Kangaroos and wallabies don’t reproduce the way most of their fellow mammals do — they keep their pregnancies short and to the point, with young crawling out of the womb and up to their mother’s pouch after just a month’s gestation. Once there, the tiny joeys spend about nine months nursing and growing before they’re ready to actually climb out of the pouch into the world.

This is the kind of thing you’ve probably been hearing vaguely on nature programs about Australia for years. But what you might not have heard is that the joey in the pouch is not the only offspring in its mother’s body. Almost all kangaroos and wallabies have two separate uteruses, and they usually contrive to have extra, undeveloped embryos waiting in the wings — or rather, in whichever uterus was unused in their most recent pregnancy. Often they get pregnant again within days of birth, and their bodies keep the new embryo from developing for months at a time, until its older sibling has reached sufficient maturity.

But researchers report in a study published in the Proceedings of the National Academy of Sciences on Monday that the swamp wallaby, a small, dark-furred creature, has an even more peculiar way of doing things. It gets pregnant again before the first pregnancy is even over, suggesting that female swamp wallabies may be pregnant continuously for their entire reproductive lives.

Swamp wallabies are delicate, skittish creatures, said Brandon Menzies, one of the paper’s authors. While researchers had suspected for decades that they were doing something unusual, answers were not forthcoming until he and his co-authors, Thomas Hildebrandt of the Leibniz Institute for Zoo and Wildlife Research in Germany and Marilyn Renfree of the University of Melbourne, managed to use ultrasound scanners on pregnant females.