11 Animals That Can Change Their Gender

Gender fluidity
Photo: Bernard DUPONT from FRANCE [CC BY-SA 2.0]/Wikimedia Commons

For Mother Nature, gender isn’t always an either-or proposition. In fact, the animal world is filled with creatures that occupy an intriguing gray area between male and female. A few species come already assembled with simultaneously functioning female and male organs. Others (like the moray eel pictured) change from female to male or vice versa, depending on need or surrounding conditions. Still others wear their half male, half female status in a more visible way, literally displaying the colors and physical traits of both genders.

The reasons behind this gender mobility are as varied as the gender-benders themselves. Some are natural processes that offer a species reproductive flexibility. Others aren’t so natural, often sparked by rising global temperatures or chemical pollutants.

Here are 11 creatures that offer a fascinating glimpse into the many ways gender can develop.

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Clownfish

Photo: Ritiks/Wikimedia Commons

Bright orange with three white bars, these distinctive fish — made famous in the movie "Finding Nemo" — are also known for their gender shifting. That’s because they’re hermaphrodites, born capable of operating reproductively as both males and females. But they’re not the familiar kind of hermaphrodite that produces eggs and sperm at the same time. Clownfish are sequential hermaphrodites, born one gender but able to switch to the other if necessary. In this case, the about-face runs from male to female (called protandry).

Here’s how it works: Clownfish live in groups where only two members are sexually mature, a large male and an even larger female. The rest are smaller, sexually immature males. If something happens to the female in the breeding pair, her male mate transforms into a female and selects the next biggest male in the group to become her new partner.

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Hawkfish

Photo: Vincent C. Chen/Wikimedia Commons

These vibrantly colored harem dwellers are also protogynous, starting off as females that can morph into males when conditions call for it. Typically, this happens when the harem’s male leader takes on too many females, prompting the largest female in the group to turn into a male hawkfish and split away with half the harem.

But that’s not the hawkfish’s only gender trick. Unlike most other sequential hermaphrodites that make the switch and stick with it, hawkfish can flip-flop on their sex change yet again. Female-turned-male fish may revert to female if, say, their new harem loses too many females or a larger male challenges them.

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Bass

Photo: wsimms8518/Wikimedia Commons

Maybe nature didn’t intend for male largemouth and smallmouth bass to grow lady parts, but that’s exactly what’s happening in rivers throughout the U.S. In the Southeast, for example, research shows that between 70 and 90 percent of male bass are now “intersex”; they have immature egg cells growing in their testes. Only bass in Alaska’s Yukon River show no sign of the condition.

The likely culprit? Pharmaceutical compounds such as birth control pills and pesticides that end up in waterways and mimic the female sex hormone estrogen.

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Moray eels

Photo: Dray van Beeck/Shutterstock

While most species of moray eels are born either female or male and remain that way for life, a few are sequential hermaphrodites. For example, ribbon morays, like the one pictured here, switch from male to female. Unlike clownfish, though, all of them make the switch. That is, every ribbon moray is born male and later becomes female via a remarkable (and colorful) process. Young males start out small and black with yellow dorsal fins. As they mature and begin fertilizing eggs, they gradually turn blue with yellow faces. However, once they grow a little bigger, male ribbon morays become yellow or greenish-yellow and develop female reproductive organs, living out the rest of their lives as egg-laying females.

Zebra and dragon moray eels, on the other hand, switch genders in the opposite direction from female to male (called protogyny). This usually occurs when males are in short supply.

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Banana slugs

Photo: Franco Felini/flickr

Bright yellow and up to 10 inches long (which explains their name), these wormlike mollusks are also simultaneous hermaphrodites, meaning they use their male and female reproductive organs at the same time. No changing back and forth between genders. But that doesn’t begin to describe their strange sex lives.

Although capable of self-fertilization, most banana slugs prefer to find a partner. And here’s where it gets weird. When it comes time to mate, two slugs curl around one another, kind of like a yin-yang symbol. Each one then uses its enormous penis (often exceeding the length of its body) for a reciprocal exchange of sperm that fertilizes each other’s eggs. Afterwards, both slugs deposit their eggs under leaves or logs and glide away with no further involvement.

