Ecology


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Sexual Reproduction and Development

Introduction.

Male Reproductive System
The male reproductive system is similar to that of placentals in most respects, both in morphology and function. A major difference in morphology is that in marsupials the scrotum is held in front of the penis rather than behind.

The testes are ovular with a tough connective tissue capsule and is surrounded by a membranous sheath. The testes are kept 3 to 4C cooler than the core body temperature by their pendulous nature and the evaporation of sweat and saliva in warm weather. This is to protect the sperm, which do not function as well in warmer temperatures. The testes contain seminiferous tubules, a mass of convoluted tubes, in which the sperm are produced.

The sperm form from large spherical stem cells called spermatogonia. They consist of a small head containing the condensed DNA in the nucleus, an acrosome, flattened on one side unlike in placentals where it wraps around the apical region, a midpiece with mitochondria to provide energy, and a single flagellum which acts as a tail.

The seminiferous tubules lead on to an epididymis with 3 main regions; the first where testicular fluid is reabsorbed to concentrate sperm, the second where new fluid is added and maturation occurs so that a sperm is now capable of fertilising an egg, and the third where mature sperm are stored. From here, sperm pass into the vas deferens, a simple muscular tube which joins the urethra just below the bladder.

The prostrate gland, accessory glands and bulbourethral glands all produce secretory fluids, which are added to the sperm, to make up the rest of what is now called semen.

The penis as a whole is usually held in a preputial sheath within the urinogenital sinus when it is not erect. In most species, the penis is also split in 2 halves. This is believed to help sperm enter the 2 lateral vaginae of a female, although this is not certain as some species have secondarily lost this feature. After ejaculation during sexual reproduction, the sperm pass in the semen into the female to fertilise an egg to form an embryo. In all South American marsupials (with the exception of Dromiciops), the sperm are released singularly but pair up in the female. This is not seen in Austalian marsupials or any other vertebrate group, and the advantage to this, if any, is unknown but it is hypothesised that it may aid directional sperm movement in certain circumstances. The paired sperm later separate so that only one sperm fertilises an egg.

As in placentals, testosterone is a major male sex hormone, as it is needed for sperm production and for secretory purposes of the epididymis and accessory glands.

Seasonal Breeding.

Pregnancy, Birth and Diapause.

Lactation.