Using a feral population of chickens from Sweden, a team of Oxford University researchers has discovered that old roosters can still dominate the sexual pecking order long after their ability to fertilize eggs has withered.

Reported in this week’s Current Biology, the study suggests an evolutionary battle is fought between what’s pleasant for the old roosters and the fertility of sex with younger, more potent males that would benefit the younger females.


Observing the relationship between the status of male birds along with their reproductive performance and female reproductive successes, the study suggests that conflicts erupt between older males who hold on to younger, fertile hens. In doing so, they cause an evolutionary turmoil because the eggs from the young females do not become fertilized by the old cocks who nonetheless enforce their power of choosing the finest lady birds.

Because only younger roosters display key reproductive traits in their libido and sperm quality that are conducive to reproduction, a sexual conflict over mating naturally results with both the older, dominant males as well as with the hens that the geriatric cocks keep romantically caged in their roost.

"Old males remain capable of dominating the pecking order: When this happens they can monopolize access to females but, because of their sexual decline, they fail to fertilize all the eggs available. This implies an evolutionary conflict between ageing roosters and fertile hens," points out an author of the report, Dr. Tommaso Pizzari, from Oxford’s Department of Zoology.

Scientists are now beginning to understand the significance to successful reproduction that age-related sexual decline plays in natural animal populations. The study concludes that sexual decline is an evolutionary motor that propels sexually impelled battles among groups of animals.