Science & Environment

Hens evolve secret sex strategy

Image caption Chickens have evolved the ability to eject the sperm of unsuitable mates

Scientists have discovered that female chickens have a remarkable ability to choose the father of their eggs.

Wily hens have evolved the ability to eject the sperm of unsuitable mates say researchers working with Swedish birds.

Promiscuous roosters try to ensure that their genes are passed on by mating with as many females as possible.

But by removing the genetic material of males they consider socially inferior, the hens have managed to retain control of paternity.

Many species ranging from zebras to insects use the strategy of sperm ejection - but the evolutionary ideas behind it are often uncertain.

Among birds, male Dunnocks force females to eject the sperm of other suitors in order to protect their own genes.

But this research indicates that among chickens, the battle of the sexes seems to be all about female empowerment.

Working with feral fowl in Sweden, the scientists found that many matings were forced, as the roosters are twice the size of the hens.

To cope with the unwanted attention, females have evolved the ability to remove the ejaculate of those males they consider undesirable.

Dr Rebecca Dean from Oxford University carried out the study. She said: "It's really important for females to have the best male sperm to fertilise her eggs, so if she can't choose before copulation, then having a mechanism to choose after copulation could really increase her evolutionary fitness."

Even when unforced, the females still exercised their right to choose by opting to eject the sperm of males they considered to be at the bottom of the pecking order.

With the reproductive odds stacked against them, these low-status roosters have fought back by developing larger ejaculates in the hope of increasing their chances of passing on their genes.

Image caption Hens eject more sperm from socially subordinate males

But according to Dr Dean, the shrewd females have worked out a way of dealing with this tactic as well.

"We found that hens will eject a greater proportion of the ejaculate from socially subordinate males, so she is in this way favouring the dominant males both before and after ejaculation," she said.

The scientists explain that domestic fowl would certainly use a similar tactic, but normally they have fewer mating choices than their wild Swedish cousins.

The research has been published in the journal American Naturalist.