Reproductive System of the Canvasback Duck
The reproductive system of the Duck is similar to that of the snake. It contains the testes where sperm is produced. The sperm is emptied into the epididymis, and then carried by the vas deferens to the cloaca. The Duck however has some other specializations in addition to this basic layout that the snake does not have. In the Duck, the testes contain a network of interconnected seminiferous tubules. Seminal vesicles are small, flat structures that open into the vas deferens near the terminal end. The duck has a single intermittent organ called a phallus. The testes drastically increase in size during the mating season and are very reduced when not in the mating season, as seen in the first picture below (Chiasson 1959). The cloaca is present in most primitive gnathostomes and persists in embryos of almost all vertebrates. For this reason the cloaca in birds as well as reptiles seems to be a primitive vertebrate feature. External fertilization seems to be most common in early vertebrates, while internal fertilization using an intermittent organ such as the phallus in the duck seems more prevalent in later vertebrates. This may be due to the increase in sperm transfer success by using an intermittent organ in conditions where external fertilization would be disadvantageous (Kardong 2002).
Different color text denotes system.
Different color text denotes system.
Phallus of the Duck
The phallus is the single intermittent organ of the Duck, and is similar to the penis of other taxa and the hemipenes of the Copperhead snake. The phallus is known as the “true penis,” and is only found in the domestic birds such as the duck, chicken, turkey, and geese. It is not present in the many other birds. The phallus is responsible for directing sperm into the urodaeum of the female from the cloacal region of the male (Chiasson 1959). The phallus contains a shaft around the penis itself. When not erect, the penis is found coiled along the ventral wall of the cloaca. It is thought that erection occurs by the filling of internal chambers instead of muscular means. This projects the penis from the cloaca causing it to bend forward (Kardong 2002).