DUBLIN: Female zebra finches improve their sons' sex-appeal through nurture in the nest, leading researchers to suggest that genetic superiority is not a factor in their selection of 'extramarital' partners.

Promiscuity is not uncommon among female zebra finches (Taeniopygia guttata), who sometimes delve into the world of extramarital sex, so to speak.

It was thought that they strayed from social partners and selected males based on advantageous genetic traits, but a new study published today in the journal Proceedings of the Royal Society B suggests otherwise.

The male chicks born from such scandalous liaisons - known as extra-pair mating - were shown to sport better ornamental feathers - a rufuous cheek patch, and a feature linked to the birds' sexual attractiveness.

But the origin of this ornamentation is motherly care rather than genes, according to the Australian and Swiss researchers who led the study, in a case of nurture outperforming nature.

Moms increase the attractiveness of their sons by increasing their prenatal reproductive investment, said lead author and evolutionary biologist Barbara Tschirren from the University of Zurich in Switzerland.

Offspring of infidelity more attractive

Female zebra finches bond with a male partner and share the duties of parental care. Most experts believe female infidelity occurs to allow a superior male contribute good genes to her offspring.

The study found extra-pair offspring (EPO) and with-pair offspring (WPO) differ in attractiveness, but this was most likely due to differential maternal investment in EPO rather than genetic differences in the fathers.

We found that the more a female invested in her eggs the more attractive her sons were, said Tschirren, who carried out the research while at Macquarie University in Sydney.

The researchers used genetic markers to determine the genetic parenthood of zebra finch broods and found that EPO had larger cheek patches and were thus more attractive than their social half brothers.

"Most studies stop here and conclude that this is support for the good gene hypothesis. However, we went one step further and compared the social and extra-pair partners of a female directly," Tschirren explained.

First hatchlings favoured

In turned out, the dads were not so different. There is a strong bias in motherly care in zebra finch clutches, Tschirren said, with eggs laid early in a sequence being bigger and containing more nutrients and hormones.

"This maternal investment bias makes sense because the chicks that hatch first are more competitive and have higher survival chances," she added. "They are more valuable to the mother, irrespective of who sires them."

Males compete to fertilise the eggs early in the laying sequence to ensure a head start for their own offspring. They are exploiting an underlying bias in mothers, Tschirren explained.

Persuasive results challenge genetic influence

What's novel about this study is that they looked at offspring quality in terms of an adult trait that is relevant to sexual selection, whereas the majority of studies compare offspring quality at the nestling or juvenile stage, commented Nicole Gerlach, a biologist at Indiana University in the U.S.

"This study also takes a nicely comprehensive look at cheek patch size, comparing it not only between within-pair and extra-pair males, but also between fathers and sons, and between within-pair and extra-pair offspring."

Their argument that the difference in cheek patch size between EPO and WPO is most likely due to differential maternal investment rather than differences in paternal genetic quality is certainly persuasive, she added.

"This argument does raise the question of why females mate with EP males at all, and of what females gain from investing more heavily in EPO, if those offspring are not already inherently - genetically - of higher quality."

Genetic combos could explain infidelity

Gerlach suggests an alternative explanation for the group's results. Females may choose an extra-pair male not because his genes are sexy, but rather because the pair's genes would form a more favorable combination.

The offspring may then be healthier, and thus better able to invest in large ornaments. Moms may still favor EPO, with preferential maternal investment amplifying a difference that is already there.

Wolfgang Forstmeier of the Max Planck Institute for Ornithology in Germany said he has limited confidence in the study and questions whether there is a mechanism whereby mothers could target specific extra-pair eggs to give them more resources.

"I also doubt that males with larger cheek patches are more attractive than others," he commented. "So I can neither see how mothers could do it, nor why they should do it."

The study population was made up of wild zebra finches caught in Western New South Wales and domesticated zebra finches obtained from three different finch breeders around Sydney. Each male was photographed in order to measure the area of the rufous cheek patch.