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The Biomechanics of Sex

UMass Amherst evolutionary biologist Diane Kelly uncovers the nuts and bolts of how baby animals get made.

Sex is something we take for granted: familiar enough to joke about, and a cultural priority, but how much have you actually thought about the fact that when a species evolves, its male and female representatives have to evolve the interlocking bits of their morphologies together

Diane Kelly


1. Calipers
2. Rat baculums (Penis bones)
3. Raccoon baculum
4. Walrus baculum
5. Model of a dolphin vagina
6. Force tranducer used to measure the force with which a penis pushes against female tissues during copulation

John Solem

You would be surprised how much is left to be learned about sexual anatomy. “There aren’t that many people who study genitalia,” says Diane Kelly, a lecturer in biology who specializes in reproductive tissues and evolutionary design. 

As a matter of course in her research, Kelly has created 3-D silicone molds of dolphin vaginas. Engineering a device to inflate cetacean penises, she used a beer keg because the keg could deliver the exact right psi. 

Besides being thrilling fodder for cocktail-party conversation, reproductive anatomy is key to understanding evolution. Kelly explains that males and females are two different halves of one physiological function—how to deliver sperm to an egg—and a species’ survival demands that those elements fit together with critical points of contact between them and little margin for error. So when a species evolves new genital shapes with evolution, Kelly says, the big question is: “How do we get from one morphological system to another, while still having babies?” 

Much of Kelly’s current work is with crocodilians: caimans, crocodiles, and alligators. “Nearly half of all crocodilians are endangered,” she points out, so understanding their anatomy helps species recovery efforts. Kelly has also been contributing to studies of cetacean reproductive anatomy: specifically, the corollaries in whales and dolphins between genital morphology and mating behavior (which is why she built the molds). 

One question she asks is if mating strategy—whether with single or multiple partners—has bearing on how a species’ reproductive tissues are structured. Kelly says of dolphin and whale vaginas: “Some species are simple. Some have multiple folds. Some turn into a 3-D spiral.” For a female dolphin who mates with many different males during a heat run, the lavish folds and spiral stair-cases inside the cetacean vagina may determine which suitor’s sperm makes it to the egg—the mother selecting the father of her calf after the deed has been done and putting the also-rans in a holding tank. This is called postcopulatory selection. 

“When you have only one calf a year, you want to make sure you have the best genetics,” says Kelly. 

Understanding how reproductive tissues work in internal fertilization—for instance, how erectile tissue can be flexible yet become stiff, which, Kelly points out, is a “big properties change!”— encompasses both mechanical design and a grasp of evolution. Kelly’s research involves innovative problem solving as well as gritty detective work. She smiles: “You would not want to see my search history.”