Penis: structure, function, and variation across animals

Human anatomy and function

In humans the penis consists principally of the root, body (shaft) and glans. Internally it contains erectile tissues—the paired corpora cavernosa and the corpus spongiosum that surrounds the urethra—which fill with blood to produce an erection. The urethra runs to the tip, carrying semen during ejaculation and urine at other times. The glans may be covered by a fold of skin called the foreskin (prepuce) unless removed by circumcision. Sensory innervation, vascular supply and connective tissues support sexual function and urination.

Variation across animals

Animal penises show wide diversity. Many mammals have a penis and some species possess a bone within it called a baculum. Some reptiles (snakes and lizards) have paired hemipenes, which are eversible structures used one at a time. Birds generally lack an external penis and most transfer sperm by cloacal contact, though exceptions exist (e.g., waterfowl). Invertebrates often have specialized intromittent organs or complex genitalia adapted to their mating systems.

Development, evolution and adaptations

Penile structures develop from embryonic tissues that differentiate under genetic and hormonal influence. Evolutionary pressures such as sexual selection, mating systems and sperm competition have shaped size, shape and complexity. For example, species with intense sperm competition sometimes evolve elaborate shapes or mechanisms that influence sperm transfer or mate removal.

Health, medical and social notes

In clinical contexts, conditions that can affect the penis include infections, congenital differences, erectile dysfunction and, rarely, cancers. Cultural practices and legal frameworks influence attitudes and procedures such as circumcision. For information about male animals and hermaphrodites in biological contexts see male animals and hermaphrodites.