For decades, evolutionary biologists have suggested that mothers can influence whether they give birth to sons or daughters, in order to maximize the genetic payoff for future generations. But does that strategy really work? Newly published animal research shows for the first time that the benefits of "choosing" the sex of offspring are extended to the offspring of those offspring as well.
"This is one of the holy grails of modern evolutionary biology — finding the data which definitively show that when females choose the sex of their offspring, they are doing so strategically to produce more grandchildren," Joseph Garner, a professor of comparative medicine at Stanford University, said in a news release about the research. Garner is the senior author of a study on the long-term reproductive game published Wednesday in the open-access journal PLOS ONE.
The study is based on an analysis of breeding records for more than 2,300 animals and their progeny, representing 198 different mammalian species at the San Diego Zoo. Garner told NBC News that the research marks the first time the effects of sex-ratio bias have been tracked methodically over three generations.
Transgenerational poker
"We are talking about a transgenerational game of poker, where the grandchildren are chips, and the cards are sperm," Garner said. The newly published data suggest that it's primarily the mothers who decide how best to play the game.
However, there are lots of mysteries surrounding this game. The mechanism that leads mothers to favor conceiving males or females is not yet known. Some researchers have suggested it may have to do with influencing the development of the egg so that it's more easily fertilized by sperm bearing the female X chromosome or the male Y chromosome. Or the mother may secrete substances that favor X-bearing or Y-bearing sperm during their race to fertilization. Or the mother may unconsciously use hormones or glucose levels to promote the development of male or female embryos.
The analysis laid out in PLOS ONE sidesteps those questions about the mechanism, and looks instead at whether a strategy that favors males or females actually works. Previous research has shown that among wild red deer, dominant females produced significantly more sons than deers that held a suborbinate position in the herd. But that study couldn't track how well those sons did in the next round of reproduction.
Evolutionary theory suggests that for most mammalian species, giving birth to males is a riskier bet with higher potential rewards, while giving birth to females is a safer, more conservative bet.
Garner cited the example of elephant seals. "In that species, every female produces a pup a year, while most males never breed. But the males that have a harem produce tens or maybe even hundreds of pups, just in one year," he said.
If the mother is dominant, she might be programmed to take more of a risk on having sons that will acquire their own harems — thus winning a genetic lottery. "It's essentially insider trading," Garner said. "If I can rig the bet, then I should. If I can't, then I shouldn't make that bet."
Tracing the payoff
To find out whether the bet pays off, Garner and his colleagues analyzed decades' worth of records on 38,000 animals. They ended up constructing family trees for 1,627 matriarchs and 703 patriarchs, representing species ranging from lions and tigers and bears, to rhinos and horses and hogs.
They found that when the matriarchs produced mostly sons, those sons had 2.7 times more offspring per capita than the males who were born to matriarchs with a 50-50 record on bearing sons vs. daughters. The daughters of female-favoring matriarchs had 1.2 times as many offspring as the daughters of the 50-50 matriarchs.
"In any 'war of the sexes' that's going on, the females will win. ... They're taking that sperm, but they're doing what they will with it," Garner said. "They're almost like voting with their offspring, and telling you what they think about the quality of their males."
Garner said the phenomenon applies to humans as well. For example, he pointed to a survey of 400 U.S. billionaires that found they were more likely to have sons than daughters. He also cited claims that the top-ranking wives in polygamous societies are more likely to give birth to sons than the lower-ranking wives.
"Humans are definitely responding to ancient cues, and modern cues," Garner said. "Does that mean that humans can to a certain extent, control the sex of their offspring? It seems as if they can, and it seems as if that works to the extent of 60 to 70 percent."
He cautioned, however, that biology sometimes works in mysterious ways. Any mechanism to influence the sex of offspring is probably hidden deep in the wiring of the human brain and reproductive system. "Evolution is not going to let you override that just by wishing really, really hard for sons, or wishing really hard for daughters," Garner said.
In addition to Garner, the authors of “Winning the Genetic Lottery: Biasing Birth Sex Ratio Results in More Grandchildren” include Collette M. Thogerson, Colleen M. Brady, Richard D. Howard, Georgia J. Mason, Edmond A. Pajor and Greg A. Vicino.