The more fertile a male red deer is, the more likely he is to sire a son

Debora MacKenzie:

It is the first study to demonstrate that male mammals can influence the gender of their offspring through sperm quality.

Red deer stags go to spectacular lengths to mate, defending harems of females and fighting off competitors. Now scientists have discovered that those who win at the rut have more sons, to carry on their fathers’ winning ways. The losers at the rut play it safe with daughters. Or they try to – how much the females influence this remains unclear.

It has long been theorised that males who are most successful at mating would produce more sons – to inherit their fathers’ brilliant plumage or big antlers – than daughters. Sons produce more offspring than daughters can.

While this has been shown for blue tits (see It's the healthy bird that fathers more sons), where the egg determines the sex of the offspring, it has never been shown for mammals, where sperm do.

Montserrat Gomendio and colleagues at the National Museum of Natural Sciences in Madrid, Spain, have now demonstrated this in red deer.

The team collected testes from stags hunted during the rutting season, and assessed their sperm quality. Then they inseminated well-fed females. “We were very surprised to see a large difference in fertility,” Gomendio told New Scientist. The proportion of females a stag could make pregnant (the stags' fertility) ranged from 24% to 70%.

Surprisingly, the sex ratio of a stag’s offspring varied from 72% male, to 75% female. and the most fertile males had the most male offspring, the team found.

The most fertile males also had the most complex antlers, which attract females and defeat other males. So, as predicted, successful males have more sons, which inherit their fathers’ traits and spread his genes. Less successful males produce daughters – their reproductive success is not affected by their fathers’ lack of male charms.

This could potentially lead to female shortages, as the dominant stags do most of the breeding. Fortunately, says Gomendio, most animals are in the middle of the range, and the overall sex ratio in the population comes out at around 50:50.

But how does the stag influence his offspring’s sex? Either successful males produce more sperm bearing the male-determining Y chromosome, or their Y-bearing sperm are more competitive, says Gomendio.

Complications arise when the interests of the females come into play. Big, well-fed females tend to have more sons – they spread her genes more effectively. But male calves need more and better milk (see Males get the best milk) so females in poorer shape have daughters, to ensure mother and offspring survive.

A poorly-off female may mate with a high-fertility male, but each has different interests in what gender of offspring to have. “We now have to find out how that works out in nature,” says Gomendio. “It may be that there’s a bit of an arms race.”

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