The Birds and the Bees part 2: Gender benders

Just like my last the-birds-and-the-bees post, this is probably Not Safe For Work. I will, again, be discussing the sex lives and sexual equipment of various animals, and may occasionally use words considered rude. You Have Been Warned.

Let’s talk sex changes. With mammals (including us humans), sex (as a category) is usually fairly simple. Barring some sort of developmental malfunction, you’re born with either an “innie” or an “outie“, and barring traumatic accident or intentional surgery, the set you start out with will be the set you have until your dying day. But for a lot of other critters, especially aquatic ones, it’s not nearly so simple. And I’m not just talking about the various full-time hermaphrodites.

The term is sequential hermaphrodism. That basically means starting out as one sex and ending up as the other, or even switching back and forth throughout life. It is particularly popular among bony fish, especially reef fish, as well as gastropods.

Sequential hermaphrodism occurs in several basic forms. You can have protoandry (most or all individuals are born male), protogyny (most or all individuals are born female), or bidirectional sex change (where individuals can switch back and forth).

Probably the best known sequential hermaphrodite is the clownfish. Yes, as in Nemo. Clownfish are protoandrous, and are all born male, but the largest individual in a group will become female, and the second largest will become a sexually mature male, while all the others remain immature males. If the female dies or is taken away, the mature male becomes a female, and the largest juvenile male matures. Which, yes, means that Finding Nemo was biologically inaccurate.

Wrasses are a good example of a protogynous fish. They form harems, with a single showy “terminal phase” male, along with either a group consisting entirely of females, or a group of females and similar-looking “initial phase” males who were born male (depending on the species). If the terminal phase male dies or is removed, the largest individual (male or female) will generally become the new terminal phase male.

Bidirectional sex change is the rarest of the 3 strategies, though in some cases a normally protogynous or protoandrous fish can “switch back” in artificial circumstances (like, say, an aquarium with only 2 same-sex fish in it). Most of the fish that do bidirectional sex change are smaller, shyer fish that might find it too risky to move around looking for mates in a busy reef, like the yellow clown goby.

There are several other odd strategies in animals, when it comes to sex binaries. I mentioned full-time hermaphrodites previously, and while most simultaneous hermaphrodites are invertebrates, there are at least a few fish that use that strategy, as well.

There is also parthenogenesis, where a species dispenses with males altogether, either some of the time or all of the time. Parthenogenically produced offspring are generally clones, though some species, like bees, produce haploid (one set of chromosome) males through parthenogenesis, and others use complicated mechanisms to recombine the mother’s egg cells or the like, so that the offspring are not a direct clone.

And, of course, there are a few random things, like pregnant male seahorses, that aren’t technically any alteration of the sex binary, but are certainly a challenge to our perceptions of “normal” sex roles.

And then there are at least a few species that genuinely seem to have gender, rather than sex, differences. That is, social behavior or the like where males appear to be and/or act like females or (rather more rarely) females appear to be and/or act like males.

Perhaps the most notorious of these are the crossdressing cuttlefish. I will discuss the amazing color changing abilities of cuttlefish, and other cephalopods, whenever I get around to the relevant cephalopod article (at which point I will edit this sentence with a relevant link), but for this article all you really need to know is that cuttlefish can change color and pattern basically at will.

In multiple species of cuttlefish (the behavior has been observed in at least two species within the genus Sepia), smaller males will, presumably to avoid aggression by larger males, adopt the color patterns and posture more typical of females, sometimes even pretending to be carrying an egg. Then, when the time is right (generally when they are in the presence of at most one other male, as well as their target female), they will flash typical male patterns *only* on the half of their body that is facing the female.

And evidence suggests that female cuttlefish, who store the male’s sperm in their bodies and can select which they use to fertilize a given egg, will somewhat preferentially use the sperm from these “cross-dressers”, presumably because the strategy involves, for a small mollusc, a pretty impressive display of intelligence.

Happy Pride!