Gender fluidity
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.
Clownfish
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.
Hawkfish
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.
Bass
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.
Moray eels
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.
Banana slugs
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.
Butterflies
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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