Marsupials are unique mammals in that the females have a pouch on their abdomen to carry their underdeveloped babies. The young spend a relatively short time in the womb. After they’re born they crawl into the pouch where they nurse and grow for months before they develop enough to permanently leave the pouch and grow old enough to live independently of their mothers.
Wallabia bicolor (common name: swamp wallaby) is even more unique than other marsupials, because the female of the species can become pregnant with a second joey before giving birth to a current joey. (Joey is the term for a baby marsupial).
Swamp wallabies inhabit the most eastern portion of Australia from Cape York in northern Queensland down to Victoria. An average adult male measures about 150 centimeters (60 inches or 5 feet) long from nose to tail, and weighs 17 kilograms (37 pounds). The typical adult female is slightly smaller at about 140 centimeters (55 inches) long from nose to tail, and weighs 13 kilograms (29 pounds). Their tails are approximately half their body length. For comparison (if you exclude their tail) they are about the size of a small to medium-size dog.
In their reproductive cycle, a single swamp wallaby embryo gestates in the womb for about one month before birth. The hairless bean-size joey then crawls to the mother’s pouch where it nurses and continues its development for eight to nine months. After leaving the mother’s pouch, the joey will continue suckling for up to another six months, until reaching approximately 15 months old. Young adult females are ready to begin mating between 15 and 18 months old.
A recent study by biologists at the University of Melbourne in Victoria
showed that the swamp wallaby’s two uteruses allows it to do something completely unique, even among marsupials. A pregnant female will mate and conceive a second embryo a day or two before giving birth to the one-month-old joey. While that first-born joey is developing in its mother’s pouch, the embryo in the second womb is in ’embryonic diapause’. In other words, its development is paused – literally put on hold – until the first joey leaves the pouch 8-9 months later.
When the first joey leaves the pouch, the second embryo can continue its development until it, too, is ready for birthing and relocating to the pouch. Days before birth of the second joey, the mother mates again, and again is pregnant with two embryos at once. The second joey is born and crawls to the pouch, while the third joey is put in embryonic diapause, and the first joey is still being nurtured by mom. This cycle continues throughout the female swamp wallaby’s fertile years.
And so, for the first time we were able to actually prove that the swamp wallaby is pregnant its whole life. It’s never not pregnant.
Professor Marilyn Renfree, University of Melbourne, VIC
Notably, the European brown hare is the only other animal in the world known to mate and concieve a new litter a few days before the birth of a current litter. However, the female European brown hare only mates January through August, and thus cannot be pregnant all year round. Continuous overlapping reproductive cycles are unique to the swamp wallaby.
By the way, while swamp wallabies are adorable critters you probably wouldn’t want one as a pet: they also go by the name of stinker or black stinker due to their natural, but unpleasant, odor which may be somewhat reminiscent of stagnant swamp water.
Question of the night: Would you like to visit Australia, or if you’ve already been there what did you like best?
