Hippos seem to have particular tastes. They prefer the short, tender grass over longer, stalkier grass. Even though tall grass, capable of filling a hippo's stomach quickly, may rest on either side of the hippo lawn, they will work hard to maintain the short grass they prefer. These long but narrow areas of short grass are important because they have been found to limit the extent of brush fires in Africa.
A hippo lawn. Notice the tall grass on either side.