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mLine 17:
Line 17: − Like all marsupials, kangaroos lack a true placenta, the structure in a pregnant female's womb that provides nutrients for and removes waste from the growing embryo. Instead of a fully developed placenta, kangaroo females form a type of yolk sac in the womb. The embryo absorbs nutrients from this yolk sac for four to five weeks and then emerges from the birth canal, still in an embryo-like form but with well-developed forelimbs. The tiny animal crawls along its mother's body and into an abdominal pouch. Once inside, the offspring attaches firmly to a nipple, suckling milk. It remains there for several months until it is more fully formed and can move about on its own. The young kangaroo, sometimes called a joey, may remain with the mother, climbing into her pouch for nourishment or safety, until it is more than a year old. + − Most kangaroos produce a single young per litter. Depending on environmental conditions and food resources, one litter is typically born each year. Some kangaroo species living in dry regions have evolved the ability to breed whenever food and water resources are favorable. This may result in normal reproduction or no litters for several years if conditions are poor. −
Reverted edits by Tnacrrrrr (Talk); changed back to last version by Philip J. Rayment
===Reproduction===
===Reproduction===
Female kangaroos usually only have one baby kangaroo (called a "joey") at a time. The newborn joey weighs as little as .03 ounces when first born, after which it crawls into its mothers pouch where it will nurse, grow, and develop. Red Kangaroo joeys will stay in their mothers pouch for about eight months and Grey Kangaroo joeys stay in there for about one year.
The unpredictable nature of the availability of resources has led to the evolution of another specialized reproductive feature in some kangaroos. As soon as the tiny offspring emerges from the womb, the female is able to mate again. While the newborn suckles a nipple in the mother’s pouch, a fertilized egg begins developing in her womb. The development of the fertilized egg stops abruptly when the egg becomes a simple, hollow ball of about 100 cells, called a blastocyst. The blastocyst remains in a state of suspended development in the female's womb while the offspring in the pouch continues to grow. After the pouch offspring is more mature and is able to leave the mother’s pouch (up to six or seven months later), the blastocyst resumes development and soon emerges from the womb and moves into the now empty pouch. This reproductive feature, called embryonic diapause, enables kangaroos to care for up to three litters simultaneously during desirable conditions: an older, weaned offspring still under the mother's care, another maturing in the pouch, and a third in suspended development in the uterus. Should adverse conditions cause the weaned young to die, other offspring are still available for rearing.
===Development of the Young in the Pouch===
===Development of the Young in the Pouch===