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SEX REVERSAL IN CHICKEN

August 25, 2024

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Sex reversal in chickens is a fascinating phenomenon. Did you know that a hen has the ability to transform into a cock?

A hen is initially equipped with two ovaries: the right ovary, which remains undeveloped, and the left ovary, which grows and matures. The left ovary is responsible for producing essential estrogen in the hen’s body, regulating the ovum production process.

Sex reversal occurs when the left ovary sustains damage, leading to a decrease in estrogen production. This reduction in estrogen levels causes an increase in testosterone levels, resulting in the hen exhibiting physical characteristics typical of a rooster. These changes may include the growth of large combs, the development of male-patterned plumage, and the adoption of aggressive rooster behaviors such as crowing.

In cases where the left ovary fails to function and testosterone levels reach a certain threshold, the dormant right ovary in the hen is activated. This ovary undergoes development into an ovotestis, a male reproductive organ capable of producing sperm. As a sexually reversed hen, may attempt to mate with other hens within the flock.

It is possible for a female chicken to take on external characteristics of a male, a phenomenon referred to as sex reversal. (To date, spontaneous sex reversal from male to female has not been reported.) Genetically and anatomically, a chicken that has experienced sex reversal is still a hen, but it has secondary characteristics similar to those of a male.

Typically, several observable differences in the secondary sex characteristics of roosters and hens exist (as shown in Figure 1). These include the following distinctions:

The male has a larger body, comb, and wattles than the female.

In single-comb chickens, the male’s comb is turgid and stands erect, whereas the female’s may flop over to one side.

The male has larger, more developed spurs than the female.

Although both sexes have the capacity to crow, typically only the male does.

In multicolored varieties, the male has more variety of coloring in his plumage than the female (except for breeds of which the males are hen-feathered, such as the Sebright and Campine breeds).

The male has longer and more pointed hackles than the female.

Both sexes have main tail feathers, but only the male has saddle feathers and sickles.

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How does spontaneous sex reversal occur? Typically, a female chicken has only one functional ovary, the left one. The right ovary and oviduct are present in the embryonic stages of all birds but usually do not develop in chickens. Most cases of spontaneous sex reversal result from a disease condition that damages the left ovary. This condition could be an ovarian cyst or tumor or diseased adrenal glands, which cause the left ovary to regress. Residual tissue in the right ovary develops in the absence of the functional left ovary. This regenerated right gonad is known as an ovotestis and may contain some tissue characteristics of the ovary, the testes, or both. Steroidogenically functional, an ovotestis secretes androgen as well as estrogen. As a result, the hen develops male secondary sex characteristics. So, although the bird is genotypically female, it is phenotypically male. (Genotype refers to the actual traits coded for in paired genes; phenotype refers to the observable expression of those traits.) There are reports of hens with ovotestes producing semen, making them capable of fathering offspring. However, most chickens that have experienced sex reversal never lay eggs or sire offspring.

Differences between the sexes

A young chicken is called a chick. A male chicken is a cock or a cockerel, depending on its age. Similarly, a female chicken is called a pullet or a hen. The age at which a pullet becomes a hen and a cockerel becomes a cock depends on what type of chicken is being raised. Purebred poultry producers have very age-specific definitions. A chicken is a cockerel or pullet if it is less than one year of age. After one year of age, the chicken is referred to as a hen or cock. In the commercial chicken industry a female chicken is called a hen after it begins egg production (around five months of age). A sexually mature male chicken (again, around five months of age) is referred to as a rooster.

The observable differences in secondary sex characteristics between the male and female chicken are referred to as sexual dimorphism.

Typical differences between a rooster (Figure 1) and a hen (Figure 2) include:

     * The male has a larger body, comb, and wattles than the female.
     * In single-comb birds the male’s comb will be turgid and stand erect, whereas the female’s may flop over on one side.
     * The male has a larger, more developed spur than does the female.
     * Roosters crow, while hens do not.
     * In multicolored varieties, the male will have more variety of coloring in his plumage than the female.
     * The male has longer and more pointed hackle feathers than the female.
     * The male and female both have main tail feathers, but only the male has saddle feathers.
As often happens, there are exceptions to these differences. Males of two breeds of chicken, the Campine and the Sebright, for example, have female plumage. The female plumage pattern is dependent upon the presence of estrogen to femininize the feather follicle and direct feather formation to produce more rounded feathers in the hackles and tail. These two breeds possess a single gene mutation that codes for excessive aromatase production in several tissues, including the feather follicles. Since aromatase is the enzyme responsible for the conversion of androgens to estrogens, the feather follicles of males of Sebright and Campine breeds have excess estrogen production. The level of estrogen in the feather follicles is sufficient to femininize the growing feathers. Castration of Sebright or Campine males removes the source of androgens for conversion to estrogen in the feather follicles causing their female plumage to revert to the male phenotype.

Genotype or Phenotype?

Sexual differentiation in birds is directed by the presence or absence of the W chromosome, similar to the Y chromosome in humans. Genetically, male birds are homozygous, ZZ, and the female bird is heterozygous, ZW. This is opposite from mammals where the male is heterozygous, XY, and the female homozygous, XX.

Within a few days after fertilization, the female chicken embryo can be identified by the accumulation of primordial germ cells in the left gonad (primary sexual gland). By the tenth day of incubation, the gonads are sufficiently differentiated to be recognized upon autopsy. Throughout embryonic development, however, there are no external characteristics that identify the sex of the chick. At hatch, male and female chicks are the same weight and, with the exception of specific genetic variants in down color or feather length, neither males nor females exhibit any distinguishing secondary sexual characteristics.

The acquisition of secondary sexual characteristics as birds mature is a consequence of the hormonal secretions from the testes and ovary. The development of secondary sexual characteristics, representing the phenotypical expression of the sex genes, is dependent upon the production of both androgens and estrogens.

Androgens are required to induce growth of the comb and wattles in roosters. They are also responsible for full expression of the characteristic voice of the rooster. The growth of spurs, however, is independent of testicular and ovarian secretion. Their growth is determined by the genetic sex of the bird.

The female plumage color is dependent upon the presence of estrogens to femininize the feather follicle and direct feather formation to produce a more rounded feather in the hackles and tail. In breeds of chickens that are sexually dimorphic for feather color, the estrogens are also responsible for the reduction in pigmentation in many of the feather tracts.