Do Reptiles Have Testicles?

Testicles are sex organs, or gonads, whose primary job is to produce and store sperm. Unlike human testicles, which produce sperm on a relatively consistent basis, many reptiles' only do so while undergoing a seasonal testicular enlargement that coincides with the breeding season. Testicles also function as endocrine glands, producing testosterone. This is important for the development of secondary sexual characteristics, such as the dewlaps, crests and flamboyant plumage characteristic of many male reptiles.

Reptiles carry their testes inside their lower abdominal cavities, often directly attached to the kidneys. Snakes and elongate lizards stagger their testes longitudinally, such that one rests anterior to the other. Each testis empties into a tube called the vas deferens, which transports the sperm to the copulatory organs, if present. Male copulatory organs do not have a urinary function for reptiles; instead, the waste products from the kidneys pour into a shared excretory chamber called the cloaca. Male turtles and alligators have penises, while most birds and tuataras lack such organs entirely, instead transferring sperm through direct cloacal contact. Snakes and lizards have paired copulatory organs termed hemipenes. In all cases, male reptiles who possess intromittent organs carry them internally. Usually, reptiles evert their sexual organs only during breeding attempts, but sometimes males evert their hemipenes during defecation.

The processes by which reptiles become male or female are varied. Genes determine the gender of many species – especially snakes, birds and some lizards. However, most turtles, crocodilians, tuataras and geckos utilize environmental stimuli to determine the gender of developing eggs. Called temperature dependent sex determination -- usually abbreviated TDSD or TSD -- the eggs of such species begin developing testis or ovaries in response to temperature-based triggers.

Scientists have discovered that some pollutants – called endocrine disrupting chemicals or EDCs – can cause reptiles to exhibit traits associated with the opposite gender. This masculinization or feminization can cause serious problems for wild populations, as affected animals are not as likely to reproduce. Males exposed to such chemicals may develop small penises or dysfunctional testes, or may fail to produce secondary sexual characteristics.