The idea that you could conceive a second pregnancy while already pregnant is definitely weird (and probably creepy for any woman in her last trimester). This is all but impossible in humans, but what about other species? Aristotle suggested more than two thousand years ago that the hare—the rabbit's relative—could do this:

Of all animals the woman and the mare are most inclined to receive the commerce of the male during pregnancy; while all other animals when they are pregnant avoid the male, save those in which the phenomenon of superfetation occurs, such as the hare. Unlike that animal, the mare after once conceiving cannot be rendered pregnant again, but brings forth one foal only, at least as a general rule; in the human species cases of superfetation are rare, but they do happen now and then.

Now scientists in Germany have confirmed that Aristotle got it right: the European brown hare (Lepus europaeus) can get pregnant while it's pregnant. Their study appears this week in Nature Communications.

The researchers used selective breeding and high-resolution ultrasonography to demonstrate that a male hare can fertilize a female during late pregnancy. The resulting embryos will develop around four days before delivery of the first pregnancy. The embryos don't have any place to go at that time, however, since the uterus is occupied by the embryos' older brothers and sisters. So the embryos hang out in the oviduct, rather like when you wait in your car for a parking space to open up. Once the uterus is free, the embryos move in.

The result is that a female hare can shorten the time between litters from 42 to 38 days and deliver up to 35.4 percent more offspring during a breeding season.

It is not yet know if other members of the hare genus (Lepus) can go through superfetation (also known as superconception). Rabbits, however, may be less likely to share this trait with their hare relatives. Rabbits and hares belong to separate subfamilies that diverged evolutionarily around 11.8 million years ago and rabbits take care of their offspring for a longer period than hares do.