A litter of four lion cubs arrived over the weekend at the National Zoo. Shera, a 9-year-old African lion, gave birth over seven hours as staff from the zoo watched over a closed-circuit webcam. The zoo reports the cubs are all healthy and nursing, moving and vocalizing well.

The four cubs join two more born in January to Shera's sister Naba (you can see her cubs via the zoo's Lion Cam.)

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"Shera successfully raised her previous litter of four in 2010, so we're cautiously optimistic that these cubs will thrive," said Kristen Clark, an animal keeper at the Great Cats exhibit. "Like any new mom, she needs some peace and quiet to bond with her cubs, so we're giving her the solitude she needs. From what we've observed on the cam, her behaviors are right on point, and there's no need for us to intervene."

The cubs will be introduced to the 8-year-old father, Luke, and the rest of the pride after about six weeks, following a similar timetable to lions in the wild. The new cubs will won't be on exhibit until this summer, giving the animals keepers and veterinary staff time to examine them.

Video: Smithsonian National Zoo