Comparative Anatomy: Female Reproductive Systems in Birds, Rabbits, and Reptiles

The reproductive systems of birds, rabbits, and reptiles exhibit significant anatomical differences, reflective of their unique evolutionary pathways. A closer examination of these systems in Calotes (garden lizard), Columba (pigeon), and Oryctolagus (rabbit) provides insight into the diversity of reproductive structures across species.

Ovaries

In Calotes, the reproductive system includes two asymmetrical ovaries located on either side of the body. These ovaries are irregular in shape and are suspended from the dorsal wall of the body cavity by mesovaria. Notably, the right ovary is positioned slightly ahead of the left. Similarly, in Oryctolagus, there are two small, compact ovaries on both sides of the body, located just behind the kidneys and attached to the dorsal abdominal wall by mesovaria. In contrast, Columba (pigeon) has only one functional ovary—the left one—while the right ovary exists only as a vestigial structure.

Oviducts

The oviducts, responsible for transporting eggs from the ovaries, also vary considerably among these species. In Calotes and Oryctolagus, paired oviducts are present, while in Columba, only the left oviduct is developed, with a vestige of the right oviduct sometimes present. The oviducts in Calotes are notable for their striated appearance, extending well ahead of the ovaries and following a straight course. By contrast, the oviducts in Columba and Oryctolagus do not display striations and follow a more convoluted path.

The oviducal funnel, which helps guide the egg into the oviduct, also differs between species. In Calotes, the funnel is large with externally directed ostia, while in Columba, the single left oviducal funnel is very large, membranous, and features fimbricated margins. In Oryctolagus, the oviducal funnels are smaller with internally directed ostia.

Uterus and Vagina

The structure of the uterus and vagina is another area of distinction. In Calotes, the oviduct dilates to form an oval shell gland, and the oviducts enlarge into small ovisacs. The animal has two vaginae that open into the urodaeum. In Columba, the oviduct is divided into several parts: the magnum, which secretes albumen; the isthmus, responsible for secreting the shell membrane; and the uterus, which produces a calcareous shell. The vagina in pigeons is thick-walled and muscular. In rabbits (Oryctolagus), the reproductive system includes a pair of uteri, which are widened and muscular, leading into a median chamber known as the vagina. This chamber is associated with the urethra to form the urino-genital canal or vestibule.

External Openings and Associated Glands

The external reproductive openings also differ. In Calotes and Columba, the urodaeum opens to the outside via a transverse cloacal aperture. In rabbits, the urino-genital canal opens externally through the vulva, which is located in front of the anus and features a small structure called the clitoris. Additionally, while Calotes and Columba lack special glands associated with the reproductive system, Oryctolagus has Cowper’s and perineal glands.

Fertilization and Milk Glands

Fertilization is internal across all three species, but their reproductive modes differ. Calotes and Columba are oviparous, meaning they lay eggs, while Oryctolagus is viviparous, giving birth to live young. Finally, only rabbits have functional milk glands, with 4-5 pairs located on the ventral surface of the trunk. Neither Calotes nor Columba possess milk glands.