WASHINGTON (AP) - The National Zoo is celebrating 40 years of giant pandas. Monday was the 40th anniversary of the day pandas Hsing-Hsing and Ling-Ling landed at Andrews Air Force Base in Maryland.

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The pandas were gifts to the United States from China following President Richard Nixon's historic visit to the country.

The pandas were officially presented to the zoo on April 20, 1972. They were the first pandas in US zoos in modern history.

There are now 11 pandas at four American zoos. Ling-Ling lived at the National Zoo until 1992, when she died. Hsing-Hsing died in 1999. The zoo's current two pandas, Tian Tian and Mei Xiang, arrived in December 2000.