Some species, mostly fish, amphibians, and invertebrates, are capable of changing sexes under certain circumstances. In fiction it is sometimes possible for other creatures, even humans in some cases, to do this as well. Subtrope of Gender Bender, may involve Bizarre Alien Sexes. Anime and Manga
- In One Piece, Emporio Ivankov ate the Horm-Horm Fruit which allows him to inject hormones from his fingertips, among its many applications include male/female hormones which can turn himself or any recipient into an opposite sex.
- In a different way, Mr. 2 Bon Clay ate the Clone-Clone Fruit which lets him alter his body into someone else's that he had touched once, including women.
- The Skrulls in the Marvel Universe can change their sex as well as appearance. In Runaways Super Skrull in training, Xavin, is engaged to Karolina for an Arranged Marriage set up by their parents as a peace keep measure between their races. Karolina isn't interested not because she isn't sold on the marriage thing, but because she's a lesbian. Xavin handily solves this by showing her his/her female form and and Karolina has no complaints.
- In Jurassic Park the cloned dinosaurs were able to get around the genetic manipulations that made them all female thanks to the frog DNA used to complete their genomes.
- Virginia Woolf's Orlando more or less spontaneously turned into a woman. The League Of Extraordinary Gentlemen reinterpretation has that happen every few years, owing to his/her father Tiresias.
- In C.J. Cherryh's Chanur Novels stsho are somewhat prone to "phasing" with little warning, assuming a new gender and personality.
- M.C.A. Hogarth's Jokka can change genders at two points in their life, once in the first seven years of their life and again in the second seven. Some never Turn, others Turn once, some Turn then Turn back to their original gender, and a few rare individuals have the chance to be all three sexes.
- In F.M. Busby's The Breeds of Man geneticists searching for a cure to an infertility plague caused by an AIDS cure Gone Horribly Wrong accidentally turn some people into serial hermaphrodites that alternate between male and female monthly based on menstruation.
- The Left Hand Of Darkness has a planet where the human-descended inhabitants are asexual most of the time, except when they're in "kemmer" and become either men or women, as well as very lustful.
- The Monster Of The Week in The X Files episode aptly titled "Gender Bender" is a member of an (apparently) alien humanoid species whose biological gender changes every time after they have sex.
- Mork from Mork And Mindy states that Orkans' "parts are interchangeable"
- In GURPS "hermaphromorph" is a five-point trait that allows a character to do this.
- Eclipse Phase has a "sex switch" BioMod that enables one to do this (and comes standard with Pleasure Pods), though the process takes a week.
- Incubi and succubi have shapeshifting powers in Amber Panyko Williams' Dan And Mabs Furry Adventures universe. This includes gender changing at will, though it takes much time and practice to do well. Abel Rewanz demonstrates the boy-to-girl morphing in strip #690, and again in strip #1038, much to Daniel's chagrin.
- The artificial lifeform Penelope from Ben Nu~nez's Just Another Web Comic was built as a sentient sex toy by Mad Scientist Eryl Keydon. Penelope can change gender and species on command, though she seems most comfortable as a female feline. See volume 2, page 32 where Penelope recites her tech specs, though be advised: this web comic is patently Unsafe For Work.
- In the Chakona Space universe skunktaurs, a sister project to the chakats, can switch between male and female at will. Though they also have an annual cycle that forces a sex change.
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