MALES: ANATOMY AND PHYSIOLOGY
Externally, the feature of reproductive anatomy of male kangaroos that distinguishes them from most placentals is the positioning of the penis behind the testes. When relaxed the penis is S-shaped in structure and enclosed in a preputial sac formed by an invagination of the skin at the base of the cloaca. When erect during the sexual arousal it is extended downward and forward by eversion of the sac through the cloaca.
The paired testes and epididymides are located in the pendulous scrotum. The surface of the scrotum is lightly furred and the skin has a large number of sweat glands. these characteristic, together with the counter-current arrangement of blood supply to the scrotum, assist in temperature regulation of the scrotum. It is essential for normal functioning that the testes be kept two to five degrees cooler than the body temperature. The reason for this is not clear. A strong cremaster muscle surrounds the spermatic cord and holds the scrotum tight against the body wall during cold weather and in times of stress. In warm conditions and after strenuous activity the muscle is relaxed and the scrotum hangs well away from the body wall.
The basic structure of the reproductive tract of male kangaroos is similar to that of placentals but there are differences, especially with the accessory glands. The testes of kangaroos are ovoid. Spermatogenesis occurs in the convoluted semeniferous tubules within the testes. The tubules are separated by small clumps of interstitial tissue which contain the testosterone-secreting Leydig cells. This male hormone is responsible for many of the secondary sex characteristics of the male kangaroo. Its production is controlled controlled via gonadotrophins produced in the pituitary gland at the base of the brain. Environmental factors may affect testicular function in kangaroos. Red kangaroos have impaired spermatogenesis in severe droughts.
The overall process of spermatogenesis has been studied in the tammar wallaby where the initial production in the testes takes seventy-two to seventy-five days. The sperm then passes into the epididymis where maturation and storage take place. During maturation, changes occur which give the sperm the capacity for motility and the ability to recognise the ovum. This takes another thirteen days. The epididymis is a much coiled duct that les within the scrotum and is enclosed with the testes by membranous tunica vaginalis. The total length of the ductus epididymis in tammar wallabies is thirty-five metres. Three functional segments are recognised: the first concentrates the dilute spermatic fluid from the testes, the second is involved in maturation and in the final segment storage occurs. Spermatozoa may remain viable in the final segment for a month.
The semen of the ejaculate is derived from a series of accessory glands in addition to the contents of the epididymis. These accessory glands are the prostate and the Cowper's glands. The prostate in kangaroos is an obvious feature of the reproductive tract. It is a large, diffuse carrot-shaped gland that surrounds the urethra. It has three distinctive segment which produce viscous fluids, presumably for aspects of maintenance of the spermatozoa. Three pairs of Cowper's glands occur in the kangaroos. They bulbous structure covered in muscle and joined to the urethra by ducts. Their secretions are mucous in nature. Kangaroos are among the few marsupials that produce a true copulatory plug in the female tract: the semen coagulates shortly after its ejaculation. Whether the function of the plug is to block the passage of spermatozoa from subsequent matings with other males or to help in sperm transport in the female tract is unresolved.