This thread is stimulated by a thread by Oleander on the Platypus. The Platypus is a monotreme, an earliest type of mammal that has hair, and produces milk, but has the genital, urinary and anal tracts of the female exiting the body in a single opening, the cloaca. This is the opening through which eggs are laid, as is also true of the reptiles and birds. What evolutionary disadvantage is present that would have given an evolutionary advantage to the marsupials [and later carried forward by the placentals] to have three separate openings, instead of just one? Was there a stage in which there were two openings, then later three? This is part of the murky origins of mammals of which we are still lacking knowledge, I believe. Any insight?
The evolution of the mammalian vagina. http://pharyngula.org/index/weblog/comments/evolution_of_the_mammalian_vagina/P50/ Still no proper answer of course.
I thought of that, but if so, why do the reptiles and birds have a cloaca, and have not evolved a three-exit system like the mammals?
You would have to look at it's appropriateness within the context of that particular animal's lifestyle. What's good for one creature may not work well for another.
So the next question is - Did giving live birth come first, followed by a separate exit-tube later to avoid the s-covered newborn? That seems to make some sense.
I've got it, the advantage is you could still have sex with someone and not get them pregnant (use your imagination). This could be advantageous in a social species that cements the social bonds through physical contact.
"The order Monotremata (one-holed creatures) is comprised of two families, the Ornithorhynchidae, including the platypus, and the Tachyglossidae, including the long- and short-beaked spiny anteaters or echidnas. Monotremes are found only in Australia, Tasmania, and New Guinea. Monotremes are a derivative of an ancient mammal stock but there is no direct evidence of what it might have been. Monotremes are not closely related to marsupials or placental mammals, but rather they evolved from a distinct group of reptilian ancestors. Despite sharing some reptilian features, monotremes possess all the major mammalian characteristics: air breathing, endothermy (i.e., they are warm-blooded), mammary glands, a furred body, a single bone in the lower jaw, and three bones in the middle ear. Monotremes have a reptilian-like shoulder girdle with distinct coracoid bones and a T-shaped interclavicle. Other reptilian-like skeletal features are present, including certain ribs and vertebral processes, as well as epipubic or "marsupium" bones. These bones are rudimentary and analogous to those that support a pouch in present-day marsupials. However, it seems more likely that these bones are a vestige from reptilian ancestors, associated with the attachment of strong abdominal muscles to support large hindquarters. Unlike higher mammals with separate reproductive and excretory systems, monotremes have a cloaca, with only one external opening for excretion and reproduction, as in birds and reptiles. In male monotremes, the penis is used only for the passage of sperm and not for urination as in other mammals. The overall pattern of reproduction is mammalian with a brief, vestigial period of development of the young in an external, soft-shelled egg. Once fertilized in the oviduct, the egg is covered with albumen and a tough, leathery shell forms. The egg is rounded, large-yolked, and compressible, rather than brittle like the eggs of birds. Echidnas develop a temporary pouch to incubate the egg and care for the young. The platypus does not develop a pouch and typically lays a single egg in a leaf nest. The mammae lack nipples, so the young lick milk from two lobules in the echidna's pouch or from the abdominal fur of the platypus. A three to six month period of maternal care is typical for monotremes. Certain shrews and monotremes are the only venomous mammals. In echidnas, the poison gland is present, but non-functional. Only the male platypus is capable of producing the venom and conveying it to a horny spur on the back of the ankle. Delivered by a forceful jab of the hindlimbs, the venom is powerful enough to cause agonizing pain in humans and can kill a dog. Although the exact nature of the venom system is unknown, it may have originated as a defense against some long extinct predator. Today, dingoes occasionally prey on echidnas, but in historical terms, dingoes are relatively recent arrivals in Australia. Because echidnas are widely hunted as food and the platypus is quite sensitive to changes in its habitat, monotremes are considered vulnerable in status." quoted from: http://science.jrank.org/pages/4435/Monotremes.html
I now read on a different theory. It could be that we are looking at it from a wrong perspective. Mammals reached a high homeothermic temperature of 38C or around that. This could have resulted in a rather obvious change of the reproductive system. The lowering of the testicles to a position outside the body: the nut sack. Interestingly some tenrecs allegedly have undescended testes ... and ... a form of cloaca. Coincidence? false report? Or a real clue? Changes in the reproductive system in a seemingly unrelated area have resulted in the loss of the cloaca, and the formation of a proper penis? reference: Lawrence W. Swan 'The concordance of ontogeny with philogeny. Bioscience 40, p376 1990.
