The Denver Zoo was started with the donation of an orphaned American black bear. With the construction of Bear Mountain, it became the first zoo in the United States to use naturalistic zoo enclosures rather than cages with bars. It expanded on this concept with Primate Panorama, featuring huge mesh tents and open areas for apes and monkeys, and with Predator Ridge, which has three separate areas through which animals are rotated so that their overlapping scents provide environmental enrichment. Toyota Elephant Passage, which opened on June 1, 2012, is divided into five areas for rotating the various species.