For a lioness, the safest bet would be to have a girl cub, according to a new study that says mammals have the ability to select the gender of their offspring for the benefit of their species. With sons, a lioness could end up with zero grandcubs or hit the genetic jackpot. (Ilnar Salakhiev/AP)

Mammals appear to have the ability to select the gender of their offspring to maximize their number of grandchildren, according to a new study that followed three generations of more than 2,300 animals from the San Diego Zoo.

It is not a conscious choice, but in some way, a female’s biology has the capability to assess her health, the quality of her mate, and her environment to determine which sex to go with, according to study author and Stanford evolutionary biologist Joseph P. Garner.

For a lioness, for example, the safest bet would be to have a girl. Even if her daughter isn’t the strongest or highest-ranking female, she will most likely have at least the average number of cubs.

Sons, on the other hand, are a high-risk but potentially high-reward bet. Most male lions don’t reproduce because they are beat out by the few alpha males that father the vast majority of cubs. With sons, she could end up with zero grandcubs, or hit the genetic jackpot.

Which direction the pendulum swings is not random but depends on exactly those factors of mother’s health, mate quality and dominance. If those swing in her favor, the researchers say, her body will bias toward having a boy.

From her point of view, “if I have insider-trading knowledge that my son will be the harem-holding male, then I should have males,” Garner said. “I should essentially cheat.”

Sex-ratio ma­nipu­la­tion, or the idea of creatures being able to select the sex of their offspring for the benefit of their species, is not new and has been confirmed in certain insects and birds.

“It’s especially interesting to find evidence in mammals,” said Yale evolutionary biologist Stephen C. Stearns, who was not involved in the study. Stearns finds the study “interesting and provocative” but said it needs further confirmation.

To really prove this idea works, you need a perfect, complete pedigree across three generations — near impossible to get in the wild. Garner and his team tested whether, for a female that produces only sons, those sons are actually outcompeting the majority of their male peers in the population by having more offspring.

That is, in fact, what they found. Those sons from a mostly male brood ended up having an average of 2.7 times more children as those whose mothers bore equal numbers of male and female offspring.

Garner said his team’s research “stands on the shoulders of giants,” referring to a revolutionary 1984 study in red deer showing that dominant females had more sons than their subordinate counterparts. But it only looked at two generations, so it was unknown whether those sons indeed went on to have more grandchildren.

The team analyzed 90 years of breeding records from 198 mammalian species to prove what Garner said has been a fundamental theory of evolutionary biology. The study was published online in the journal PLOS One on Wednesday.

The researchers looked across many different species — such as lions, tigers, hogs and gazelles — rather than a single one. Garner cites this as a limitation of the study, in that different species are most likely to have varying abilities.

Stearns also expressed reservations about the data being from zoo animals rather than gathered in the wild.

In mammals, how sex selection works remains a mystery. Early ideas regarding male influence, such as alpha males somehow developing a higher percentage of male-oriented sperm, were largely debunked.

“Which means females are really the ones controlling the situation,” said Garner, who calls the strategy “sneaky Machiavellian girl power.”

The egg could make itself more conducive to a certain sex of sperm, allowing one type in more easily than another, even if they reach the egg at the same time. Others speculate that male and female sperm have different shapes and can be slowed down or sped up via mucus in the female reproductive tract.

But sperm-stage theories are inherently flawed, Stearns said, since all sperm is molecularly very similar and thus very hard to tell apart. He believes selection happens at the zygote stage — after the egg is fertilized — when the mother’s body could more easily distinguish between tiny embryonic males and females.

And because so many mammalian species select for offspring sex, what about humans?

Opinions seems to differ among biologists. “Human beings are definitely doing this, too, there’s no question,” Garner asserted. Stearns is more skeptical, saying research up to this point does not give a conclusive answer.

Some evidence of sex-ratio ma­nipu­la­tion in humans exists. One study found that members of the Forbes billionaires list tend to have significantly more sons than the general population. Also, in certain polygamous societies, first wives are more likely to have sons than lower-ranking wives.