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For Roosters Comb Size is Big Cue for Sex

USDA photo by Stephen Ausmus.

Farmers and other breeders of poultry have long known that the comb, that reddish display of spiky skin on top of chicken heads, can be a reliable indicator of health and vigor. Now scientists have demonstrated that hens with the largest combs produce the most eggs — and roosters have it all figured out.

“Hens with the largest combs are like to get a bigger dose of sperm from roosters,” according to a paper presented this week in the science journal PLoS Genetics by scientists at Linköping University in Sweden.

“The hen with the largest comb gets a bigger dose of sperm — and thus more chicks.”

“A lone rooster sees a lot of all the hens in the flock, but the hen with the largest comb gets a bigger dose of sperm — and thus more chicks,” says a Linköping news release about the research.

Enlargement of the female comb is apparently the outcome of human selection of birds for breeding over countless generations. Breeding the best egg producers resulted in the more showy headdress. Roosters noticed.

“For thousands of years, people have tinkered with the development of domestic chickens, Linköping University said in its statement. “Through selective breeding for a few characteristics such as large muscle mass and increased egg-laying, we have at the same time caused numerous other radical changes in appearance and behavior. A research group at Linköping University … has now shown how the size of a hen’s comb is bound up with the ability to lay more eggs.”

Domestic hens have larger combs as well as denser bones than their wild ancestor, the jungle fowl, Linköping explained. “This influences egg-laying, as the hen’s bone tissues provide calcium for the eggshells. The greater the bone mass, the more eggs she can lay.”

Here’s how the university explained the experiment:

“After having spotted a clear correlation between comb size and bone mass in chickens from a cross between red junglefowl and domestic chicken, the research group – under the leadership of evolutionary geneticist Dominic Wright – set up a study where such chickens were crossed for several generations. In this way the genome was split up into smaller and smaller regions, which allowed the ‘mapping’ of the functions of individual genes.

“In the eighth generation, the researchers found an area that had a strong effect on the weight of the comb — but also on bone mass and fertility.

“The genetic variation has gradually decreased over the course of domestication. In domestic chickens there are now some 40 known small regions with stable genes that potentially govern their typical ‘domestic’ characteristics. LiU researchers have now discovered two ‘pleiotropic’ genes: two genes connected to each other that influence several characteristics simultaneously. By regulating the production of cartilage, they influence combs (which consist of cartilage throughout) as well as bone growth (where cartilage is the base material) and, ultimately, egg production.”

Said Dominic Wright, the lead researcher: “The original hens have smaller combs, thinner legs, and lay fewer eggs. When people bred for the characteristic of laying many eggs, the comb grew automatically.”

Concluded the Linköping news statement: “In nature, the comb is an example of a sexual ornament. Individuals — often males — with the most impressive ornaments are favored by females, thereby obtaining more numerous offspring than their competitors. In domesticated animals, sexual selection — like natural selection — has lost its role, as it was humans who determine breeding.”