed buffalo weaver, the cassowary, a ratite, exhibits a pseudo-penis in both males and females. The male's pseudo-phallus is used to "invaginate", or to push the female's pseudo-phallus inside-out, and then ejaculates from the cloaca to ensure a successful mating.
Insects
One of the more known examples of a pseudo-penis to occur in the insects is found in the barklouse genus Neotrogla. In this genus, the pseudo-penis plays a part in their rarely-seen sexual reversal. In this case, the male has a vagina-like structure while the female has a pseudo-penis. Some may be confused, wondering why the individuals with a vagina-like structure are considered male while the ones with the pseudo-penis are considered female. This is because even though they have visible outer structures of the opposite sex, their inner structures are consistent to their sex. More important, the males produce sperm and the females produce eggs.