Reproductive System

1. Male Reproductive Organs
1. Penis
2. Testes
2. Female Reproductive Organs
1. Ovaries
2. Uterus

The penis of the dolphin originates from the rear pelvic bones and from the pelvic region, the penis extends to the penis slit, just to the back of the umbilicus. The penis is coiled or curved within the sheath (prepuce) except when erect; it is held in this position by a pair of strap-like retractor muscles. The penis slit exposes a part of the penis and the rest of the penis is firmly attached to the retractor muscles. This part of the penis which is covered with skin, is called the terminal cones, similar to the "glans penis" of most mammals.

When the penis is erected, the folded skin (penis sac) can be stretched and the dolphin can protrude approximately two-thirds of its penis from the slit. Just to the rear of the terminal cone, the retracted penis shows a loop. Since during erection, the skin does not allow stretching to occur easily, this loop enables the dolphin to protrude the penis.

A flat "retractor penis" muscle runs from just above the terminal cone to the rectal wall. This muscle is well-developed in species with a "fibro-elastic" penis. The muscle is not necessarily essential in the retraction of the penis, as studies have shown that the loop may naturally recur. One possible function is to prevent the penis from erecting too much.

The penis is made up of three layers, the outer layer being a thick, tough skin, the middle layer containing connective tissue and the innermost layer containing elastic fibres. Unlike most mammals whose three layers are usually made up of spongy tissue, the dolphin's penis contain a large amount of tough, fibrous tissue. During erection the inner layer fills with blood, but this produces no noticable swelling.

It has been suggested that erection results simply from the elasticity of this fibrous tissue when the retractor muscles relax. However, the mechanism is probably more complex than that.

The testes of dolphins are elongated and cylindrical in shape. They are not found in an external scrotum as in other mammals. Instead, the testes are located in the abdominal cavity just behind the kidneys. The absence of a scrotum is part of streamlining, reducing water resistance. It is also believed that ancestors of cetaceans may have had an external scrotum and evolution took a part in remodelling the reproductive structure of the cetaceans.

During puberty, the testes of a dolphin increase significantly in size. The testis of an young dolphin is about half the size of finger and weighs about 20 grams. After puberty, the testes may grow to an extent that they are as long and as thick as the forearm and also weigh several kilograms.

Sperm are produced in the testis, then pass into the epididymis. Maturation of the sperm occurs as they pass along this tube to be stored near the tail of the epididymis. From here, the short vasa deferentia conducts the sperm to the urethra. The vasa deferentia (sperm ducts) are highly convoluted. It has been suggested that the shortness of the tubes, due to the internal position of the testes, are made up for by the convolutions.

The mammalian semen is made up of many different fluids secreted by a few sex glands. However, the only accessory sex gland present in the dolphin is the prostate, which surrounds the urogenital canal. This gland is protected by a thick layer of muscle, called the compressor prostatae. This gland produces most of the necessary fluids in the semen.

The female reproductive system of dolphins is similar to that of other mammalian species. The ovaries are located in the abdominal cavity behind the kidneys, which is around the same place where the male testes are located in the male reproductive system. The ovaries of dolphins are spherical and elongated in shape, with a fairly smooth surface. Immature ovaries have a flat and elongated shape, with grooves on the surface. The ovaries resemble "a bunch of grapes" when the ovaries mature.

During ovulation, the mature ovaries contains oocytes (egg cells) surrounded by other cells to form follicles. Some of these follicles enlarge and develop a fluid-filled space. One of the follicles eventually enlarges further and bursts, releasing the oocytes into the uterine tube. The cells that remain in the collapsed follicle after ovulation multiply and form a greyish yellow structure called the corpus luteum. The corpus luteum degenerates if the oocyte is not fertilised but if fertilisation occurs, the corpus luteum persists to the end of the pregnancy.

Dolphins seem to ovulate and can become pregnant again even immediately after giving birth, although lactation seems to prevent this sometimes. The female seems to have one period each year but if the first ovulation if not followed by pregnancy, she may experience more periods. It is generally believed that dolphins can retain reproductive ability until they die. But some studies have shown that this may not be true as some species have been seen to experience regression in parts of the reproductive system.

The uterus of the dolphin has two main uteri called the cornua. The two cornua contain the uterine glands and a muscled connective tissue containing numerous blood vessels. The internal layer of the uterus thickens during pregnancy. The muscular tissues provide peristalic contraction during labour to push the infant out of the mother's womb.

The inner walls of the vagina have well-developed annular folds that look like a chain of funnels. The function of the folds is not known yet. The inner part is largely glandular.

The vulva is a slitlike aperture with labia majora and minora (inner and outer folds, or "lips"). A strand of tough, fibrous tissue can be found at the front end of the vulva. Many immature animals also show a band of fibrous tissue running from the front to the rear of the opening, placed so that it is likely to be ruptured during intercourse.

Bibliography

Harrison, Sir Richard, et. al. Whales, Dolphins and Porpoises. New York: Facts on File, Inc., 1994.

Tursi. "Tursi's Dolphin Page" Available http://www.neteng.bc.ca/~tursi/dolphins/index.htm, 4 September 1997, Accessed 4 April 1998