True hermaphroditism is when a single individual has both ovaries and testes. It's very common among invertebrates -- almost all flatworms and sponges are hermaphrodites as are most snails, all earthworms and others, too. But once you get to animals with skeletons, it's a different story. Hermaphroditism is very rare in reptiles. One known species has a hermaphroditic third sex, and individuals in other species are born that way on very rare occasions.
Cause for Confusion
Parthenogenetic reptiles are sometimes confused with hermaphrodites. These animals produce live young in the absence of males, but not because they have sperm to fertilize their own eggs. Parthenogenetic animals reproduce with unfertilized eggs. Obligate parthenogenetic species have only females and always reproduce without fertilization. They include the brahminy blind snake, Ramphytyphlops braminus, at least one species of Armenian rock lizard, Darevskia dahli, and the mourning gecko, Lepidodactylus lugubris, which occasionally produces sterile males. Facultatively parthenogentic species have two sexes, but can reproduce without sperm in a pinch. They include such well-known species as the Komodo dragon and some boas.
Further confusing the matter is sexual parthenogenesis. Aspidoscelus (formerly Cnemidophorus) uniparens is a whiptail lizard of the western U.S. Couples pair up during breeding season, mate, then both lay clutches of eggs and hatch their young. Some observers concluded these lizards are hermaphrodites who fertilize one another -- not so. A. uniparens is entirely female, but variations in their hormonal balances can cause stereotypically male or female social and mating behavior, even though all of them hatch from unfertilized eggs and lizards who don't join the party still reproduce.
Getting Hot in Here
True hermaphrodites are incredibly rare in reptiles -- fertile ones are even more rare. Two developmental factors work against it. In the first place, incubation temperature determines sexual development in most reptile embryos. Details vary with species, but above a certain temperature you get one sex, below it another. There isn't really a middle ground that makes both, due to the second factor: Male and female genitals develop from the same embryonic cells, so you have to start with duplicate cells to end up with two sets of gonads. This usually only happens in cases of failed twinning. Conjoined twin embryos can turn into two-headed babies, with or without two sets of genitalia, or they can turn into one animal with multiple sets of organs.
A Class of Their Own
The exception is the golden lancehead snake, Bothropoides insularis. Widely touted in popular media as a hermaphroditic species, the real story is a bit more complicated. This isolated island animal has developed an odd characteristic. While the species contains males and females who breed with one another and produce viable offspring from fertilized eggs, it also contains a common third sex. The third sex has female and male genitals, but zoologists call it intersex -- stopping short from labeling it a hermaphrodite because the male genitals are smaller than in full males, and these individuals may have reduced or absent fertility in one or both sets of gonads.
- IUCN Red List: Darevskia dahli
- Endocrinology: Regulation of Pseudosexual Behavior in the Parthenogenetic Whiptail Lizard, Cnemidophorus uniparens
- San Diego Zoo Global Wildlife Conservancy: Brazil's Golden Lancehead - A Treasure to Be Protected
- Arkive.org: Golden Lancehead (Bothropoides insularis)
- Herpetology Notes: First Record of the Mourning Gecko (Lepidodactylus lugubris) on Guadaloupe, French West Indies
- Advanced Topics in Zoology; H.V. Kashyap
- Academia.edu: Unisexual Rock Lizard Might Be Outcompeting Its Bisexual Progenitors in the Caucasus
- TS-Si.org: The Hermaphrodite Albino Black Snake
- Florida Museum of Natural History: Brahminy Blind Snake, Flower Pot Snake (Non-Native)
- Biology Letters: Facultative Parthenogenesis Discovered in Wild Vertebrates
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Angela Libal began writing professionally in 2005. She has published several books, specializing in zoology and animal husbandry. Libal holds a degree in behavioral science: animal science from Moorpark College, a Bachelor of Arts from Sarah Lawrence College and is a graduate student in cryptozoology.