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Can you imagine being pregnant for your entire adult life? Is it even possible? According to scientists from the University of Melbourne, the answer is yes – but only if you happen to be a swamp wallaby (Wallabia bicolor). As it turns out, this iconic Australian marsupial is the only known animal on the planet that can be “permanently” pregnant throughout adulthood.
While most female mammals have one uterus, marsupials have two. Yet unlike other kangaroo and wallaby species that mate after a pregnancy is over, swamp wallabies go into estrus (become fertile) and mate just one or two days before the end of a pregnancy, creating a new embryo before the fetus in the other uterus is born. The researchers discovered this surprising finding by conducting ultrasound scans on 10 swamp wallabies in captivity.
When a newborn swamp wallaby begins to drink its mother’s milk, the recently-fertilized embryo’s development will go dormant until the newborn becomes less reliant on milk. Without this period of dormancy, which lasts around 11 months, swamp wallabies would give birth to a new baby (known as a joey) approximately every month.
A swamp wallaby mother can give her offspring different types and amounts of milk simultaneously, a process known as asynchronous concurrent lactation. So not only is the swamp wallaby capable of constant pregnancy, but she can also lactate throughout her entire mature life, as well.
- Kangaroos and wallabies can support three offspring at three different stages of development. A female typically has a fetus developing in the uterus, a newborn in the pouch, and an older joey who is out of the pouch but is still dependent on her for milk.
- Most mammals require at least a short period between pregnancies, mainly so they can provide milk to their new offspring without also trying to support a pregnancy. In other cases, some mammal species don't mate during the winter months due to the lack of resources.
- Marsupials can also enter a period of embryonic diapause in unfavorable conditions such as a limited food supply or bad weather.
- The European brown hare (which is not a marsupial) also has the ability to become pregnant while already supporting a developing fetus – in a single uterus. However, they only mate for certain months of the year (January to August), so unlike the swamp wallaby, they are not "permanently" pregnant.