Gouldian Head Color Affects Sex Of Chicks

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

Posted Fri Mar 20, 2009 11:40am AEDT
Updated Fri Mar 20, 2009 11:53am AEDT


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".

This basically the same information but a different source.

Finches choose sex of offspring
By Victoria Gill
Science reporter, BBC News






<p>Female Gouldian finches "decide" to have more male chicks if they are less compatible with their mate.
<p>The birds, which have either red or black heads, prefer to mate with males with the same head colouring, as this signifies a better genetic match.
<p>Chicks from a mismatched mating - particularly the females - are weaker and more likely to die very early.
<p>A report in the journal Science says that the birds compensate for this by having more male chicks in their brood.


Colourful Gouldian finches can judge if a mate is genetically compatible just by looking at its head.
<p>A female that mates with a male with the same colouring lays eggs that hatch much healthier chicks.
<p>This new study has found that, when the female finches mate with a male that has a different head colour, they select the sex of their offspring - giving their chicks a better chance of survival.
<p>Parental control
<p>In birds, the sex of an egg is already determined before it is fertilised by the male.
Sarah Pryke, a biologist from Macquarie University in Sydney, led this study. She found that when female finches mate with mismatched males, 70% of their chicks are male.

“ Females really don't want to mate with a male with a different head colour ”
Sarah Pryke Macquarie University

<p>This is beneficial for the birds, because male chicks from genetically mismatched parents are more likely to survive than females.
<p>"It is pretty amazing to think that the female herself has so much control - subconsciously of course - over this basic physiology," said Dr Pryke.
<p>The results were particularly striking because colour-matched matings, which result in much healthier broods, always produce roughly equal numbers of male and female chicks.
<p>"Females really don't want to mate with a male with a different head colour.
<p>"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."
<p>Birds of a feather
<p>Dr Pryke's team disguised some of the male finches to show that this "sex bias" is entirely controlled by the females.
<p>They blackened the head feathers of red males, using a non-toxic dye, and paired them to both red and black females to allow them to breed.
<p>"It's actually quite hard to tell the experimentally blackened birds apart from natural black males," explained Dr Pryke.
<p>The birds were fooled, and the team found that black females that mated with the "disguised" red males produced an equal ratio of male and female chicks.
<p>"This is the clearest and perhaps most extreme example of sex biasing that has been found," said Dr Pryke. "It's really black and white - or in this case black and red."
<p>She said that exactly how the birds select the sex of their eggs is still a "big mystery".
<p>"We have an idea that hormones may play a role - but that's a working hypothesis we're looking to test."
<p>Dr Ruedi Nager, a biologist from Glasgow University who specialises in avian reproduction described this as an "excellent experiment".
<p>"It's now clear that the control is driven by the females," he told BBC News.
"Somehow the female recognises the sex of the follicle [or egg cell] and selects it based on how much she likes the male.
"Hopefully, this will reinvigorate the debate about how this works."

Story from BBC NEWS:
http://news.bbc.co.uk/go/pr/fr/-/2/hi/science/nature/7953467.stm

Published: 2009/03/20 13:36:32 GMT