13 Animals That Can Change Their Gender

When naturalist John Muir said (via Goodreads), "The clearest way into the Universe is through a forest wilderness," he probably wasn't thinking of sequential hermaphroditism or bilateral gynandromorphism. But these exceptional traits contribute to what makes animals such marvels of our peculiar world. Indeed, there is no greater source of inspiration than nature and all its furry, feathery, and scaly inhabitants.

Did you know there's a fish that can change gender back and forth, according to the needs of the group? Or that there's also a type of sea snail that changes gender when males touch each other. Incredibly enough, this works in their favor. Or perhaps it's not that incredible — animals have an amazing way of adapting to new challenges and helping their species along the way. As per National Geographic, by reversing their gender, frogs are slowly clearing out negative mutations. But there's so much more to sex swaps in the animal kingdom — let's look at 13 animals that can change their gender.

As per this PeerJ article, gender change in frogs is not considered a usual phenomenon. Yet, it's been noticed more and more frequently, especially in green tree frogs. According to National Geographic, pollution is one of the factors that has been observed to contribute to gender change in frogs — particularly, the presence of herbicides and synthetic estrogens. Strangely, these only cause male frogs to turn female and not the other way around.

However, more recent research (including the PeerJ article) found that frogs reverse their gender in natural environments, too. Researcher Rick Shine explained: "This isn't just a story about pollution — instead, it suggests that frogs can adjust their sexual destiny to local circumstances." After studying green frogs in 18 different ponds, scientists observed an average of just under 5% of frogs changing gender. No number one cause has been identified, and it's unclear why one pond had 10% sex-reversed frogs (a much higher number than the other 17).

But researcher Max Lambert points to a potential goal: Gender change (which only happens in the frog's infancy) can develop genetic diversity and remove a few bad mutations along the way. Whatever the causes and effects of gender switches in frogs, there's still much to learn about the cute amphibians.

As per Forbes, a mammal's gender is determined by their sex chromosomes: XX for females, and XY for males. In reptiles, it's usually the outside temperature that influences the embryos' gender. But bearded dragons are a special kind of reptile. At regular temperatures, they develop as per the chromosome determination system. But when it gets very hot (above 32° Celsius or 89.6° Fahrenheit), some bearded dragons with male chromosomes turn female. And if it's above 36° Celsius or 96.8° Fahrenheit, all genetic males reverse their gender to female.

It gets even more interesting. According to ABC Science, females with male chromosomes (ZZ) can produce twice the amount of eggs as the females who didn't change gender (ZW). By the way, ZZ females paired with ZZ males have healthy babies. The only difference is that their babies are more prone to switching genders at high temperatures — bearded dragon see, bearded dragon do.

While fascinating, this phenomenon begs the question: with global warming being a well-known fact today, how long until all bearded dragons turn female?

As per Reef Builders, blue ribbon eels are rarities to the human eye, because they spend most of their lives tucked in holes at the bottom of the reef waters. But when they do come out, it's a real spectacle, as shown in this Dodo video. But their beautiful colors and amazing swim patterns are not the only traits that make ribbon eels special. They're protandrous hermaphrodites — similar to clownfish, ribbon eels can turn from male to female during their lifetime.

But unlike clownfish, all ribbon eels undergo this change, as Ocean reports. They are born male, and all juvenile males are pitch-black, later developing a yellow dorsal fin. Then, as they become adults, they become blue with yellow accents, which is how they spend most of their lives. As per Australian Geographic, the males keep growing in size with age, but they also start developing female organs in the meantime. Soon enough, the ribbon eels become female, sporting a bright yellow. But according to Adventures of an Aquaholic, the change only happens some 20 years into the eel's life, and the female only gets to live for about a month — just enough time to lay eggs and ensure another batch of tiny, pitch-black ribbon eels.

Hawkfish are one of the very few species that can change gender back and forth. As per Science and the Sea, many fish can only change from female to male, and this only happens once they have reached a certain stage in their lives (either sexual maturity or the moment they meet potential mates). Others, like southern flounders, change from female to male if the water temperature increases. But when it comes to hawkfish, gender changes are socially motivated.

As New Scientist explains, hawkfish live in harems, made up of one dominant male and multiple females. But sometimes, the male takes too many females to reproduce with all of them. That's when one of the larger females in the group switches gender. Interestingly enough, if the harem loses a lot of females, or if the newer male is challenged, he can reverse his sex back to female. As you might expect, this behavior works in the favor of the species, as scientist Tatsuru Kadota explained: "The ability to undergo bidirectional sex change maximises an individual's reproductive value."

Much like cardinal birds, butterflies sometimes exhibit bilateral gynandromorphism. While gynandromorphic butterflies are a marvel to look at, the specifics of how this happens genetically are still, largely, a mystery. As per the New York Times, there is a probable explanation, at least when it comes to gynandromorphism in birds: The female lays a double-nucleus egg cell, containing both a Z and a W chromosome. When each is fertilized by a Z chromosome sperm, some cells become ZZ and others ZW within the same hatchling. According to Natural History Museum U.K., some scientists believe the process is quite similar in butterflies — basically, two sperms enter the same egg, one fusing with the nucleus and the other one developing in the cell fluid.

In butterflies — just like in some bird species — gynandromorphism isn't always bilateral. This means that the male and female cells aren't always distributed evenly, left and right. These individuals are called mosaic gynandromorphs, and their patterns can be quite astonishing. About one in 1,000 butterflies will show a form of gynandromorphism — they are very rare, but they are larger than life. Today, several doctors and scientists are studying gynandromorphic butterflies in order to gain insight into human sex chromosome-related diseases and how some of these affect one gender more than the other.

Although oftentimes seen in restaurant menus, sea bass are mysterious creatures with complex lives. As per Rutgers University, black sea bass are protogynous hermaphrodites, so they are born female and are able to turn male soon after reaching maturity, somewhere between 2 and 5 years old. While not all scientists have agreed on a theory regarding the reason for gender change in sea bass, the most likely reason is to ensure the species thrives by balancing out the numbers. Assistant professor Olaf Jensen commented, "It sounds crazy, right? But from an evolutionary perspective, it's a perfect way to keep balance in a population. If it's operating out in nature, maybe we don't have to worry so much about fishing pressure removing the big males and skewing the sex ratio." Assistant professor David Berlinsky confirmed that female sea bass turn male when there are no males around, in a study published by Science Daily. But Berlinsky also noticed that a constant temperature is more likely to trigger an early sex change. 

Sea bass gender change research was partially motivated by the need to better assess the stock for the fishing industries. Because sea bass are born female, population sizes have been estimated erroneously for decades, leading to season closures or sea bass population declines. Jensen wrote: "This research represented an important first step toward properly accounting for the complex life history of black sea bass in the species' assessment and management."