The Pittsburgh Zoo & PPG Aquarium opened on June 14, 1898, after Christopher Lyman Magee donated $125,000. It is one of only six major zoo and aquarium combinations in the United States. Located in Pittsburgh, Pennsylvania's Highland Park, the zoo sits on 77 acres (31 ha) of park land where it exhibits more than 4,000 animals representing 475 species, including 20 threatened or endangered species. The first gorillas kept at Pittsburgh Zoo were a couple imported from Osnabruck in May 1965; they were called Duala and Bossel. One of the current silverbacks of the zoo, Mrithi, was the first baby gorilla of the zoo. He was born in June 1992 to parents Zakula and Mimbo. The Tropic Forest Complex is the name given to the area where gorillas are held. It was opened in 1991. The outdoor enclosure is a large grassy prairie with a waterless moat surrounding it. There is a thin river and a lake crossing the exhibit. Trees are provided to offer some shadow to the great apes and tree trunks are located in different points. Visitors can observe the gorillas from windows and on viewing points. The indoor enclosure is used on cold winter days.