The Evolution of Sex and Gender: Chase Mendenhall on the power of diversity

Eli Savage contributing to The Evolution of Sex and Gender lecture at the Carnegie Museum of Natural History


The Evolution of Sex and Gender: Chase Mendenhall on the power of diversity

Author: Eli Savage

Undergraduate student and CMNH content contributor

Chase Mendenhall’s talk on March 7 in the Carnegie Museum of Natural History (CMNH) provided a broad and informative overview of the biological basis for sex and gender diversity. Mendenhall began at the cellular level, detailing how some bacteria extend fibers to one another that serve to transmit information that helps to promote each bacterium’s continued survival. Though this may seem irrelevant to us as humans, leagues removed from our distant, unicellular predecessors, he explained how this is the most basic framework for why diversity is crucial for groups to thrive. He used children playing board games as a human example: if one child finds a new game to play, it is in the best interest of the group for that child to introduce the new game to the group, expanding everybody’s options for play and allowing each child to pick a game best suited to their skillset, maximizing the group’s efficiency and well-being.

Mendenhall then explained that an organism’s sex, as defined in biology as a near-universal rule, comes down to whether its gametes are relatively large or small. That, he said, was it. Everything else we tend to associate with biological sex—ovaries, testes, penises, vaginas, pregnancy, breasts, etc.—varies wildly, and are viewed in contemporary biology as gendered characteristics rather than sex characteristics. Gender, while a difficult term to define in any concrete and all-encompassing way, can be defined as everything that is associated with or extrapolated from an organism’s sex, from its body parts to its role in a family to the way it does or should act. 

To illustrate these principles, Mendenhall listed numerous cases of non-human animals that diverge from human ideas of “the norm” when it comes to gender and sexuality. One of these was the long-tailed manakin, a species of tropical bird in which males exhibit almost exclusively homosocial behavior—two males will pair up for life and spend years choreographing an elaborate, beautiful courtship dance to perform for a female during the brief annual mating season. An interesting mystery in this dynamic, though, is that only one of the two males does the vast majority of the mating, and no difference has yet been found between the roles. It is unclear what the other male gains from this arrangement, aside from one chance in a hundred to mate with a female, but their dance, a stunning and mesmerizing display, would not be possible with only one bird.

Another example was the clownfish. Mendenhall joked that if Finding Nemo had been accurate to the actual behavior of the species, the film would have gone very differently. Clownfish, he explained, live in groups of many males and one egg-bearing female. If the female dies, the males have a solution: one among them changes sex and becomes the new female. This process is beneficial to the survival of the group, and it’s hardly unique to clownfish—many species of fish, amphibians, and other animals practice sex-switching.

Mendenhall finished his talk by returning to humans, providing two anthropological cases of gender and sexual diversity being beneficial to groups. One being the bissu of the Bugis society in Indonesia, an inter-gender priestly class, and the other being Native American two-spirits, or individuals viewed as containing male and female spirits in one body. Stressing that diversity is a key feature, if not the key feature, of the survival and development of all life. It is not something to be feared or discouraged, but embraced as a natural continuation of how we got to where we are now.

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