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.