Roosters, the male of the chicken species, do not possess a penis in the way mammals do. This anatomical distinction is a result of a specific evolutionary path that has shaped their reproductive strategy. The absence of this organ is not a defect, but is explained by their distinct anatomy, mating behavior, and the genetic instructions that guide their development.
The Cloaca and Avian Reproductive Anatomy
Instead of separate openings, birds like chickens possess a single, multi-purpose orifice called the cloaca. This chamber serves as the exit for the digestive, urinary, and reproductive tracts. For reproduction, a rooster has a small, internal bump known as a papilla on the back wall of his cloaca. This structure is not a penetrative organ.
The rooster’s internal reproductive system consists of two bean-shaped testes located high in the abdominal cavity. These testes produce sperm that travels through tubes called the deferent ducts to the papilla. From this small protuberance, sperm is transferred to the hen during mating. This setup is efficient for the bird’s compact anatomy.
The hen also has a cloaca, which serves as the entry point for sperm and the exit for eggs. She can store sperm from a single mating in specialized glands within her reproductive tract, allowing her to fertilize eggs for several days or even weeks afterward. This storage capability ensures fertilization can occur without daily mating.
The Rooster Mating Process
The reproductive act in chickens is a rapid event referred to as a “cloacal kiss.” The process begins with courtship behaviors, where a rooster may dance or drag his wings to attract a hen’s attention. If the hen is receptive, she will squat down, signaling her readiness for the rooster to mount her.
To maintain balance, the rooster holds the feathers on the back of the hen’s head or neck with his beak and uses his feet to tread on her back. He then lowers his tail as the hen lifts hers, bringing their cloacas into contact. In this moment, the rooster’s cloaca everts slightly, pressing his papilla against the hen’s cloaca to transfer sperm.
This entire process is swift, often lasting only a few seconds. The efficiency of the cloacal kiss allows for successful fertilization without a penetrative organ. The hen has some control over the process; if she is unwilling, she can eject the sperm from an undesirable mate.
The Genetic Reason for a Missing Phallus
The absence of a penis in roosters is caused by a specific genetic event during embryonic development. Research has shown that male chicken embryos initially begin to develop a phallus, much like their waterfowl relatives. However, this development is actively halted before the chick hatches.
The primary driver of this regression is a gene known as Bone Morphogenetic Protein 4 (Bmp4). Around the ninth day of a chicken embryo’s development, the Bmp4 gene becomes active at the tip of the budding phallus. This gene triggers programmed cell death, causing the developing organ to shrink, leaving only the rudimentary papilla.
In contrast, in birds like ducks, the Bmp4 gene is not activated in the same way, allowing their penises to develop into complex, corkscrew-shaped organs. Scientists confirmed the role of Bmp4 by artificially blocking its signaling pathway in chicken embryos, which resulted in a more complete phallus. This genetic switch explains why chickens evolved to reproduce without a penis.