Animal IQ

Nature Is Full Of Amazing Hermaphrodites

In the beginning of Disney's Finding Nemo, a father clownfish is left to care for his one remaining egg—which grows up to be Nemo—after the mother was eaten by a barracuda. If this were real life, though, Nemo wouldn't be without a mother for long. Nemo's dad would just change his sex to female. That's because clownfish, like many plants and animals, are hermaphrodites.

The Birds And The Bees—And The Self-Fertilizing Snails

Hermaphroditism happens a few different ways in nature. Simultaneous hermaphrodites—snails, for example—are born with both male and female sex organs, so any two members of the species can usually mate, or even fertilize themselves. There are also what are known as sequential hermaphrodites, which is an organism that's born with one set of sex organs but can change them at some point in its life. If it's born as a male and switches to female, that's called protandry; the reverse, and it's called protogyny.

Clownfish like Nemo are protandrous, which means they're born male. They live in groups of two large fish—a breeding male and a breeding female—and many smaller fish. If the breeding female dies, her mate changes his sex to female and the next largest fish becomes a sexually mature male. And then they mate. (Don't think about that for too long, Disney fans.) Many flowers, on the other hand, are protogynous, meaning their female organs (the pistil) mature before their male organs (the stamen).

WTF, Evolution?

Those examples are just a sampling of nature's sex-changing smorgasbord: earthworms, sea stars, sea cucumbers, frogs, newts, salamanders, and many, many other types of fish all have some ability to change their sex. Why does this happen? Doesn't it seem like nature had it pretty much figured out with the male-female dichotomy?

Some scientists pose a different question: why isn't hermaphroditism more common? Isn't the ability to mate with literally anyone, including yourself, a win-win for evolution? A 2009 study in The American Naturalist sought to answer this question by examining the "costs" of hermaphroditism—that is, how much energy and risk is required for an animal to change its sex. The answer? Not enough to make it this rare. If changing your sex made you less likely to procreate, or more likely to be eaten, or required too much energy, it would make sense that not many organisms would take this path. But the study didn't find that—in fact, they found that a hermaphrodite could spend a third of its life just in the process of changing its sex, and would still thrive. We don't know why so few organisms can change their sex. But we can still be fascinated by those that can.

Bonus fact: Hermaphroditism doesn't always happen for natural reasons. Especially in amphibians, environmental factors such as pollution and a warming environment can cause animals to spontaneously change their sex. For example, a 2009 study found that Atrazine, a common herbicide, could make male frogs develop female sex organs.

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Watch And Learn: Our Favorite Videos About Sex In Nature

These Animals Can Change Their Sex At Will

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Gynandromorphs: Dual-Sex Animals

Male or female? All of the above.

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Key Facts In This Video

  1. Gynandromorphs have both male and female traits. 00:32

  2. Sex chromosomes are found in tightly coiled bundles of DNA. 01:40

  3. Gynandromorphism is not the same as hermaphroditism. 02:44

Intersexual Animals

Wait till you get to the hyenas.