11 Animals That Can Change Their Sex

A green sea turtle swimming on the Great Barrier Reef.
A green sea turtle swimming on the Great Barrier Reef.

Diane Keough / Getty Images

For Mother Nature, biological sex isn’t always an either-or proposition. Some species come with simultaneously functioning female and male organs. Others change from female to male or vice versa, depending on need or surrounding conditions. The reasons behind this fluid mobility are varied: Some are natural processes that offer a species reproductive flexibility, while others aren’t so natural, often sparked by rising global temperatures.

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

1
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Clownfish

Three orange and white clownfish in a sea of green anemone.

 © RAZVAN CIUCA / Getty Images

Bright orange with three white bars, clownfish are sequential hermaphrodites, born one sex but able to switch to the other if necessary. In this case, the about-face, which is called protandry, runs from male to female.

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

Two hawkfish with orange spots on a reef in the Maldives.

atese / Getty Images

These vibrantly colored harem dwellers are 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 to turn into a male hawkfish and split away with half the harem.

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

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Sea Bass

A giant black sea bass in a kelp forest underwater.

Gerard Soury / Getty Images

Black sea bass, found throughout the U.S. from Maine to the Florida Keys, are protogynous hermaphrodites, animals that can change from female to male. Because the sea bass population is spread over a large range, it is difficult for scientists to observe their reproductive behavior in their natural habitat. However, research of sea bass in tanks has revealed that the sex change may be related to supply and demand. When female sea bass observe a decrease in the male population in an adjacent tank, they switch.

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Humphead Wrasse

A blue-faced humphead wrasse on a colorful coral reef.

Vladimir Mladenovic / Getty Images

Another protogynous hermaphrodite is the humphead wrasse. Beginning at about 9 years of age, the humphead wrasse is able to change from female to male. Along with sex, the coloration of the humphead wrasse will change from reddish orange (female) to blue-green (male). Although they can live for 30 years, humphead wrasse are endangered due to overfishing, export trade, and threats to their coral reef habitat.

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Banana Slugs

A bright yellow banana slug on a green fern in the Redwood National Forest.

Ed Reschke / Getty Images

Bright yellow and up to 10 inches long, these wormlike mollusks are simultaneous hermaphrodites, meaning they don't change back and forth, but use their male and female reproductive organs at the same time. Although capable of self-fertilization, most banana slugs prefer to find a partner. When it comes time to mate, two slugs curl around one another and engage in a reciprocal exchange of sperm that fertilizes each slug's eggs.

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Butterflies

A butterfly on a lavender flowering plant displaying gynandromorphism, or the coloration of both male and female genders.

Burkhard Hinnersmann / Wikimedia Commons / CC BY-SA 3.0

In some creatures, like butterflies, the split is visible over their entire bodies. Some Lycaeides butterflies display a rare dual condition called gynandromorphism that can cause male and female traits to be arranged either haphazardly or bilaterally with one side male and the other equally female. Gynandromorphism is found in crustaceans, insects, birds, and perhaps most spectacularly, in butterflies. This unique phenomenon occurs in approximately one in 10,000 butterflies.

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Cardinals

Shirley Caldwell took this photo of a rare gynandromorphic cardinal through her kitchen window.

Courtesy of Shirley Caldwell

Bilateral gynandromorphism also occasionally shows up in northern cardinals. Since male and female cardinals have different coloration, it’s easy to spot a gynandromorph—which has brown-gray "female" feathers on one half and bright red "male" feathers on the other. According to a study, gynandromorph cardinals not only look different, but they also act differently, at least the one researchers observed from 2008 to 2010. During that time, the “half-sider” was never seen vocalizing or mating. On the plus side, other cardinals seemed to accept it: The researchers never witnessed it being mistreated.

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Frogs

A green frog sitting on a bed of green and brown moss.

Contrabaroness / Wikimedia Commons / CC BY-SA 3.0

For years, researchers have observed frogs spontaneously changing sex in the lab; now they have done the same studies in the wild. Their work suggests that sex change, complete with fully functioning reproductive organs, may be fairly commonplace among green frog populations. While prior research indicated that sex reversal in frogs may be related to pollution introduced by humans, the same scientists’ current research suggests that the change may be a natural occurrence in amphibians.

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Copperhead Snakes

A copperhead snake with its head facing upward laying on dry leaves with green plants behind it.

Joe McDonald / Getty Images

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

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Bearded Dragons

An Eastern bearded dragon stretched out on a fallen tree with its head upright.

jjron / Wikimedia Commons / CC BY-SA 3.0

The delightful bearded dragons actually perform 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 complete switch. They actually remain male genetically, but act and reproduce like females. What's more, these non-binary lizards lay twice as many eggs as normal females. 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

A green sea turtle swimming in shallow water over a reef.

Brocken Inaglory / Wikimedia Commons / CC BY-SA 4.0

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 study, sea turtle hatchlings from beaches near Australia’s Great Barrier Reef, where global warming is particularly intense, were 86.8 to 99.8 percent female. On cooler beaches to the south, female hatchlings ranged from 65 to 69 percent.

What impact might this dramatic sex imbalance have? The researchers conclude that females may seek mates in cooler climates so reproduction continues. However, if too many females can’t find a mate, sea turtle populations, which are already endangered, could severely decline.