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 and change.