Swamp 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. The swamp wallaby does not have this constraint, allowing for indefinite pregnancies throughout its reproductive life. Kangaroos and other wallabies also have two uteruses, but they are not used simultaneously.

“Potentially, these...

Researchers noted around 50 years ago that swamp wallabies tend to mate in late stages of pregnancy, giving rise to the hypothesis that pregnancies could overlap. The current study teased out the details by performing regular ultrasounds and vaginal swabs on female swamp wallabies, searching for signs of copulation. Nine out of 10 pregnancies featured both uteruses in use and nearly all couplings occurred in the final days of pregnancy.

The new embryo enters diapause, allowing its older sibling to be born and move to the mother’s pouch. This latency lasts several months until the older joey begins to wean, signaling that it is time to continue development. Then, a few weeks later, the fetus is born and another pregnancy has already started.

Staggering the birth schedule allows the mother to time the joey’s development with the boon of grass that appears every spring so that once they emerge nine months later from the pouch, there is plenty to eat and the young wallaby has its best chance of success. 

“The female reproductive body is amazing,” Ava Mainieri, an evolutionary biologist at Harvard University who was not involved in the research, explains to Smithsonian. “Any strategy a [female body] can capitalize on . . . to increase her fitness, she will use.”

Many questions remain as to why swamp wallabies reproduce this way, given the fact that they only give birth once a year and this approach doesn’t boost the overall number of births the mother could have over her lifetime. It would also appear to be very energetically taxing for a female to be perpetually pregnant and lactating. The researchers suppose that it could be due to a lack of mate availability on a regular basis, Menzies tells Smithsonian

“Swamp wallabies are actually very solitary,” Menzies explains to the Times. “We thought, maybe the swamp wallaby is pushing estrus back into pregnancy so it has a longer period of receptivity to find a male in the wild.”

Understanding the mechanisms behind this unique process could possibly give rise to new ways of approaching human fertility and the endocrinology of pregnancy.

“If we could resolve this complex process, we may not have to cryopreserve embryos, but rather keep them in stasis within the laboratory,” David Gardner, who studies in vitro fertilization at the University of Melbourne and was not involved in the study, tells National Geographic. “There’s so much still to learn from marsupials.”

Lisa Winter is the social media editor for The Scientist. Email her at lwinter@the-scientist.com or connect on Twitter @Lisa831.

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