Great apes in U.S. zoos

What makes an ape a great ape

The great apes form the family Hominidae, which includes humans. Its eight living species sit in four genera: Pongo (the orangutans), Gorilla, Pan (the chimpanzee and bonobo) and Homo. Orangutans branched off first, about 12 million years ago, while gorillas, chimpanzees and humans share a more recent ancestor.

All great apes are tailless, and males are on average larger and stronger than females, though the gap varies a lot between species. Most move on all fours but use their hands to gather food and nesting material, and some use tools. Every night they build sleeping platforms, or nests, in trees; chimpanzees and gorillas also make nests on the ground. The gibbon is an ape too, but it belongs to a separate family, the lesser apes, that split away 18 to 12 million years ago.

Four apes, four ways of life

The orangutan of Borneo and Sumatra is the most tree-bound and the most solitary of the group, with lasting bonds mainly between mothers and their young. It spends much of its foraging time eating fruit and builds a fresh nest each night.

The gorilla is the heaviest. Western gorillas live in groups of two to twenty animals built around a dominant silverback male. In the dry season, when fruit is scarce, they fall back on tougher leaves and bark.

The chimpanzee and the bonobo together make up the genus Pan, humans' closest living relatives. They are easy to confuse. Bonobos are more slender, with narrow shoulders, long legs, a black face, pink lips and hair that often parts neatly on the head. Chimpanzees are bigger and more robust, with a pronounced brow ridge. Their societies differ most: chimpanzees keep a male-dominated hierarchy and are famous for making and using tools, while bonobo communities are led by coalitions of females, and lethal violence among bonobos is essentially unknown.

Why every great ape is threatened

Every wild great ape is at risk. Both gorilla species and all three orangutan species are listed as Critically Endangered, and the chimpanzee and bonobo as Endangered. The Sumatran orangutan and the Cross River gorilla have stayed on a list of the world's 25 most endangered primates since 2000.

The threats overlap: forest loss to logging, farming and palm oil plantations, hunting for bushmeat, the pet trade and disease. Ebola cut western gorilla numbers in protected areas by a third between 1992 and 2007. Large primates such as great apes are especially vulnerable because they mature late and breed slowly; a female gorilla usually raises only one offspring to maturity every six to eight years, so depleted populations recover slowly.

Cooperative breeding programs such as the Species Survival Plan manage threatened species across many zoos as one population, so that slow-breeding animals like these stay genetically healthy in human care.