Superconception: European brown hares, while pregnant with one litter, can start a new pregnancy

Scientists of the Leibniz Institute for Zoo and Wildlife Research (IZW) in Berlin, Germany, elucidated the phenomenon of "superfetation (superconception)," a second pregnancy whilst already pregnant, using high resolution ultrasonography in female living European brown hares (Lepus europaeus). The results are now published in the latest issue of the scientific journal Nature Communications.

Applying an experimental approach within a captive colony of hares, scientist found that ovaries of hares close to parturition had active corpora lutea from an initial pregnancy plus a more recent, less developed set of corpora lutea indicating a second, simultaneous pregnancy. Corpora lutea develop after an ovulation and produce progesterone, the pregnancy hormone. The ovulation in female hares does not occur spontaneously and regularly but only after copulation with a male hare.

Copulation in late pregnancy is initiated by the female hare. If copulation occurred after the 37^th day of pregnancy it was unclear whether semen could cross the uterus in late pregnany or was stored from a previous insemination in the reproductive tract. "Using different males as fathers, genetic paternity tests revealed that semen must have passed the uterus with the previous fully developed litter inside," says Dr. Kathleen Roellig, scientist and veterinarian at the IZW. The European brown hares were fertilized during late pregnancy and developed a second pregnancy around four days before delivery. The embryos develop in the oviduct and migrate to the uterus. After parturition of the previous litter the new embryos were already four days old and the next implantation could occur immediately. Hence, the interval between two successive litters was 38 days instead of at least 42 days which is the normal duration of a pregnancy in hares."

"New in our study was the collection of data using the latest examination technique in living females. Therefore we examined female hares repeatedly using high-resolution ultrasound to make the "invisible visible" ," says Roellig. With this method, criteria could be developed for the first time to demonstrate superfetation in living individuals. The findings suggest that using this reproductive mechanism of superfetation, female hares can deliver up to one third more offspring per reproductive season. "Therefore we think that superfetation might be an evolutionary adaptation to increase the reproductive success," says Roellig.

Since the times of Aristotle the European brown hare was assumed to have the mechanism of superfetation. However, findings were not conclusive and the functional mechanisms of superfetation were not clear. Shortened intervals between successive parturitions have previously been observed in several captive populations, thus fertilization before delivery was assumed to occur. The systematic examination coupled with an experimental approach applied to their captive population enabled the researchers of the IZW to elucidate the phenomenon of superfetation in the European brown hare.