March 20, 2009

Finches Can Decide The Sex Of Their Offspring

A report in the journal Science says that female Gouldian finches "decide" to have more male chicks if they are less compatible with their mate, BBC News reported.

Finches with either red or black heads prefer to mate with males with the same head coloring, as this signifies a better genetic match, researchers said.

The study found that chicks from a mismatched mating - particularly the females - were weaker and more likely to die very early. The birds compensate for this by having more male chicks in their brood.

It seems as if the finches are able to judge if a mate is genetically compatible just by looking at its head.

Females that mate with a male with the same coloring lays eggs that hatch much healthier chicks, experts said.

If the female finches mate with a male that has a different head color, they select the sex of their offspring - giving their chicks a better chance of survival.

The sex of a bird egg is already determined before the male fertilizes it, according to Sarah Pryke, a biologist from Macquarie University in Sydney, who led this study.

She said some 70 percent of chicks are male when female finches mate with mismatched males"”a benefit for the birds, because male chicks from genetically mismatched parents are more likely to survive than females.

Pryke said it was amazing to think that the female herself has so much subconscious control over such a basic physiology.

Additionally, color-matched matings, which result in much healthier broods, always produce roughly equal numbers of male and female chicks.

"Females really don't want to mate with a male with a different head color. But there simply aren't enough compatible males, so later in the mating season they seem to use this control to make the best of a bad situation," Pryke said.

Pryke's study was even able to disguise some of the male finches to show that the "sex bias" is entirely controlled by the females.

The team used a non-toxic dye to blacken the head feathers of red males and paired them to both red and black females for breeding.

Pryke said it was actually quite hard to tell the experimentally blackened birds apart from natural black males.

The trick did confuse the birds, however, and researchers found that black females that mated with the "disguised" red males produced an equal ratio of male and female chicks.

Pryke called it the most extreme example of sex biasing that has been found. However, researchers are still unsure exactly how the birds select the sex of their eggs.

They said hormones might play a role, adding it was only a working hypothesis.

"It's now clear that the control is driven by the females," said Dr. Ruedi Nager, a biologist from Glasgow University who specializes in avian reproduction.

The female somehow recognizes the sex of the follicle [or egg cell] and selects it based on how much she likes the male, Nager told BBC News.

"Hopefully, this will reinvigorate the debate about how this works."


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