The absence of external reproductive organs in the male chicken, or rooster, is a biological curiosity. This applies to the vast majority of all bird species, as approximately 97% of male birds lack the structure found in most other vertebrates. While mammals use external genitalia for reproduction, the rooster relies on an entirely different anatomical approach. The reason for this difference lies in a precise developmental mechanism that halts the growth of the structure before the chick even hatches.
The Cloaca and the Avian Reproductive Method
Male and female chickens both possess a single posterior opening called the cloaca. This common chamber is where the digestive, urinary, and reproductive tracts terminate. The term “cloaca” comes from the Latin word for sewer, reflecting its multi-purpose function. Unlike mammals, birds pass uric acid instead of liquid urine, which contributes to a lighter body weight beneficial for flight.
Reproduction occurs through a brief physical maneuver known colloquially as the “cloacal kiss.” The rooster balances on the hen’s back, and both birds momentarily evert or push out their cloacal tissue. This rapid touching of the two openings allows for the transfer of sperm into the hen’s reproductive tract without penetration. The entire process is extremely fast, sometimes lasting less than a second. Inside the rooster’s cloaca is a tiny internal bump, or papilla, which serves to deliver the sperm during this quick exchange.
The Genetic Switch That Stops Development
The answer to the missing structure lies in embryonic development. All vertebrate embryos, including chickens, begin to form a genital tubercle, the precursor to external genitalia. In the chicken embryo, however, a specific genetic program is activated that causes the budding structure to regress.
This developmental self-destruction is triggered by the expression of the gene Bmp4 (Bone Morphogenetic Protein 4). The Bmp4 protein acts as a cell death factor, instructing the cells of the genital tubercle to undergo apoptosis, or programmed cell death. This gene’s expression is widespread along the entire length of the developing tubercle, causing it to wither away early in development.
In contrast, birds that retain a developed structure, such as ducks, express Bmp4 only at the base or not at all in the distal tip. The activation of this single gene is sufficient to stop growth and reduce the structure to a rudimentary nub.
Why Evolution Favored Genital Regression
The loss of external genitalia in most bird lineages is considered evolution’s most puzzling change, given the structure’s importance in internal fertilization. The leading hypothesis suggests this change was driven by female choice and the avoidance of forced copulation. Since the rooster cannot penetrate the hen, successful sperm transfer requires the hen to cooperate by everting her cloaca.
This mechanism gives the female greater control over the act of mating and over paternity. By choosing to whom she presents her cloaca, the hen can select mates with desirable traits, a process known as cryptic female choice. The absence of the structure prevents males from overcoming female resistance, thereby increasing the fitness of selective hens.
A secondary hypothesis suggests that the lack of external bulk provided a minor advantage in reducing body weight, which is beneficial for flight.
Birds That Break the Rule
While most male birds lack external genitalia, the rule is not universal across the avian class. A few notable exceptions possess a functional structure. The most prominent exceptions are species within the waterfowl group, such as ducks, geese, and swans, along with the large, flightless ratites, including ostriches and emus.
These species often exhibit structures that are complex and highly elongated. The Argentine lake duck, for instance, is famous for its corkscrew-shaped structure that can be nearly half a meter long.
In ostriches and emus, the structure becomes erect not through blood flow, as in mammals, but through the rapid influx of lymphatic fluid. These exceptions often correlate with high rates of male-forced copulation, which reinforces the theory that the structure was lost in other birds to promote female control.