Science in the Triangle

Ryan Calsbeek

I didn’t feel kinship with female lizards until I listened to Ryan Calsbeek talk about women having a say in whether their children will be boys or girls.

Calsbeek has studied natural selection among lizards and spoke about his research Thursday at N.C. State University’s biology department. He is an assistant professor of biological sciences at Dartmouth College and a visitor at the National Evolutionary Synthesis Center in Durham where he is working on a book.

Experiments he and his team have run with brown anoles, lizards native to Cuba and the Bahamas, suggest that the female lizards sort sperm to have the fittest males father more male offspring. It’s unclear how female brown anoles do that, Calsbeek said, but they’re not the only ones doing it.

“It’s even true in humans,” he said.

The remark generated quips from the audience about the exclusively female offspring of three consecutive U.S. presidents and the girl that Calsbeek’s pregnant wife is expecting. But Calsbeek argued that statistically four examples don’t mean much. Throughout history, women whose mates were presidents and kings tended to overproduce sons, he said.

Still, why should I care about sperm sorting among reptiles the size of my finger? Because brown anoles are, as Calsbeek put it, “the drosophila of lizards.” Both are model organisms. Just as the drosophila fruit fly has been extensively used to understand genetics, brown anoles can tell us something about the role female choice plays in the evolution of organisms, including ours, Calsbeek said.

Brown anole

More than 150 years ago, Charles Darwin picked up on female behavior patterns that ensure reproductive fitness down the generations.

Nature is full of examples. To a peacock hen lustrous tail feathers on a male signal he’s not parasite-ridden. Size and strength help male elephant seals drive away competitors and attract harems of up to 100 cows. Size is also important for female brown anoles to determine male fitness.

After World War II, another Brit, Angus Bateman, determined through drosophila experiments that female choice makes sense, because the number of offspring a female fruit flies can produce is limited more by how many eggs she generates than by how many mates she has. Bateman concluded that eggs are more precious than sperm, Calsbeek said.

Calsbeek and his team conducted breeding experiments with brown anoles to learn more about the choices the female lizards made. The findings they reported last year suggested the dams were very sophisticated.

The experiments showed that the size of the father only played a role in the number of male offspring that hatched.

Males are either losers or winners while females do pretty well regardless, Calsbeek said. “If you’re a loser in the animal kingdom, you’re probably a male. Sorry guys.”