THE GIST

- Mothers can adjust the sex of their offspring to match the quality of the rearing environment.

- Genetics are a factor, but it's believed that mothers exert far more control than dads do.

- Stress hormones circulating in the mother may help to drive the process.

Mothers can adjust the sex of their unborn children in response to the environment where they live, according to new research.

The study, published in the latest Proceedings of the Royal Society B, finds that mothers exert far more control than fathers do over whether or not the couple has a son or daughter. The goal is to improve the child's survival.

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"It seems likely that when there are large and predictable costs associated with producing and/or rearing either sons or daughters in a given environment, females should bias offspring sex ratios to produce the sex that will perform best in the given environment," co-author Sarah Pryke told Discovery News.

"Altering offspring sex ratios in response to the quality of the local environment is likely to be highly advantageous to any species, as it should allow mothers to best match the phenotype of their offspring to the prevailing condition, and thus maximize their own fitness," added Pryke, a researcher in Australian National University's Research School of Biology.

Prior studies on birds, reptiles and mammals -- including humans -- has long suggested that this was the case, but scientists were unclear on what factors triggered the son or daughter outcome. Some researchers, for example, speculated that the overall body condition and health of the mother affected the outcome of her child's sex.

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To help eliminate that possibility, Pryke and colleague Lee Rollins studied a bird, the blue-faced parrot finch, whose body condition appears largely insensitive to changes in nutritional quality.

The researchers randomly assigned 56 of the female birds either a high-quality or low-quality diet. The former contained 20 percent protein, with egg, wheat germ, a seed mixture and more, while the latter contained only 8 percent protein. After 12 weeks on the diet, the birds were weighed and underwent blood tests to measure various aspects of their health. Based on these tests, all of the females were in comparably good and equivalent shape both before and after the 3-month study period.

Mother birds fed the lower quality diet, however, later produced far more sons than daughters.