Better Sperm, More Sons

30 November 2006 (All day)

Julian Garde

Super dad.
Red deer fathers with high-quality sperm sire more sons.

Pregnant moms have no control over the sex of their offspring, but things work a bit differently in the wild. In some species, the healthier the mother, the more likely she is to produce sons. Now, a team of Spanish researchers has shown that fathers, too, can play a role in this sex selection: Among European deer, males with high-quality sperm are more likely to sire sons than daughters.

The measure of a parent's success is its offspring's ability to reproduce. That's because more descendents means more of the parent's genes in the gene pool. Under certain conditions--say, a skew in the sex ratio of a population--however, parents can benefit by producing more sons than daughters, or vice versa. Although a mother's role in sex selection is well-documented, little attention has been paid to the father's role in this phenomenon.

That led ecologist Montserrat Gomendio and her colleagues at the Museo Nacional de Ciencias Naturales in Madrid to investigate sex selection in a southern Spain population of wild red deer, a group of animals that has been used as a model system for studying sex ratios. The researchers collected semen from 14 males and artificially inseminated a captive herd of 344 females. Each female received semen from only one male. The researchers calculated each male's fertility rate by comparing the number of females he inseminated to the number of females that became pregnant. The researchers found that more-fertile males produced more sons, whereas less-fertile males produced more daughters. Highly fertile males also had a higher proportion of normal-shaped sperm, as opposed to less-fertile males, which had more misshapen sperm, the team reports tomorrow in Science.

The findings make sense, says Gomendio. "Sons inherit the fertility of their fathers," she says, so fathers with high-quality sperm maximize their genes' presence in the gene pool by siring sons. Fathers with lower-quality sperm, meanwhile, keep up their representation in the gene pool by siring daughters, who don't inherit their father's fertility problems. These competing strategies balance out to produce a 50:50 sex ratio in the wild red deer population, the researchers report.

"I think the paper is going to change our field," says evolutionary ecologist Patricia Adair Gowaty of the University of Georgia, Athens, who notes that, until now, most researchers have focused on the mother's role in skewing sex ratios. But evolutionary biologist Stuart West of Edinburgh University in the United Kingdom says that although the study is both novel and useful, the research should be repeated on a larger group of animals before any definitive conclusions are drawn.

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