6.4: Marsupials
- Page ID
- 62160
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In contrast to monotremes, no marsupial lays a shelled egg. The embryo develops for a short period inside the uterus (or womb) before transferring to (in most species) a pouch; hence marsupials are sometimes termed 'pouched mammals'. The newborn is tiny and very undeveloped. It looks like an embryo because it is; the embryo continues development inside the pouch. Marsupials are therefore said to give embryonic birth. To describe fully these complex events requires some new vocabulary.
In many kangaroos, females mate very soon after giving birth. In the event of conception, the tiny ball of dividing cells, called a blastocyst, stops developing after a few days and the process of attachment to the inner lining of the uterus is prevented. In most forms of mammalian reproduction, a blastocyst would undergo such implantation without significant delay - indeed, in humans it's seen as marking the beginnings of true pregnancy. But in kangaroos the blastocyst remains 'frozen in time' in what is technically termed embryonic diapause. Sometime just before the youngster in the pouch is ready to leave, the blastocyst implants and development proceeds to the point of birth. At about that point, the mother actively encourages the older offspring to spend less time in the pouch and prepares the pouch for the new arrival. Soon after the birth, mating is likely to lead to a further conception, and so on.
