From the 1920’s through the late 1950’s, Denver Zoo continued to grow. In 1936 Monkey Island was added to the zoo with the support of federal funds. In 1950 Denver citizens supported the zoo through contributions to bring the first elephant, Cookie, to the facility. She lived for nearly a decade in the City Park Pump House before the new Pachyderm Building was completed in 1959.
In 1956 the Denver Zoological Foundation signed an agreement with the City and County of Denver to oversee the administration, management and development of Denver Zoo. In 1959 the zoo’s first master plan was released and provided a map for future enhancements at Denver Zoo. The opening of the Feline House in 1964 trumpeted a new beginning for the future of Denver Zoo and was the most dramatic new exhibit opened since Bear Mountain in 1918.