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Butterflies

Photo: Burkhard Hinnersmann/Wikimedia Commons

Gender fluidity doesn’t confine itself to hermaphroditism, where male and female mixing is entirely centered on the genitals. In some creatures, the gender split is visible over their entire bodies. Yup, they literally display both female and male colors and characteristics. This rare dual-gender condition is called gynandromorphism, which results from a genetic error during early cell division that can cause male and female traits to be arranged either haphazardly over the body or bilaterally with one side male and the other equally female.

Gynandromorphism is found in crustaceans, insects, birds and maybe most spectacularly in butterflies, like the common blue butterfly pictured here. Its "male" side has the male’s blue wings with a black-brown border along the outer edge, and its “female” side features the female’s brown wings with orange spots.

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Cardinals

Photo: Shirley Caldwell/Facebook

Bilateral gynandromorphism occasionally shows up in northern cardinals, too. The one in this photo, taken by Shirley Caldwell of Erie, Pennsylvania in 2019, sports brown-gray "female" feathers on one half and bright red "male" feathers on the other.

According to a study by Brian Peer of Western Illinois University and his colleague Robert Motz, gyandromorph cardinals not only look different, they also act differently, at least the one they observed from 2008 to 2010. During that time, their “half-sider” was never seen vocalizing or mating. On the plus side, other cardinals seemed to accept it: The researchers never witnessed it being bullied or mistreated.

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Frogs

Photo: Contrabaroness/Wikimedia Commons

Researchers have observed frogs spontaneously changing genders in the lab. Now they’re observing it in the wild too, and it’s not a natural process. More and more male frogs are becoming females, complete with fully functioning reproductive organs. The frog-feminizing culprit? A common weed killer called atrazine.

Banned in Europe, atrazine ends up running into rivers and lakes where it wreaks havoc on frog sexual development, suppressing production of testosterone (the male sex hormone) and boosting the female sex hormone estrogen. Not good news for amphibian populations that are already declining from climate change, loss of habitat and invasive species.

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Snakes

Photo: Ltshears/Wikimedia Commons

Some female snakes, such as copperheads and cottonmouths, are capable of virgin birth, or parthenogenesis, meaning they fertilize their own eggs without a male sexual partner. While not technically a gender reversal, it is an ability to carry out the reproductive functions of both genders at once — and not as a hermaphrodite. Rather, with parthenogenesis a special cell called a polar body that’s produced with an egg sometimes acts like a sperm to “fertilize” it.

Scientists once believed that parthenogenesis occurred only among animals in captivity when males were in short supply. Captive female sharks, turkeys and lizards have all reproduced sans sex. But researchers have since observed virgin births among wild snakes even when males are plentiful. Why? You’ll have to stay tuned for further research.

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Bearded dragons

Photo: jjron/Wikimedia Commons

These delightful Australian gender-benders actually do a sex reversal in the egg. Studies show that when warm temperatures occur during egg incubation, male bearded dragons often reverse course to become female. But it’s not a clean switch. They actually remain male genetically, but act and reproduce like females. Even more extraordinary, these he-she lizards are supermoms, laying twice as many eggs as a normal female. Male bearded dragons are currently undergoing sex reversal at a rising rate, likely due to the spike in global temperatures.

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Green sea turtles

Photo: Brocken Inaglory/Wikimedia Commons

Like bearded dragons, green sea turtle embryos are also temperature sensitive. The warmer the sand where eggs are laid, the more females are born. In fact, according to a new study, sea turtle hatchlings from beaches near Australia’s Great Barrier Reef, where global warming is particularly intense, are now 99.8 percent female. On cooler beaches to the south, the gender ratio is 65 percent female to 35 percent male.

What impact might this dramatic gender imbalance have? The researchers conclude that females may seek mates in cooler climates so reproduction continues. However, if too many females hatch and can’t find a mate, sea turtle populations could collapse.