Chicken Rank: Hen social position shifts egg hormones

A study of leghorn chickens has linked hormone concentrations in a hen’s eggs to her rank in the pecking order.

The yolks of low-ranking hens’ eggs harboring males have about the same concentrations of testosterone and one of its chemical precursors as do yolks of eggs containing female embryos, says Wendt Müller of the University of Groningen in the Netherlands. The male-to-be eggs of top females, however, have an extra dose of these hormones, Müller and his colleagues report in an upcoming Proceedings of the Royal Society of London B. The researchers speculate that parental investment of extra male hormones may make chicks more robust, competitive, and therefore more likely to reproduce.

The new finding complicates other scientists’ notion that such an extra dose of hormone might make an egg turn out male instead of female.

Theories of parental favoritism rest on the idea that, in bad times–such as when a hen is near the bottom of the pecking order–sons may not grow up to win the mating game. Then, investing in daughters is a safer bet because in many species, including chickens, low-ranked females are more likely to have offspring than low-ranked males are. “If you’re a bad male, you don’t get any success,” says Müller. “But if you’re a bad female, you still have matings.”

The study by Müller and his colleagues adds to a growing body of evidence supporting shifts in maternal favoritism. Some females even adjust the ratio of male to female offspring depending on the circumstances.

Just what biochemical mechanism might be behind such shifts has been quite a puzzle.

Scientists have considered hormones in the yolk, which is produced before the sex of the chick embryo is determined. A 1993 study hadn’t found differences in male hormones between yolks of male and female canary eggs. But Marion Petrie of the University of Newcastle in Newcastle upon Tyne in England and her colleagues last year reported finding that the concentrations of four male hormones in the egg yolks of 10-day-old male peafowl embryos differed from those in yolks of females. Petrie’s study didn’t consider pecking order.

In their recent work, Müller and his colleagues checked testosterone and a hormone precursor, androstenedione, in 3-day-old eggs from five leghorn groups. When the researchers didn’t take into account the rank of the mothers, the two sexes didn’t show consistent differences in yolk hormones. However, when the investigators examined only eggs from plump, high-ranking mothers, differences between the sexes appeared.

This result dashes any hopes scientists might have had that sex-ratio manipulation could be as straightforward as adding a dash of testosterone to the yolk to make males. “It’s not so simple,” Müller concludes.

There may not be a direct link between egg sex and yolk hormones, but the Müller experiment can’t completely dismiss the idea, says Petrie.

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