Call it "sneaky Machiavellian girl power," as the lead researcher does in the Washington Post. His study in PloS One concludes that female mammals have the innate ability to determine the sex of their offspring. It's not a conscious decision—the expectant moms somehow factor in a host of environmental and societal factors. The upshot is that if they think a male would grow up to play a dominant role in the pack, they'll have a boy and thus lots of offspring. If they're not so sure, they'll go with a girl, who likely will have at least a few babies even if she's not a dominant member of her group.
“Which means females are really the ones controlling the situation,” says Stanford biologist Joseph Garner, whose team analyzed nine decades of breeding records from mammals at the San Diego Zoo. "If I'm producing nothing but daughters, I'm making a safe bet—I'm going to make the average," he explains to Science Daily. Sons, however, are "high risk, high reward" because non-dominant males might get shut out from breeding entirely. High reward, indeed: Females who produced the most males had up to 2.7 times the number of grandchildren as those with even numbers of male and female offspring, LiveScience reports. Though there isn't much evidence on how the theory applies to us, Garner says humans "are definitely doing this, too ... there’s no question." (More scientific study stories.)
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