ITHACA, N.Y. -- In mammals, sperm from the male determines the sex of the offspring. In birds, however, it is the female's sex chromosome that determines offspring sex. Now, Cornell University researchers think they understand the mechanism that several bird species use to bias the sex ratios of their offspring toward female.
By experimenting with domestic chickens, they have determined that the presence of higher-than-normal levels of the hormone progesterone during the first meiosis -- the cell division that divides the sex chromosomes and genetically determines the sex of an offspring -- produces significantly more females.
"For years, behavioral biologists have been trying to figure out how the females of a few species, such as the Seychelles warbler, the zebra finch and tree swallow, adaptively manipulate the sex of their offspring before an egg is laid," says Stephanie Correa, a doctoral student in neurobiology and behavior at Cornell and the lead author of the study that was posted online recently in The Royal Society's Biology Letters (Vol. 1, 2005). "Most investigators have looked primarily at testosterone, but we decided to look at progesterone, the major hormone produced by the female bird's preovulatory follicle."
To test the influence of progesterone on the sex ratio of offspring, Correa, along with Elizabeth Adkins-Regan, professor of neurobiology and behavior and of psychology, and Patricia Johnson, professor of animal science, both at Cornell, experimentally manipulated levels of progesterone in female chickens during the first meiosis. They injected either a low dose or a high dose of progesterone (dissolved in sesame oil) or a control dose of pure sesame oil.
The low dose of progesterone produced 61 percent males, which was very close to the control group rate of 63 percent males. The high dose of progesterone, however, produced only 25 percent males.
"Thus, we think that how much progesterone the female follicle produces during the first meiosis is the mechanism in birds that manipulates the sex ratio of offspring," Adkins-Regan says.
"Researchers think that birds such as the Seychelles warbler may bias the sex of their offspring when, for example, their territory has plenty of food and the mother bird needs help in feeding the next generation," says Correa. Since males disperse, the mother bird may adaptively bias her offspring to female so that plenty of females from one generation will be available to help raise the next generation.
Although the finding is not of practical use in the near future for the poultry industry, Correa and Adkins-Regan say that understanding the basic mechanism of biasing sex ratios in birds could provide the foundation for learning how to manipulate the sex ratio of avian offspring in the future. Research in this area is just beginning, they note, because the molecular methods used to determine the sex of the eggs have only recently been made available.