Swamp wallabies, a small member of the kangaroo family, have been found to take the remarkable marsupial reproductive system to an even more extraordinary place, with females often pregnant and lactating their whole adult lives.
Instead of a single uterus, kangaroos and wallabies have two wombs, and alternate between them. Both are quite small, since, like other marsupials, they give birth to underdeveloped young, who finish their development in the pouch.
Some kangaroo species can stall the development of embryos until the timing is suitable to bring a young one into the world – a trick many humans might wish they could emulate. This can create a baby queue. The eldest will be hopping around, but still dependent on its mother's milk. A younger joey will be confined to the pouch, fed from a different teat with milk more suited to its stage of development, while an embryo is paused in one uterus. When the oldest moves on, the second joey leaves the pouch, reactivating the embryo's development. Soon after the youngest is born, the other uterus becomes ready for conception.
Dr Brandon Menzies of the University of Melbourne reports in Proceedings of the National Academy of Sciences that swamp wallabies prefer not to wait, ovulating in one uterus while the embryo is still developing in the other. They will then have sex and conceive in the ovulating uterus, sometimes while the older embryo is still developing.
Menzies told IFLScience this process is triggered by a joey leaving the pouch. The joey's sucking serves to halt embryo development, but once it starts to explore the world, only coming back to drink occasionally, this effect wears off and the paused embryo is reactivated, followed by ovulation in the other uterus.
Why swamp wallabies have evolved such an unusual reproductive strategy is not known, but it appears to be working for them. “They're one of Australia's most successful animals,” Menzies said, ranging from Victoria to Cape York. One tentative hypothesis is that it relates to swamp wallabies' relatively solitary nature. Mob-dwelling kangaroos always have males around, but a female swamp wallaby might want to mate at the first opportunity in case there isn't another.
Even more astonishingly, “There's new evidence in the literature [some wallabies] may be making post conception decisions based on the fitness of their mates. This could be happening in a variety of cryptic ways,” Menzies told IFLScience. Tentatively, swamp wallabies may be ensuring their two embryos are from different mates to avoid having all their eggs in one genetic basket, something that may be easier to do when the pregnancies coincide.
The only other mammal known to be capable of having simultaneous pregnancies at different stages of development is the European brown hare. More than 2,000 years after Aristotle recorded this remarkable capacity, we still don't know much about how they do it.