Many of the zoos in the past were actually parks with fenced enclosures and buildings containing caged animals. Lions, tigers and other cats were kept in one house; monkeys in another; birds in another. Single animals in bare cages were not very interesting to the visitors, however. Many animals that live in groups in the wild also did not adapt well to living alone. Modern zoos present the animals by showing them in surroundings that resemble as closely as possible their natural habitats. Visitors to a modern zoo can look across a plain and see lions seemingly free to wander. Nearby are birds, with antelopes and zebras feeding at the edge of a water hole. The other animals are not afraid of the lions because they are separated from them by wide, deep trenches, or sometimes moats. These trenches also separate the visitors from the animals in the exhibit. The trenches are often hidden with plants, and the visitor frequently does not notice them. The animals appear to be living wild in their natural settings. Often what appear to visitors as real trees, rocks and vines, however, are in fact artificial. These naturalistic parts of the exhibit are built of durable materials and designed to withstand rough treatment by the animals.