This is Scientific American's 60-second Science, I'm Susanne Bard.
Picture a small kangaroo. You're basically seeing something very much like the Australian marsupial called the swamp wallaby. They're also known as black stinkers because of their distinctive odor.
"I can't really describe it except to say it's quite a pungent smell. It's not terribly pleasant."
University of Melbourne biologist Marilyn Renfree, who says that their unique scent isn't the only thing that sets swamp wallabies apart from their evolutionary relatives. She and her colleagues recently discovered that adult female swamp wallabies can carry two pregnancies—in different stages of development—at the same time. That's possible because—like other kangaroos and wallabies—they have not just one uterus but two. But unlike their close relatives, pregnant swamp wallabies mate again a day or two before they give birth.
"We only know that strategy in one other mammal, and that's the European hare. And the European hare does this mating while they're pregnant—a phenomenon known as superfetation. So they '—the wallabies—' have effectively a fetus at an advanced stage of pregnancy and a very, very early conceptus at the same time."
Renfree and her team confirmed the simultaneous pregnancies using high-resolution ultrasound.
"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."
This revolving cycle of pregnancy means that after a female swamp wallaby gives birth to her first baby from one uterus, a second embryo lies dormant in her other uterus, while the first baby attaches to a teat within her pouch. After nine months, the first baby leaves the pouch, triggering the second embryo to start developing. And right before the second baby is born, the mother mates again, and a third embryo is implanted back in the first uterus.
What's more, while the second baby lives inside the mother's pouch, the first baby hops around outside, continuing to suckle from a teat in her pouch. But she's able to supply each of them with milk tailored to their specific developmental needs.
"The little baby is drinking a milk of a totally different composition."
And once this second baby leaves the pouch, the third embryo starts to develop, and the cycle continues.
"So the swamp wallaby has it all. It has a continuous pregnancy and a continuous lactation for most of its life. Truly amazing animal."
The study is in the Proceedings of the National Academy of Sciences.
Whatever evolutionary benefit exists to the swamp wallaby's unusual reproductive strategy remains to be seen. After all, being pregnant all the time isn't exactly a hop in the park.
Thanks for listening for Scientific American's 60-second Science. I'm Susanne Bard.
想象一只小袋鼠 。你想到的可能非常接近名为黑尾袋鼠的澳大利亚有袋类动物 。这种动物也被称为“黑色臭家伙”，因为它们身上气味独特 。
“我只能形容那是一种‘相当刺鼻的气味' 。并不是很令人愉快的气味 。”
墨尔本大学的生物学家玛丽莲·伦弗里说到，她表示，黑尾袋鼠独特的气味并不是其与其它进化亲属的唯一区别 。她和同事最近发现，成年雌性黑尾袋鼠可以在不同的发育阶段同时怀胎两次 。这是可能的，因为像其它袋鼠和小袋鼠一样，它们不仅有一个子宫，而是有两个 。但与其近亲不同的是，怀孕的黑尾袋鼠在临产前一两天会再次交配 。
“我们只知道另一种哺乳动物会采用这种策略，那就是欧洲野兔 。欧洲野兔也在怀孕期间进行交配，这种现象被称为‘异期复孕’ 。因此，黑尾小袋鼠实际上肚子里同时有处于孕晚期的胎儿和处于非常非常早期的胚胎 。”
“因此，我们第一次能够证实黑尾袋鼠整个一生都在怀孕 。没有不怀孕的时候 。”
这种循环妊娠意味着，雌性黑尾袋鼠的一个子宫产下第一个幼崽后，第二个胚胎便休眠在另一个子宫里，而第一个幼崽则其其育儿袋内吃奶 。九个月后，第一个幼崽离开育儿袋，触发第二个胚胎开始发育 。就在第二个幼崽出生之前，母亲会再次交配，第三个胚胎被植入第一个子宫 。
另外，当第二个宝宝幼崽生活在母亲的育儿袋中时，在育儿袋外蹦蹦跳跳的第一个幼崽会继续在育儿袋吃奶 。但母亲能为每个幼崽提供适合各自特定发育需要的奶水 。
“因此，黑尾袋鼠的一生都是这样 。大部分时间都在不断怀孕和哺乳 。真是神奇的动物 。”
黑尾袋鼠不寻常的繁殖策略在进化上有何好处，尚有待观察 。毕竟，一直怀孕并不是一件容易的事 。
谢谢大家收听科学美国人——60秒科学 。我是苏珊娜·巴德 。
1. give birth 生（孩子）；分娩；生育；
She's just given birth to a baby girl.
2. what's more 更有甚者；更为重要的是；此外；
What's more, they carry out programs that offer money to reduce water use.
3. tailor to 按需定制；量身打造；
We can tailor the program to the patient's needs.
4. after all 毕竟；终究；
After all, 15 minutes of exercise is better than nothing.