Among humans, the conversation surrounding sex versus gender has been incredibly heated. Everyone from activists to medical professionals to keyboard warriors with myriad opinions has weighed in on the subject, yet it continues to move very little. The critical element of the debate focuses on sex as a set of biological traits and gender as a form of social expression. The common counterargument is that “deviance” like that found among humans is not found elsewhere in the animal kingdom, but that couldn’t be farther from the truth.

It’s In Your Genes

Most of us learn that, for humans, sex is determined by X and Y chromosomes. XX makes female. XY makes male. Such an explanation makes it all seem so straightforward, but the reality is far from being so black and white. Countless variations can occur surrounding the sex chromosomes and their expression, and not all of them are outwardly visible. Individuals with elements of male and female sexual genotyping fall into the category of intersex. Intersex conditions can lead to individuals feeling like their apparent sex “isn’t right,” or it can go completely unnoticed.

When focusing solely on human biological sex and gender expression, it can be easy for someone new to the subject to become lost in the information. If we look outside of the human species, we’ll find that sex gets a whole lot weirder. Not all members of the animal kingdom have their sex determined by X and Y chromosomes, and not all biological females and males play the same role as we do when it comes to reproduction.

Changing The Game

Seahorses are possibly one of the most familiar examples. Male seahorses contribute sperm to the equation, but the females deposit the eggs into the male instead of the other way around. Male seahorses aren’t the only ones with unusual roles. Certain species of frog and fish can change their biological sex depending on environmental conditions and stages in their lifecycles. Other creatures, like earthworms and some snails, get the best of both worlds, carrying both male and female gonads.

Birds are some of the most baffling examples. Ducks have been known to choose same-sex partners, along with about 10% of species in the animal kingdom. Homosexual pairing is extremely common, from birds to mammals, and though it does not further procreation, scientists believe it must offer some form of evolutionary advantage. On the physical front, intersex characteristics also appear outside of humans.

The Janus-Faced Cardinal

Sightings of two-tone cardinals have been popping up across the United States. Cardinals are one of the animal kingdom’s more obvious examples of sexual dimorphism or a difference in appearance between males and females. Male cardinals sport bright scarlet plumage while females wear a more easily-camouflaged tawny. These unusual gynandromorphic cardinals, however, are split down the middle, wearing red on one side and tan on the other. They form when a pair of would-be fraternal twins become fused during the developmental process and become one embryo, bearing both male and female biology.

Many two-tone cardinals have been spotted, but only a few have been studied. One of these birds seemed to have difficulty finding a mate, leaving scientists unsure whether or not it was capable of reproduction. Another seemed to blend right into the flock, landing themselves a mate and bonding like any other pair would. Other avian gynandromorphs have displayed unusual blends of sex characteristics as well, including one where the female gonads were on the opposite side of the body but corresponding to the phenotypically female side. Another presented a combined ovary/testis. Based on these studies, it seems safe to conclude that genetic variation and sex chromosome expression is just as complicated and diverse across the animal kingdom as it appears to be in humans. Perhaps we’re not so different after all.