A German study has found that European brown hares are capable of being pregnant twice at the same time, and can potentially be permanently pregnant.
Researchers at the Leibniz Institute for Zoo and Wildlife Research in Berlin fertilised female hares from a captive colony in late pregnancy and found they developed a second pregnancy about four days before delivery, a phenomenon called "superfetation" (superconception).
Simultaneous pregnancies mean female hares can give birth to as many as a third more offspring each reproductive season due to the interval between delivery of litters being reduced from 42 days -- the normal length of pregnancy for hares -- to 38.
"Therefore we think superfetation might be an evolutionary adaptation to increase reproductive success," said Dr. Kathleen Roellig, who led the study.
via Hares can be permanently pregnant - German study - Yahoo! News.