The process by which a male chicken, or rooster, contributes to the creation of a fertile egg involves distinctive biological steps that differ significantly from mammalian reproduction. Fertilization supplies the necessary genetic material from the rooster, allowing the egg to develop into a chick. This contrasts with the unfertilized eggs commonly purchased for food, which are simply the result of a hen’s natural reproductive cycle and would never produce an embryo.
The Mating Process
Sperm transfer in chickens is achieved through a rapid and unique physical act known colloquially as the “cloacal kiss.” Unlike mammals, the rooster lacks an external copulatory organ, and instead relies on the cloaca, a single opening that serves as the exit for the digestive, urinary, and reproductive tracts in both sexes. During mating, the rooster mounts the hen, stepping onto her back and using his feet to position himself.
The transfer of semen occurs when both the rooster’s and hen’s cloacal openings are briefly everted and pressed together. This contact is extremely fast, often lasting only a few seconds, which is a characteristic adaptation for a prey species. The male reproductive tract terminates in a small papilla within the cloaca, from which sperm is delivered directly to the hen’s cloaca. This efficient maneuver successfully delivers the sperm into the hen’s reproductive system, beginning its journey up the oviduct.
Sperm Storage and Viability
Once inside the hen, the sperm must navigate the oviduct, which is a twisted, approximately two-foot-long tube, to reach specialized storage structures. The hen possesses a remarkable biological adaptation that allows her to store sperm for extended periods, meaning she does not need to mate for every single egg she lays. Only a small fraction, less than one percent, of the transferred sperm successfully reaches the storage sites due to an intense selection process within the vagina.
The primary storage location is the sperm storage tubules (SSTs), which are microscopic, tubular invaginations found at the uterovaginal junction (UVJ), located between the uterus and the vagina. These SSTs act as reservoirs, where the sperm can remain viable and retain their fertilizing capacity for two to three weeks in chickens. This prolonged viability is achieved through specialized substances secreted by the SST cells that help maintain the sperm’s health. The stored sperm is released gradually from the SSTs, often triggered by the muscular contractions of the oviduct as an egg is laid, ensuring a continuous supply of sperm is available for future ova.
The Fertilization Event
The fusion of the sperm with the female cell, the ovum, happens at the very beginning of the egg’s journey down the oviduct. The hen’s ovary releases the ovum, which is the yolk, into the infundibulum, the funnel-shaped entrance to the oviduct. This release, called ovulation, means the ovum is only available for fertilization for a short window, approximately 15 to 17 minutes, before it moves into the next section.
Fertilization must occur in the infundibulum before the protective egg components are added. During this brief time, sperm released from the SSTs make their way to encounter the newly released ovum. A single sperm penetrates the blastodisc, a small whitish spot on the yolk that contains the female genetic material, completing the fertilization process.
Once fertilized, the blastodisc officially becomes the blastoderm, the true embryonic cell of the developing chick. While the yolk travels through the oviduct and the albumen, membranes, and shell are added over a period of about 25 to 26 hours, the blastoderm begins to undergo cell division.