Here are 11 creatures that offer a fascinating glimpse into the many ways s.ᴇ x can develop.
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.
In some creatures, like ʙᴜᴛᴛerflies, the split is visible over their entire bodies. Some Lycaeides ʙᴜᴛᴛerflies 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 ʙᴜᴛᴛerflies. This unique phenomenon occurs in approxiᴍᴀᴛᴇly one in 10,000 ʙᴜᴛᴛerflies.
3. Bearded Dragons
The delightful bearded dragons actually perform a s.ᴇ x 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 ɢᴇɴᴇᴛɪᴄally, 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 s.ᴇ x reversal at a rising rate, likely due to the spike in global temperatures.
For years, researchers have observed frogs spontaneously changing s.ᴇ x in the lab; now they have done the same studies in the wild. Their work suggests that s.ᴇ x change, complete with fully functioning ʀᴇᴘʀᴏᴅᴜᴄᴛɪᴠᴇ ᴏʀɢᴀɴs, may be fairly commonplace among green frog populations. While prior research indicated that s.ᴇ x reversal in frogs may be related to ᴘᴏʟʟᴜᴛion introduced by humans, the same scientists’ current research suggests that the change may be a natural occurrence in amphibians.
5. Banana Slugs
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 ʀᴇᴘʀᴏᴅᴜᴄᴛɪᴠᴇ ᴏʀɢᴀɴs at the same time. Although capable of self-fᴇʀᴛɪʟɪᴢᴀᴛɪᴏɴ, most banana slugs prefer to find a partner. When it comes time to ᴍᴀᴛᴇ, two slugs curl around one another and engage in a reciprocal exchange of sᴘᴇʀᴍ that fᴇʀᴛɪʟɪᴢes each slug’s eggs.
6. Sea Bass
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 haʙɪᴛat. However, research of sea bass in tanks has revealed that the s.ᴇ x 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.
Bright orange with three white bars, clownfish are sequential hermaphrodites, born one s.ᴇ x 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 s.ᴇ xually mature, a large male and an even larger female. The rest are smaller, s.ᴇ xually immature males. If something happens to the female in the breeding pair, her male ᴍᴀᴛᴇ transforms into a female and selects the next biggest male in the group to become her new partner.
8. Green Sea Turtles
Like bearded dragons, green sea turtle embryos are also temperature-sᴇɴsɪᴛɪᴠᴇ. 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 s.ᴇ x imbalance have? The researchers conclude that females may seek ᴍᴀᴛᴇs in cooler cliᴍᴀᴛᴇs so reproduction continues. However, if too many females can’t find a ᴍᴀᴛᴇ, sea turtle populations, which are already enᴅᴀɴɢᴇʀed, could sᴇᴠᴇʀᴇly decline.
9. Copperhead Snakes
Some female snakes, such as copperheads, are capable of virgin birth, or parthenogenesis, meaning the female fᴇʀᴛɪʟɪᴢes their own eggs without a male s.ᴇ xual partner. While not technically a reversal, this is an ability to carry out the reproductive functions of both s.ᴇ xes 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 sᴘᴇʀᴍ to “fᴇʀᴛɪʟɪᴢe” it.
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 ᴍᴀᴛɪɴɢ. On the plus side, other cardinals seemed to accept it: The researchers never witnessed it being ᴍɪsᴛʀᴇᴀᴛᴇᴅ.
11. Humphead Wrasse
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 s.ᴇ x, 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 enᴅᴀɴɢᴇʀed due to overfishing, export trade, and ᴛʜʀᴇᴀᴛs to their coral reef haʙɪᴛat.