Think sex and gender are simple? Nature says otherwise. Sex—one small word with huge implications. To most humans, being male or female implies a certain set of inseparable biological and sociological characteristics, but the natural world around us is rarely so black and white. For every characteristic that we associate with a particular sex, the animal kingdom harbors at least one surprising exception; concepts that we believe are inextricably linked are uncoupled, and even reversed, in other species.
Appearances can be deceiving - The spotted hyena may have one of the most arresting sexual incongruities in the animal kingdom. Female hyenas possess what scientists call "pseudopenises," since they so closely mimic male penises. These female organs are fully erectile, can be up to seven inches long, and are accompanied by a “pseudoscrotum.” Stranger still, a female hyena must urinate, copulate, and give birth through her pseudopenis. No wonder that, for centuries, people thought hyenas were hermaphrodite—watching a mom with a large penis nurse her babies can be a little confusing.
In a different kind of genital reversal, many males in the animal kingdom lack a penis entirely. In more than 97 percent of bird species, males have no external sex organ at all; instead, both males and females have vents called "cloacas." In these species, mating involves the romantic-sounding “cloacal kiss,” where the two birds line up their genital vents for the transfer of sperm.