Though roosters may think they have the upper hand during mating, female chickens exercise a trick of their own.
After having sex with multiple males, hens can selectively eject the sperm of undesired mates, often that of lower status males, according to new research. Both sexes of the domestic chicken (Gallus gallus domesticus) are promiscuous, but ultimately, such "sperm dumping" gives females the last say in who fathers their offspring.
The behavior had been observed before, but now animal experts have a better idea of how it works and when hens are more likely to show a bias toward certain members of the pecking order.
Led by Rebecca Dean of Oxford University, the study examines how proficient hens are at expelling sperm after copulation. Apparently, from measuring averages, she and her colleagues estimate that hens can eject up to 80 percent of a male's genetic load. Interestingly, larger "ejaculates" stood a higher risk of being dumped from the hens' reproductive tracts, but smaller loads still had larger proportions ejected.
Female pickiness could very well place males in an evolutionary conundrum, where their sperm ejaculate size shouldn't be too large or small.
Even more, hens were more likely to dump the sperm of subordinate males when compared to higher ranked ones. Also, the sperm of roosters that initiated sex first was less likely to be ejected than that of other birds that followed.
But it's unlikely the majority of chicken mating today occurs in the same semi-natural conditions the experiment was conducted in. The chickens' domesticated counterparts used in poultry production are likely to breed in more superficial environments with fewer males involved.
Still, the findings add to a picture of chickens as socially complex creatures with clever shortcuts well worth studying.
Other members of the animal kingdom practice sperm selection, albeit in different ways, including worms, insects and primates (maybe even humans).
Photo by Kristine Paulus/Flickr.com