Pregnant While Already Pregnant

Superfetation is a second conception that happens during a pregnancy. In the European brown hare, it occurs after copulation in late pregnancy when a sperm fertilizes an ovule to form embryos which will only implant themselves in the mother's uterus after delivery of the litter from the first pregnancy. It is basically the start of a new pregnancy during an ongoing pregnancy. Superfetation is not a new idea and has been suggested for the hare by Aristotle, in fourth century BC.

However, it has only recently been definitely proved to occur in the European brown hare by researchers of Leibniz Institute for Zoo and Wildlife Research, Germany. They found that superfetation in the hare species can occur if there is copulation during late pregnancy. They also concluded that superfetation occurs frequently and increases the litter size, ultimately accounting for a reproductive output increase of about 35% per breeding season in the hares used in the study. That's like one more leveret for every three of them. Quite a significant boost indeed.

In their experiment, the researchers divided the female hares into three groups. The "conventional" group housed a male only for a day to allow procreation of the initial litter. In the "permanent" group, the male was permanently housed and copulations during late pregnancy were observed frequently by video surveillance. The "temporary" group housed a male for one day to allow procreation of the initial litter and again in late pregnancy for five days until birth.

The study showed that the interbirth interval (time between two successive pregnancies) for females of the "permanent" and "temporary" groups, where the male was present for copulation in late pregnancy, was about 38 days. This was roughly four days less than the mean pregnancy period of females in the "conventional" group, which was about 42 days. So where did those four days go?

"The pregnancy in all cases is always a mean of 42 days. With superfetation, the interbirth interval is only 38 days, because there is a period of about four days where two pregnancies are present at the same time," says Kathleen Roellig, first author of the paper.

The researchers suggest that this phenomenon may be an evolutionary adaptation to enhance reproductive success. Not only does superfetation increase the reproductive output of European brown hares, it would appear that it also favors keeping the little ones in the safe environment of their mothers' wombs for a longer time period. By comparing the European brown hare with its closest ancestor, the snowshoe hare, the mean pregnancy period of the former was found to be around four days longer than its ancestor's. The researchers postulate that superfetation played a role in increasing the pregnancy period of the European brown hare by pushing birth to the latest possible day before implantation of the successive litter.

Evolutionary adaptation or not though, I'm pretty sure the female European brown hares are cursing evolution right now.

Image Credit: Ansgar Walk (via http://wikimedia.org)

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