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Fit females have strong male-like traits, reveals study

2 February 2011, by Adele Walker

Combs on female red grouse are bigger during the breeding season and among fitter, more mature adults – just like they are in males – a new study led by scientists from the University of Aberdeen shows.

Female grouse

A female red grouse, Lagopus lagopus scoticus

This is the first experimental evidence that suggests these male-like traits in females are dependent on the individual's fitness.

Most scientists have associated these conspicuous features with sexual selection in males, and put the female versions down to shared genes – a by-product of sexual selection pressures on males with no purpose in females at all.

But not everyone agreed. 'I've always wondered: if that's the only reason, then why do females show conspicuous traits at all? What's the need for it?' says Dr Jesus Martinez-Padilla of Spain's National Museum of Natural History in Madrid, who was at the University of Aberdeen when he led the research.

An alternative theory, tested by this new study published in Biology Letters, suggests that these elaborate features could help potential mates assess the quality of the female, as well a play a role in competition between females.

'The first step to test this theory is to show that these traits are condition-dependent; in other words, that they provide some information about the quality of the bearer,' explains Martinez.

'The most important result of our study is that we've shown female conspicuous traits are condition- and parasite-dependent.
Dr Jesus Martinez-Padilla, National Museum of Natural History, Madrid

The researchers studied the combs of 647 female red grouse, Lagopus lagopus scoticus over both spring (breeding) and autumn (non-breeding) seasons. They mapped comb size against the birds' weight, wing-length, age and condition.

The team found that females had bigger combs during the breeding season, and adults had larger combs than young females. Birds in better conditions also had larger combs.

The birds' condition – and therefore their 'fitness' to reproduce – is in part determined by the number of parasites they're infected with. So the researchers also measured the level of infestation with the intestinal parasite Trichostrongylus tenuis, and dosed 27 female grouse with a chemical to remove the parasites.

When they recaptured the treated birds three weeks later, they found that removing the parasites had had a small but measurable effect on the birds' comb size. Put simply, those birds that lost the most parasites had a greater increase in comb size.

'The most important result of our study is that we've shown female conspicuous traits are condition- and parasite-dependent. This makes it possible that they play a role in sexual selection among females as they do in males,' Martinez explains.

Male grouse

A male red grouse

In male red grouse, comb size basically shows others how fit they are and signals opponents to keep away. We do not know yet if that applies to females as well, but comb size certainly provides some information about their quality.

The scientists don't rule out the influence of other factors too – like genetic correlation between sexes – and point out that they haven't established the function of these features in females.

'We do not yet know the function that female conspicuous traits have in red grouse,' says Martinez. 'But our results mean we can now start exploring what it might be.'

Condition- and parasite-dependent expression of a male-like trait in a female bird. J Martinez-Padilla, P Vergara, L Pérez-Rodríguez, F Mougeot, F Casas, S C Ludwig, J A Haines, M Zeineddine & S M Redpath. (2011) Biology Letters doi: 10.1098/rsbl.2010.0991

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