Why you should care

Because animal sex is perennially relevant. 

It’s a question we’ve surely all been dogged by: Why do whales have pelvic bones?

The quandary popped into biologist Matthew Dean’s mind one day six years ago. Dean had taken a stroll across the street from his office at the University of Southern California, where he teaches, into the Natural History Museum of Los Angeles, seeking some mouse specimens. He got to chatting whales with the mammals collections manager, Jim Dines. Dines was interested in how whales are classified. He told Dean that he’d been wondering about the whole pelvic thing: Swimming mammals haven’t needed legs since they ditched the land life some 40 million years ago, and pelvic bones seemed like they were there for legs.

Dean listened thoughtfully. One answer arrived: sex. Specifically that whale pelvic bones anchor the stuff that runs whale penises. And obviously, evolution wanted the whales to keep their penises, which happen to be the world’s largest — as sizable as eight feet long, depending on the type. “No one had ever considered that before, but that’s where his mind is,” Dines says. Five years later, that casual conversation broke through to a paper in the prestigious journal Evolution.

It’s not because Dean has a dirty mind or anything: Dean studies sex to study evolution, pulling aside the curtains during that racy moment when the genetics get handed over to the next generation. Though he’s just 41 and an assistant professor, Dean’s work has landed in enviable journals. Dean’s work is unique and up-and-coming because of his “integrative” approach, combining sex and evolution, says Patricia Brennan, an evolutionary biologist and research professor at the University of Massachusetts Amherst. Because of his focus, Dean gets to tackle bizarre traits, like ground squirrel penis bones shaped like sporks or sneaker jack salmon that steal sex partners from their aggressive male competitors by secretly slipping into the sexual fray at the critical moment, expelling faster sperm and more of it near the egg-laying female to overwhelm all other comers. (“They’re like a swimming testicle,” Dean says.)

This kind of evolutionarily focused work is crucial to understanding how animals function today, Brennan says. Studying sexual selection can help us fight antibiotic resistance and disease, breed new crops and save ecosystems, according to Brennan. Plus, she adds, with a twinkle of fun, “sex is intrinsically interesting.”

Whale scientists

Matthew Dean, left, and Jim Dines searched through more than 10,000 boxes of unsorted cetacean bones in search of pelvic bones.

Source Gus Ruelas/USC

Dean didn’t start out in life knowing he would be surveying whale penises for science. As an undergraduate at the University of Wisconsin at Madison, he majored in insects, thanks to his love of fly fishing. Growing up in Novato, California, just over the Golden Gate Bridge from San Francisco, he was so dedicated that he regularly drove four hours to the Sierra Nevada mountains to go fly fishing. “It’s mostly the hunt,” Dean says about why he loves it. “From the moment you’re stepping into the stream, you’re pretending to be a mayfly.” With an oval face and regular grin, Dean, a father of two, is all energy, favoring tees or occasional Hawaiian shirts. Dines describes Dean as hugely enthusiastic: “Even in this stage in his career,” Dines says, ”as a professor heading up a laboratory, he’s like a little kid.”

If you’re wondering what you do with a B.A. in entomology … you get a Ph.D. in biology. The insect focus carried Dean through grad school, but it wasn’t until his postdoctoral time at the University of Arizona that he started thinking sex. His very first paper there showed that, in a fifth of pregnancies, female house mice carry babies from more than one dad. Meaning those lady mice get around, mating with multiple males in a short time span. Potential dads have to produce tough, competitive sperm. Dean uses funnier language: “It’s a constant arms race.”

No, really: Dean dug up, in another paper, an exposé on the competition those female mice induce in males — a veritable variety of chemical warfare. For example: Male mice have evolved ejaculate that coagulates in the female mouse cervix and forms a plug, serving as a shield, blocking other males’ sperm. The female mice have not let this lie. In retaliation, they’ve evolved proteins to break down that plug so they can keep on mating with other males. Retaliating, the males’ sperm have evolved inhibitors that stop the female’s proteins from breaking the plug.

Of course, like so much in scientific research, there’s far from consensus on Dean’s research. For instance, not everyone agrees that the purpose of the plug formed from ejaculate is to block other sperm. Some think it functions to trap sperm inside the female. And, fascinatingly enough, some of Dean’s work has actually gone in the exact opposite direction of what he’d prefer — in what he calls a “bastardization of science,” creationists have picked up on his studies as “proof there is a god,” he says, claiming that the whale paper has been cited by more creationists than evolutionary biologists.

But the details aside, Dean has a big vision for his work. “It’s why you exist,” he says. “If you’re not interested in life, you’re not interested in sexual selection.”

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