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Boy or girl? Female finches make the choice

Black female Gouldian finch choosing between black and red males.
[Photo by Sarah Pryke]

In some species, females can adjust the sex of their offspring when they are pregnant. Depending on the quality of their mate, females can sometimes decide if they will have a male or a female baby. This is a very adaptive trait that allows the females to make the most out of what they have their offspring, their mates, and food.

Now, researchers have shown that female Gouldian finches, a type of bird, can decide whether to have male or female babies, depending on the genes of their mate. They can also adjust the level of resources they provide to their offspring in order to continue passing on good genes to the next generation of finches.

These Gouldian finches have either red or black coloration on their heads, and females always try to find a mate who matches their color. But, when they cannot find a mate that looks like them, they might decide to have more sons than daughters in order to limit the number of daughters that could have more babies with the mixed genes.

Black Gouldian finch family: male, female, and juveniles.
[Photo by Sarah Pryke]

In an experiment, researchers Sarah Pryke and Simon Griffith painted the red finches black so they all looked the same regardless of their actual genes. Then, they mated the "colored" finches with both red and black females to see how the females responded. Since all of the male finches looked the same (black), the females could not tell their genetic makeup simply by looking at them.

The researchers found that black females that mated with red males (painted black) had approximately the same number of male and female offspring. However, when red females mated with red males that were painted black to hide their true identities, the females gave birth to many more males (72 percent of their eggs), even though their color genes were actually the same.

When females were mismatched with a male of a different color, even if the color was painted on, they also laid fewer and smaller eggs.

These results suggest that female finches were actually helping to determine their offspring's sex, rather than actual genetic differences. They knew (or thought they knew) when their mates genes were different from theirs, and they adjusted the sex of their offspring accordingly.


This research appears in the 20 March 2009 issue of Science.