Hen Social Position Shifts Egg Hormones. (Chicken Rank)
Milius, S., Science News
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 Muller of the University of Groningen in the Netherlands. The male-to-be eggs of top females, however, have an extra dose of these hormones, Muller 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 Muller. "But if you're a bad female, you still have matings."
The study by Muller 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. …