A list of some animals with reproductive patterns that deconstruct the male-female binary:
Animals That Can Change Their SexMany can switch from male to female or vice versa, depending on environmental or social conditions. (No mammals, though.) Famously, the clownfish, first on the list, reveals that Nemo's dad in the movie should have become his mother. Some animals are hermaphroditic, with both sexes in one body, or nonbinary. Some (e.g., the hawkfish) can even change back and forth rather than shifting to the opposite sex and sticking with it for life. Parthenogenetic species also exist, whose females can give birth without having their eggs fertilized by males. Humphead wrasse are females in youth but can change to males later in life.
Interestingly, Heinlein adopts this reproductive pattern for his Martians in STRANGER IN A STRANGE LAND. The fertile young -- "nymphs" -- are female. All fully mature Martians are male. Although only hinted at in RED PLANET and PODKAYNE OF MARS, apparently the Martians in those books have the same biology.
Such transsexual phenomena can't be classified quite the same as human transgender identity, since the physical changes are automatic in animals but voluntary in humans. Still, the examples mentioned, only a few among many, illustrate that life on Earth is stranger than we often realize. Consider the extreme sexual dimorphism of the anglerfish, where the male, much smaller than the female, attaches himself permanently to her body. He fuses with her so that he atrophies into simply a parasitic sperm cell producer.
For a deep dive, with copious references and statistics, into unconventional animal sexual behavior, check out the exhaustively thorough BIOLOGICAL EXUBERANCE (1999), by Bruce Bagemihl. He explores transsexualism, transvestism, parthenongenesis, and homosexuality, mainly the latter, in numerous species (mostly mammals and birds). The six chapters of Part I, "A Polysexual, Polygendered World," offer an overview of the field and refute arguments that have attempted to explain away homosexual behavior in nonhuman animals. Part II, "A Wondrous Bestiary," comprises a catalog of particular species observed to exhibit the behaviors discussed in Part I, with lists of sources for the information on each. The book's appendix, bibliography, endnotes, and index take up almost 100 pages.
Through natural phenomena such as these, we can find inspiration for bizarre alien reproductive biology without ever having to leave Earth.
Margaret L. Carter
Please explore love among the monsters at Carter's Crypt.