Spontaneous Sex Reversal – Is That My Hen Crowing?!

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Have you heard a hen crowing when you know you don’t keep roosters? Jeff Goldblum’s groovy scientist character in the 1993 blockbuster, “Jurassic Park,” comments that “life finds a way” and that somehow the all-female population of cloned dinosaurs would reproduce. Well, life is stranger than fiction and your backyard chicken CAN undergo a spontaneous sex reversal and become a rooster!

A hen is born with two ovaries like a female human (sort of). The left ovary in a hen grows and develops. It is this left ovary that produces all of the necessary estrogen in a hen’s body that regulars the production of ova (though these are called oocytes in chickens) and their release into the oviduct tract. The right “ovary” in a hen doesn’t actually develop at all as the bird grows. Rather this gonad sex organ (i.e. right “ovary”) remains small, dormant and undeveloped.

A spontaneous sex reversal occurs in a hen when her left ovary becomes somehow damaged or fails to produce the necessary levels of estrogen. A hen’s left ovary is the only organ in her body producing estrogen. Without the left ovary properly functioning in a hen, the estrogen levels in her body will drop to critically low levels and conversely testosterone levels will rise. Without proper estrogen levels, how do chickens lay eggs? The hen will no longer produce eggs.

More disturbing though, a hen who’s left ovary has failed and consequently has elevated testosterone levels in her body, will actually physically transform to take on male characteristics. Such a hen will grow a larger comb, longer waddles, male-patterned plumage, and spurs. Moreover, this hen will also adopt aggressive rooster behaviors — such as a hen crowing.

You might be thinking to yourself, just because a hen with high testosterone levels grows spurs, long waddles and takes to crowing like a rooster — does not make her, in fact, a rooster. It just makes her a very butch hen. If that was all that happened in a spontaneous sex reversal of a hen — you would be correct. There is more to it though!

When a hen’s left ovary fails and sufficient testosterone levels are reached in her body, the hen’s dormant right side gonad becomes activated. When the dormant, right-side gonad is switched on, it develops into a male sex organ, called an ovotestis. Scientists have found that an ovotestis will produce sperm. A sexually reversed hen with a “turn-on” ovotestis, will actually try to mate with the other hens in the flock. There is conflicting information as to whether a hen that has undergone a spontaneous sex reversal and developed an ovotestis can sire offspring. At least one account of a sex-reversed hen fathering chicks exists on the web.

Dr. Jacqueline Jacob, a poultry expert (whose Ph.D. is in Poultry Science) wrote a very informative paper on the spontaneous sex reversal phenomenon in chickens. Dr. Jacobs discusses this rare condition in episode 018 of the Urban Chicken Podcast. Listen HERE to learn more about this truly fascinating and bizarre chicken phenomenon. There are links to several news articles about spontaneous sex-reversed chickens in the show notes of this episode.

Recently, a couple of Urban Chicken Podcast listeners wrote to report about hens crowing and behaving like roosters all of the sudden in their flocks. You can read more about these stories sent to me about spontaneous sex reversal in backyard hens HERE.

One last thought on this subject, there are rare cases of roosters reportedly also being able to undergo sex reversal — thereby becoming hens and even laying eggs. The cases of rooster to hen sex reversal is so extremely rare that it is not fully understood and is a topic that is still hotly debated.

Have you ever heard a hen crowing? Let us know.

Originally published in 2014 and regularly vetted for accuracy.