How can so many wild animals manage to survive in the Serengeti? Their migrations of course let part of the story. By moving from place to place with the changing seasons, they do not overuse and damage the grass in any one area. But other, less obvious factors also are involved. 　　Here on the eastern plains in January, it is clear that most of the animals are eating the abundant grass that springs up like a well-mown lawn between low clumps of Sodom apple and indigo plants. Nearly all of them, from 1,500-pound eland to tiny 10-pound Thomson's gazelle calves, are grazers, which feed on shrubs or the leaves of trees. Singly or in pairs, long lines, or little groups, they move over the green pastures, never remaining long in one place. Where the grass is all short, as it will be when it has been heavily grazed, all the animals apparently eat much the same sort of grass. But where the grass is of varied lengths and toughness, we can see that each animal copes differently with the available fodder. 　　The herds of zebras tend to roam in areas separate from the rest of the grazing multitude. Unlike all the other grazers on the plain, they have teeth in both jaws. This enables them to deal with taller, coarser grass than can the other herbivores. All the rest are various species of antelope, which nip off the grass between their lower incisors and toothless upper palates. Thus, the zebras eat down the longer grasses to a certain level and then move on. 　　Following the zebras come the wildebeests and, in better-wooded areas, hartebeests. These animals eat the grass down a stage further, until it is really short. (They also eat new growth before it has had a chance to grow tall.) Then the Thomson's gazelles take over. With their tails flicking constantly, they nibble at the individual leaves of the tussocks and on the tiny plants that grow between them. By the time all of them have finished, the plain resembles a closely but rather unevenly mown lawn. 　　Thus, one species or another of animal often predominates over a great expanse of the plain, depending on the height to which the grass has grown or has been grazed. Finally when all has been eaten down rather short, most of the grazers leave the area altogether. 　　Two or three weeks later, when more rain has brought on fresh growth, the herds may return to feed over the area again. Perhaps they move about in response to the intensity of local showers, which can vary a good deal over a distance of only a mile or two. In any case, the result of their returning again and again to the same areas is to keep the grass green and short, just as the repeated mowing of a lawn in summer does. 　　If, as a result of badly drawn park boundaries or some other cause, the migrant herd of Serengeti were confined to either the western woodlands or the eastern short-grass plains, they would be forced to return to the same areas so often and would eventually weaken the grass that it would die out. But as they eat it down, they move away and the grass recovers.