I recently learned that avocados change sex
daily, and I need you to know about this too. Some avocados spend their mornings as females, then close up their flowers only to re-open as males the following afternoon. Others laze about until the afternoon, when they’re female, then close down for the night and wake up as males the next day. This gender fluidity has some wild implications for avocado farms. But before we get into that: Let’s talk about sex, baby.
Let’s talk about you and me. We are humans, and as humans, we’re mostly born as one of two biological sexes. Most of us are born with either a penis or a vagina, which
usually corresponds to having either an X and a Y chromosome or two X’s. Some of us, though, have just one sex chromosome or kick it with three (XXX, XYY, or XXY). A few people are even born XX with just a
teensy bit of a Y chromosome that makes them, for the most part, present as outwardly male.
In summary, human sex is way more complex than most people think (and only gets more complicated because of our species’ long history of conflating it with gender identity). But fish—hundreds of species, in fact—turn the whole idea of sex on its head by swapping from one to another with ease. Moon wrasses, a type of fish native to the Indian and western Pacific Oceans, all start off female and become males over the course of 10 days based on a complex dominance system. Other fish simply spend a certain number of years as one sex and then switch as a kind of maturation.
Having this kind of sexual transition is called sequential hermaphroditism, or sequential dichogamy if you’re talking about plants, and it suggests that binaries are far from a rule in nature. Plenty of critters are plain ‘ol hermaphrodites, meaning they’re always able to fulfill either the male or female role in mating, as are the vast majority of plants (most flowers have male and female bits all in one). Those species that do it sequentially are just a little more special.
Here are some more organisms that prove sex is anything but fixed.
To the right now
One hop this time! 
