Some animals can display characteristics of both sexes, a phenomenon that raises questions about their underlying biology and how it occurs naturally.
The Phenomenon of Bilateral Gynandromorphism
A chicken can exhibit traits of both sexes, a condition known as bilateral gynandromorphism. This unique biological occurrence results in an individual visually split, with one side of its body displaying distinct male characteristics and the other female traits. For instance, a gynandromorphic chicken might have a larger comb, wattle, and spur, along with masculine plumage on one half, while the opposite half presents smaller wattles, an absence of spurs, and typical female feathering. This striking asymmetry can manifest as a clean division down the middle of the bird.
This visual distinction is particularly noticeable in species like chickens, which show clear differences in appearance between males and females, a trait known as sexual dimorphism. The male half of a gynandromorphic chicken might also possess a more muscular body structure compared to the female half. While a left-right split is the most commonly recognized pattern, other forms can exist, such as polar (front-back) or mosaic patterns where male and female cells are more randomly distributed.
How Gynandromorphism Occurs
Bilateral gynandromorphism in chickens stems from an error during early embryonic development involving their sex chromosomes. Unlike mammals, where males have XY and females have XX chromosomes, birds operate on a ZW sex-determination system: males are ZZ (homogametic) and females are ZW (heterogametic). Gynandromorphism arises when a female egg, which normally carries a Z and a W chromosome, fails to properly expel a polar body during meiosis. This results in an egg with two nuclei, one containing Z and the other ZW.
If this abnormal egg is then fertilized by two Z-bearing sperm cells, each nucleus can be fertilized separately. This leads to the formation of an embryo with a mixture of male (ZZ) and female (ZW) cells. Since chicken cells develop their own sexual identity very early—around 18 hours after fertilization—and independently of gonadal hormones, these genetically distinct cell lines can develop into their respective male or female tissues. This cellular autonomy means that even if hormones are circulating throughout the bird’s body, individual cells will express their inherent genetic sex.
Distinguishing Gynandromorphism from Other Conditions
It is important to differentiate bilateral gynandromorphism from other conditions that involve mixed sexual characteristics. Gynandromorphism is distinct because it involves a mosaic of genetically different male (ZZ) and female (ZW) cells within the same organism, leading to a visible split in physical traits. This cellular mosaicism is the hallmark of gynandromorphs, where the external appearance directly reflects the genetic composition of the cells in different body regions.
In contrast, intersexuality typically refers to ambiguous internal or external genitalia, often resulting from hormonal imbalances or developmental issues, rather than a mosaic of distinct male and female cell lines. While intersex individuals may show a blend of sexual traits, it is not due to a clear, side-by-side genetic division. True hermaphroditism, another distinct condition, involves the presence of both functional male and female reproductive organs within a single individual. Although rare in vertebrates like chickens, hermaphrodites may not necessarily exhibit the external bilateral asymmetry seen in gynandromorphs, as their external appearance might be predominantly male or female despite possessing dual reproductive organs.
Functional Implications for Gynandromorphic Chickens
For a gynandromorphic chicken, the presence of both male and female cellular identities can have various functional implications, particularly regarding reproduction. While a gynandromorphic chicken might possess an ovary on one side and a testis on the other, reproductive capability is often compromised. Female birds typically have only one functional ovary, located on the left side. Therefore, if the female side of a bilateral gynandromorph is on the left, there is a possibility of egg production. However, even if eggs are laid, they are frequently infertile.
The behavior of gynandromorphic chickens can be a mix of male and female actions, or they may predominantly exhibit behaviors of one sex. Some individuals have been observed to display male-typical behaviors, such as crowing or attempting to mate with hens, despite having female characteristics on one side. The overall health of gynandromorphic chickens appears comparable to their fully male or female counterparts, with the primary functional difference being their reduced or absent reproductive capacity.