The Denver Zoo is celebrating the birth last month of four Amur tiger cubs.

The quadruplets were born May 31 to mother, Koshka, and father, Waldemere, the zoo said in a news release.

The four cubs, the first born at the zoo since 2003, were born in a private maternity den and are not yet on public display.

Amur tigers are “critically endangered,” the zoo release said. There are fewer than 400 Amurs in the wild. Waldemere was born at the Denver Zoo in 2003, and Koshka came to Denver from the Beardsley Zoo in Bridgeport, Conn., in 2008 under a breeding program.

Amurs were once called “Siberian tigers,” but now that the animals are only found in the far eastern portion of Asia along the Amur River, they are commonly referred to as “Amur tiger.”

Amurs are the largest members of the cat family, the zoo said. An adult male can be nearly 4 feet tall and 7 feet long.