The simple answer to whether roosters possess a phallus is that they do not, at least not in the sense of external male genitalia found in mammals. Chickens belong to the vast majority of bird species—roughly 97%—that rely on a different method of reproduction. The biology behind this missing organ is a story of evolution and genetic programming.
The Rooster’s Reproductive Anatomy
Roosters, like hens, possess a single posterior opening called the cloaca, which serves as the common exit for the digestive, urinary, and reproductive tracts. During mating, the male mounts the female, and the two birds press their cloacas together in a brief, momentary contact known as the “cloacal kiss.” This contact is remarkably efficient, allowing for the rapid transfer of sperm from the rooster’s system to the hen’s.
Sperm is produced internally in two testes located high in the abdominal cavity, a necessity because avian body temperatures are too high for sperm survival. The sperm travels down a tube called the vas deferens, which terminates at a small bump or protrusion called the papilla inside the cloaca. This papilla is the rooster’s functional mating organ, though it is merely a rudimentary structure. The entire copulation process is extremely quick, often lasting only a few seconds.
The Evolutionary Mystery of the Missing Organ
The reason most male birds lack external genitalia lies in a specific genetic mechanism that triggers a process called apoptosis, or programmed cell death, during embryonic development. Ancestral birds possessed external genitalia, meaning the structure was lost over evolutionary time. In the developing chicken embryo, a structure known as the genital tubercle—the precursor to the phallus—begins to form normally.
However, a gene known as Bone Morphogenetic Protein 4 (BMP4) is highly expressed in the tip of this tubercle in chicken embryos. The heightened expression of BMP4 switches on a cell suicide program, causing the cells of the developing structure to die off, resulting in the regression of the genital tubercle. Experiments have shown that blocking the BMP4 gene in chicken embryos results in the continued growth of the phallus, confirming the gene’s function as the biological cause of the organ’s absence.
Waterfowl and the Exceptions to the Rule
While the rooster represents the majority of the avian class, a small group of birds provides exceptions to this rule. Waterfowl, such as ducks and geese, along with ratites like ostriches and emus, have retained or re-evolved a functional phallus. In these species, the BMP4 gene is not highly expressed in the genital tubercle, allowing the structure to develop fully.
The male waterfowl organ is unique, typically spiraling and everting explosively from the cloaca during copulation. The evolution of this structure is often linked to the high frequency of forced extra-pair copulations observed in these species. This has led to an evolutionary “arms race,” where female waterfowl have developed elaborate, complex vaginal structures, including dead-end sacs and counter-spirals, to help them retain control over fertilization.