Image credit: Yellowfin Goby, Acanthogobius flavimanus, in Jawbone Marine Sanctuary, Williamstown, Port Phillip, Victoria, February 2018. Source: Wayne Martin / iNaturalist.org. License: CC By Attribution-NonCommercial.

Edit (6/23/21): The book excerpt uses “gender” when it talks about morphology (body plan) in the Barlow study. Thanks to reader Breanna H. for asking about a previous draft of this post that repeated the book’s wording. I learned recently how much K-6 material leans on gender norms to explain biological things. Models are always imperfect, and Breanna refers to a teaching approach that avoids applying the human concept of gender to animals, and a discussion about the risks in conflating sex and gender in animal model studies. (-RXS)

Edit (6/8/23): Thanks to Wayne, adding this: Editor’s note: The term "hermaphrodite" is appropriate for referring to non-human animals with sex characteristics that do not fit typical binary notions of male or female bodies. For humans, “intersex” is the appropriate term—learn more here!(-RXS)

A species of goby from Lizard Island on Australia’s Great Barrier Reef [is the Australian goby].(EN12) In the Australian goby, all the juveniles mature into females, with some later becoming males. The males, however, can change back into females. In fact, the meaning of male is ambiguous here. The investigators defined a male to be any fish with at least some sperm production. All males, however, contain early-stage oocytes—cells that develop into eggs—in their gonads. So all the males remain part female. The species therefore consists of two genders at any one time: all-female fish and part-male-part-female fish.
(EN12 G. Barlow (2000) The Cichlid Fishes: Nature’s Grand Experiment in Evolution. Perseus.)
Citation: Roughgarden, J. (2013) Evolution’s Rainbow: Diversity, Gender, and Sexuality in Nature and People. University of California Press, Berkeley. pp. 137-38.

A few of the facts that biology teacher Grace Pokela shares:

sex chromosomal variety: Insects use an XO sex determination system, while birds use the ZW system.
environmental factors for sex: A reptile’s sex is at least partly determined by the temperature in which the egg develops.
hermaphroditism: Flatworms transfer sperm through a process called penis fencing (which is described in graphic detail here).
hermaphroditism & sex change: In clownfish colonies, dominance is based on size, the female being the largest and the male being the second largest. If the female dies, the male gains weight and becomes the female for that group.
- Editor’s note: The term "hermaphrodite" is appropriate for referring to non-human animals with sex characteristics that do not fit typical binary notions of male or female bodies. For humans, “intersex” is the appropriate term—learn more here!
transgender behavior: Other fish species take on female attributes while mating, and they release sperm in the process.
2+ sexes: Fungi like molds and mushrooms have 36,000 sexes.
In humans, 5-alpha-reductase deficiency is a real (albeit rare) condition, in which young women grow a penis during puberty.
In humans, androgen insensitivity syndrome (AHS) is an intersex condition in which a person who is genetically male is resistant to male hormones . As a result, the person has some or all of the physical traits of a woman, but the genetic makeup of a man.
In humans, the SRY gene is involved in male sexual development—without it fetuses can be genetically male (with XY chromosomes) but have a female body. The same is true in females (with XX chromosomes), who can develop a male body without the SRY gene.
In humans, XXY males are sterile, with small testes—while women with only one X chromosome (a condition called Turner syndrome) are infertile and don’t go through puberty. Males with two X chromosomes (called Klinefelter syndrome) are taller, with a higher risk of breast cancer and osteoporosis.

Read more at https://observer.com/2017/03/transgender-facebook-troll-biology-sexuality/

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