Promiscuity pays for female finches

By John Pickrell with AAP August 20, 2010
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These birds need a man to help about the nest, but still want options when choosing the best sperm to fertilise their eggs.

CHEATERS NEVER WIN, except when they don’t get caught. Australian researchers have found female birds go to great lengths to hide their infidelities because of the many benefits dishonesty brings.

A new study of the Gouldian finch (Erythrura gouldiae) has cracked the puzzle of why females cheat on their mates, when the costs of infidelity are so high.

The answer, published today in the journal Science, is that female finches willingly cheat on their partners to insure against the effects of male infertility and incompatible combinations of genes.

Running a risk

Though female finches are commonly unfaithful, they go to great lengths to hide this from their mates, says study co-author Simon Griffith, a biologist at Sydney’s Macquarie University. “They have a lot to lose if caught with another male, so we’re intrigued to find out why the female would run such a risk,” he says.

Male birds undertake up to half the care involved in raising offspring, but they will reduce that care or even desert a partner if they discover she’s been cheating.

From studying the Gouldian finch in northern Australia and in captivity, Simon and co-worker Sarah Pryke found that if a female cheated once with a male bird that was genetically superior to her partner, he could end up fertilising over three-quarters of her eggs.

Sperm selectors

Gouldian finches come in two genetic varieties or colour morphs – those with red heads and those with black heads. Genetic incompatibilities mean that interbreeding between these morphs can result in young birds that have low genetic fitness.

The Macquarie University team carried out tests where females mated with their partner and also an outside male. They used combinations of bird with different head colours, so they were able to tell who had fathered the female’s offspring by the resulting colour. The results revealing that the father was typically the best genetic match for siring offspring.

The female finch can select genetically good sperm from bad sperm and so, by cheating she maximises her chances of having healthy offspring, explains Simon. “One copulation with good male sperm is better than 30 copulations with bad sperm,” he says.

“Our work suggests that with this [sperm selection] mechanism, it would pay most female animals to cheat on their partner once or twice to insure against any genetic incompatibility or infertility that their partner might have.”

Gouldian finches: winged jewels