Promiscuous Males And Choosy Females? Challenging A Classic Experiment
Of the 100 "top science stories for 2012" chosen by Discover Magazine, I am most fascinated by #42: "The Myth of Choosy Women, Promiscuous Men." It reports a serious challenge to an experiment that has remained a touchstone in evolutionary biology for over 50 years.
The study, on fruitfly mating, was done in 1948 by geneticist A.J. Bateman. Bateman showed that the male insects' strategy was to mate with many females, whereas the females' strategy was to be discriminating in their choice of partners. Male reproductive success, in other words, correlated positively with number of mates, but female reproductive success did not.
Now, ecologist and evolutionary biologist Patricia Gowaty and her colleagues Yong-Kyu Kim and Wyatt Anderson have repeated that study. They conclude something startling: Bateman blew it.
Before I explain where they say Bateman went wrong, I need to show how Bateman's conclusions rippled far beyond the scholarly world of fruitfly sex. His findings — promiscuous males, choosy females — seemed to strike a cultural chord. After biologist Robert Trivers cited it in a key 1972 paper on parental investment, the "Bateman principle" turned up everywhere. In my own field of primate behavior, for instance, field researchers expected to see (and thus often did) male primates with highly active sex lives and females who were coy, verging on sexual passivity.
And don't think that humans were left out of this picture. We've all heard it, right? When some big-name male public figure sleeps around, and cheats on his wife, isn't there always someone ready with an evolutionary explanation about vigorous male seed-sowing?
It's not that these Bateman-powered expectations have gone completely uncontested. Anthropologist Sarah Hrdy's 1981 book The Woman That Never Evolved pounded away, with careful field data, at the inaccuracy and sexism of assumptions about coy or passive primate females. As recently as 2009, though, scientists were still at work overturning the idea that Bateman's findings apply universally to humans.
Then along came Gowaty's team to take on the Bateman experiment itself.
In 1948, Bateman created isolated fruitfly populations and allowed free mating to occur within each. His goal was to count up the number of mates that males had, versus females. Back then DNA analysis wasn't available, so he couldn't link offspring with specific parents in that way. Instead, he reasoned that by using adult flies with mutations, he could trace an offspring back to a specific pair of parents. As Gowaty et al. wrote in their 2012 paper, he used "heritable, dramatic, and phenotypically obvious genetic mutations to identify the parents of offspring in small, replicated trial populations." The mutations included things like curly wings, thick bristles and deformed eyes.
Offspring, then, could have a single-dose mutation from either mother or father, or a double-dose, from both parents. (If they had no mutation, their parentage could not be traced.)
But here's where things went awry, as Gowaty et al. discovered when they ran the experiment using Bateman's methods. For one thing, the double-dose mutations seriously impacted offspring survival. "The crucial assumption of Bateman's method," Gowaty et al. wrote, "is that there is no reduction of offspring viability from inheritance of parental markers." But this turned out to be untrue — and because the non-surviving offspring weren't counted in the results, the data was skewed.
Also, Bateman's methods violated Gregor Mendel's laws of genetics. As Gowaty explained in an email message, sent to me on Monday:
Bateman's method over-counted the number of offspring for fathers, because there were more single-mutant offspring that had dad's mutation than mom's mutation. It's elementary, and essential in sexual species, that the number of offspring having Dads must equal the number with Moms.
The upshot? For both these reasons, Bateman didn't, in fact, show in any convincing way that fruitfly males are promiscuous and females are choosy.
The Gowaty et al. paper offers enough technical details to warm a geneticist's heart. For me, the best part is their study's broad implications. As Gowaty told me:
If we failed as a discipline to notice flaws [in the Bateman study], it makes one wonder, do we notice the flaws in other, more modern studies that also have results consistent with status-quo expectations?
Here, then, is a direct link between science and society. Because it made intuitive sense to many people that males would be promiscuous and females choosy, they were "dazzled" (Gowaty et al.'s word) by Bateman's central conclusion. But now, Gowaty told me:
The most important experimental data for the evolutionary justification for the double standard in humans is in question.
It's about time.
You can keep up with more of what Barbara is thinking on Twitter: @bjkingape