Gouldian Head Color Affects Sex Of Chicks

22nd October 2010, 12:02 pm

Finch head colour affects sex of chicks
By Science Online's Dani Cooper

Gouldian finches: the head colour of the males indirectly affects the sex of the offspring. (Dr Sarah Pryke)
Female finches from northern Australia are controlling the sex of their offspring, according to the head colour of their male counterpart.

The finding, published today in the journal Science, is one of the first to clearly show that birds are capable of biasing the sex of their offspring to overcome genetic weaknesses.

Lead author Dr Sarah Pryke, from the department of brain behaviour and evolution at Macquarie University in Sydney, admits the mechanism by which the birds do this is not yet known.

The endangered Gouldian finch (Erythrura gouldiae), which is found in the northern savannahs of Australia, can have either black or red heads.

Dr Pryke says some genetic incompatibility between the black and red-headed birds results in high mortality in the offspring when birds of different head colours mate.

With female offspring this mortality rate can be as much as 80 per cent higher than in a same-head colour pairing. Sons in a mixed pairing have a 40 per cent mortality rate.

Dr Pryke found that if the female mates with a finch of different head colour, she attempts to overcome this genetic incompatability by over-producing sons - as much as four males to one female.

"This is the first time such a large effect has been shown," says Dr Pryke. "It is actually the female that is controlling the gender."


Changing colour

To reach this conclusion Pryke and colleague Simon Griffith took 100 red-headed and 100 black-headed female birds and mated them with a male of the same head colour and a male with the different head colour.

They found females in mixed pairs produced broods that were 82.1 per cent male, whereas females in matched pairs produced an unbiased sex ratio with 45.9 per cent males.

They then tested whether the females in mixed pairs were deliberately over-producing sons.

Female birds were tricked into thinking they were mating with an incompatible male.

The researchers did this by temporarily blackening the head colour of red males and mating them with red and black-headed females.

The black females paired to red males with blackened heads produced a sex ratio that was roughly equal.

By contrast red females paired with red males that had blackened heads over-produced sons at a ratio similar to a mixed pairing.


Wearing their genes

Dr Pryke says there is no chemical or genetic interaction between the parents at work.

"Gouldian finches wear their genes on their head so it is easy for a female to assess the genetic suitability of the male," she says.

"Change the colour of the male's head with dye and the sex ratio changes."

Through the study the researchers also found that females from mixed pairs produced fewer and smaller eggs.

Dr Pryke says the finding is important because up to 30 per cent of breeding pairs in the wild are mixed "perhaps because of constraints on preferred mate availability".

Professor Ben Sheldon, director of the University of Oxford's Edward Grey institute of field ornithology, says the results demonstrate "hitherto unsuspected degrees of control over reproductive investment by female birds".