The determination of an individual’s biological sex is often a complex process. For many species, sex is established at conception through specific chromosome combinations, such as the XX/XY system found in most mammals. However, some species exhibit sex determination influenced by environmental factors or can even change sex during their lifetime.
The Science Behind Sex Reversal in Chickens
A hen can undergo a rare form of sex reversal. This phenomenon occurs despite chickens possessing a ZW sex-chromosome system, where hens are ZW and roosters are ZZ. Unlike mammals, in birds, the female is the heterogametic sex, meaning her chromosomes determine the offspring’s sex.
A hen typically develops with only one functional ovary, located on her left side, while the right gonad remains rudimentary. Should the functional left ovary become damaged, diseased, or regress, the rudimentary right gonad can begin to develop. Conditions such as tumors, cysts, infections, or age-related issues can trigger this ovarian malfunction.
When the left ovary ceases to function, the hen’s estrogen levels drop significantly. This hormonal shift allows the undeveloped right gonad to activate and differentiate, often into an ovotestis, a gonad containing both ovarian and testicular tissue. This developing right gonad can then produce male hormones, primarily testosterone. The hen’s genetic makeup remains ZW, but the change in hormone balance leads to a masculinization process, rather than a complete genetic sex change into a fertile male.
Observing Gender Role Changes in Hens
When a hen experiences this internal hormonal shift, visible physical changes start to appear. The comb and wattles, typically smaller in hens, may grow larger and more prominent, resembling those of a rooster. Plumage can also transform, with the hen developing longer, more pointed hackle and saddle feathers, and even sickle tail feathers characteristic of a male chicken. Spurs, bony growths on the legs, may also develop.
Accompanying these physical alterations are behavioral changes. The hen may start exhibiting rooster-like behaviors, including crowing, strutting with a more upright posture, and attempting to mate with other hens in the flock. While a hen is physically capable of crowing, a sudden onset often indicates underlying hormonal changes.
A hen undergoing this transformation will typically cease laying eggs, as the functional ovary responsible for egg production is no longer active. These observed changes are external and behavioral, resulting from the altered hormonal environment within the hen’s body. The chicken’s underlying genetic sex does not change, and the masculinized hen does not become a fertile male capable of siring offspring.