Two key features in the initial development of the mammalian reproductive tract are crucial for understanding its evolution in both males and females. First, the system begins development as essentially separate left and right halves that are virtually mirror images of one another (bilateral symmetry). Second, there is a very close connection between the development of the reproductive tract and that of the kidneys and allied structures (renal system). According to species, these initial conditions are progressively reduced to varying extents in the course of development, but they provide im
portant clues for distinguishing between primitive and advanced features.
The basic features of the female reproductive tract are common to all mammals. On each side of the body, there is an ovary that discharges the egg(s) into an oviduct, which leads to a uterus that is in turn connected with the vagina. Like other land-living vertebrates, all mammals have internal fertilization, which requires insertion of the male's erectile penis through the external opening (vulva) into the vagina. One variable feature of the vaginal region of the female reproductive tract in mammals concerns the relationship with the outlet of the urinary system (urethra) on either side of the body. In most mammals, the ureter enters the female tract at some distance from the vulva and there is a combined urinary and reproductive passage (urogenital sinus). This is the case for most small mammals, such as numerous marsupials, many insectivores, tree shrews, many rodents and carnivores. Perhaps the most spectacular case is the female elephant, which has a urogenital sinus that is close to 2 ft (61 cm) in length. In contrast, there is no urogenital sinus and the ureter has a separate opening adjacent to the vulval orifice in primates (including humans) and certain other mammals. Because a close connection between the urinary and reproductive systems is known to be primitive, the loss of the urogenital sinus is undoubtedly a secondary development. It should be noted, in cidentally, that in many mammals (e.g. monotremes, marsupials, and some placentals) the reproductive, urinary, and digestive tracts all open into a common structure known as the cloaca. Indeed, the word monotreme means "one-holed", referring to the fact that there is a single cloacal opening. In several groups of placental mammals, such as hoofed mammals and primates, the cloaca has been completely lost and there is a wide separation between the anus and the reproductive/urinary outlets.
The form of the uterus also shows substantial variation among mammals. In the widespread primitive condition, there are two separate uterine chambers (bicornuate uterus), reflecting the initial development of two completely separate reproductive tracts in the female. A bicornuate uterus is found in marsupials and most placentals, although there is variation in the degree of separation between the two uterine chambers. In contrast, some placental mammals have the two original uterine chambers completely fused to form a single midline structure (simplex uterus). This relatively unusual condition is found in simian primates (monkeys, apes, and humans), but not in prosimian primates (lemurs, lorises, and tarsiers), which have retained the primitive bicornuate condition. A single-chambered, simplex uterus is also found in some edentates (armadillos and sloths) and in a few bat species. Interestingly, several bats show conditions intermediate be-
tween the typical bicomuate uterus and the advanced, single-chambered form, thus providing clues to the evolution of the simplex uterus. All placental mammals show some degree of midline fusion of the right and left female reproductive tracts in that there is a single vaginal passage (with or without a urogenital sinus). In marsupials, there are separate left and right vaginal passages and the penis is correspondingly bifid in males. This difference between marsupials and placental mammals may have arisen through a chance difference in development. In marsupials, the ureters pass between the paired vaginal tracts, thus preventing full midline fusion, whereas in placentals, the ureters are located lateral to the vagina and do not stand in the way of midline fusion.
In the male reproductive system, sperm are produced in the testis, stored in the epididymis and transported into the penis by the vas deferens. As a further consequence of the early close connection between the urinary and reproductive systems, the bladder and the vas deferens from each testis open into a common channel in the penis, which conveys both urine and seminal fluid to the outside world. An unusual phenomenon found in most marsupials and placental mammals (but not in monotremes) is descent of the testes into special scrotal sacs outside of the main abdominal cavity. Once again reflecting the close developmental connection between the urinary and reproductive systems, the testes initially develop close to the kidneys. In most mammals, they subsequently migrate into external scrotal sacs. In addition to monotremes, mammals that show no descent of the testes, or only partial migration within the abdominal cavity, notably include burrowing marsupials, insectivores and rodents, various aquatic mammals (certain marsupials, insectivores, and rodents, along with seals, sea-cows, hippopotamus, whales, and dolphins) and heavily built pachyderms (elephant and rhinoceros). It has long been suspected that descent of the testes is in some way connected with avoidance of the relatively constant, elevated core body temperature that characterizes mammals. However, recent evidence suggests that it is not the actual production of sperm (spermatogenesis) that requires a lower temperature but rather sperm storage. For instance, in some mammal species that lack descent of the testis as such, the tail of the epididymis migrates to a position close to the ventral abdominal wall. This adaptation, which ensures a lower temperature for sperm storage but not for sperm production, is found, for example, in hyraxes and elephant shrews.
A further special feature of both male and female reproductive organs in many mammals is the presence of a bacu-lum. This is commonly present both in the penis of the male (os penis) and in the clitoris of the female (os clitoridis), although the baculum is typically significantly larger in the male. Various mammals have secondarily lost the baculum. This is, for example, true of certain higher primates, including humans. The function of the baculum is still unclear, although there is some evidence that the os penis may play a role in stimulating the female during copulation. However, rather like teats in males, no function has been proposed for the os clitoridis in females.
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