The baby emerges from an opening at the base of her tail called the cloaca. The infant is very tiny, only about the size of a lima bean. The baby, which is little more than a fetus, makes this climb completely unaided and guided only by instinct.
Marsupials deliver their young at a much earlier developmental stage than other mammals, and their joeys have months of lactation and growth ahead of them. While the newborn is nursing the embryo is "paused" in the womb, and its development doesn't begin until lactation of its sibling ends and the baby leaves their mother's pouch.
This reproduction strategy "completely blurs the normal staged system of reproduction in mammals. Originally published on Live Science. Written and presented in a style that makes even the most complex subjects interesting and easy to understand, How It Works is enjoyed by readers of all ages. Mindy Weisberger is a Live Science senior writer covering a general beat that includes climate change, paleontology, weird animal behavior, and space.
Mindy holds an M. This allows them to be continuously pregnant throughout their adult life. These results appear in the journal Proceedings of the National Academy of Sciences.
Unlike human females — who give birth after 9 months of pregnancy and are unlikely to conceive again for another 6 months, if they breastfeed the baby — kangaroos and wallabies have much shorter pregnancies. They can ovulate and mate again just hours after giving birth. This reflects the diametrically opposite evolutionary strategies of eutherians which are mammals with a placenta, such as humans and marsupials which are mammals that lack a placenta, such as kangaroos and wallabies.
Although humans have evolved to maximize the length of their pregnancy in order to give birth to well-developed offspring, female kangaroos and wallabies pack as many pregnancies as possible into their adult life. They give birth to tiny young, which must then spend months completing their development in an external pouch, where they suckle.
The mothers can become pregnant again just a few days after giving birth. In fact, researchers believe that female kangaroos and wallabies can be simultaneously supporting young at three stages of development: an embryo in the uterus, a joey in the pouch, and an offspring at her feet.
For some time, however, biologists have suspected that female swamp wallabies take this evolutionary strategy to the next level — possibly conceiving during an active pregnancy. There were also reports of females mating before the end of an existing pregnancy.
S wamp wallabies are able to have two simultaneous pregnancies at different stages of gestation, indicating a unique method of mammalian reproduction that leaves them able to be pregnant and lactating for their entire reproductive lives, according to a study published Monday March 2 in PNAS.
The current study confirms what many had long suspected, that swamp wallabies Wallabia bicolor can be pregnant in both of their uteruses at the same time, conceiving a second embryo days before the first is born. As Smithsonian Magazine points out, while the European brown hare Lepus europeaus is also able to have overlapping pregnancies, it is confined to a breeding season.
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