Decision maker: Black female Gouldian finch choosing between a black and a red male (click button above for second image).
Credit: Sarah Pryke
Startling plumage: Male finch with a red head.
Credit: Sarah Pryke
SYDNEY: Gouldian finches decide the sex of their offspring based on the colour of their partner's head, reveals a study published in the U.S. journal Science today.
It appears scientists have underestimated the role of female choice in determining the sex of their offspring, said the authors.
"Our study shows female finches can have a lot more control of their reproductive success than we gave them credit for," said Sarah Pryke, lead author and a biologist based at Macquarie University in Sydney.
Genetic incompatibility
The theory that females can bias the sex of their offspring based on external factors is not new. But up until now researchers have found very little evidence that can't be explained by random fluctuations, she said.
Gouldian finches (Erythrura gouldiae) occur in three different morphs, distinguishable by head colour. Matings between different morphs sometimes occur in the wild, but due to genetic incompatibility, offspring of these pairings have a higher mortality rate than in same-morph pairings, with far more daughters dying than sons.
In their natural habitat, females show a preference for mating with a male of the same head colour. But if one isn't available, females make the best of a bad situation, mate with a different morph, and produce more sons than daughters.
This new study, using a captive breeding colony in New South Wales' Hunter Valley, lends further support to this idea, but also shows that it's not genetics that influences this gender bias, it's what the females see that counts.
Gender split
Pryke and a team of researchers investigated sex determination in the two most common morphs of Gouldian finch, red and black. They took 200 female finches (100 black and 100 red) and mated each twice, once with each morph.
As predicted, mixed matings produced significantly more male than female offspring, whilst pairings of the same morph resulted in a roughly equal gender split.
To find out whether this was due to a chemical or genetic interaction between the birds, Pryke and the team took out their paintbrushes and blackened red males to trick the females.
When red females mated with pseudo-black males they produced significantly more sons (72% males) despite in fact being genetically compatible. When black females mated with the pseudo-black impostors they produced similar numbers of males and females (55% males).
