It is difficult to find grazers in grassland when they are feeding with their heads down.
Predators lack places to hide from prey in grasslands.
It is difficult for predators to outrun grazers on grasslands.
Grazers in open grasslands tend to be very large animals.
[#paragraph1]In open grasslands there is no place for a large animal to hide. [#insert1]Thus a watchful grazing animal will see the slight movement that betrays the presence of a predator long before it is close enough to launch an attack. It sounds as though the hunters (predators) stand no chance at all. [#insert2]Unfortunately for the grazers, life is not so simple, however. [#insert3]A grazing animal must lower its head and look at the ground to feed. [#insert4]Its attention may be occupied for only a few seconds before it raises its head and resumes its watch while chewing the food it took, but hunters are patient and skillful and are concentrating intensely. Those few seconds provide time enough to advance a few steps and then freeze, body flattened against the ground. It may take hours, but eventually these repeated small advances will put the hunter within range—close enough to outrun its prey—and the long time the hunt has taken will have been worthwhile, because the resulting feast will be highly nutritious.
[#paragraph2]Clearly the grazers are at a disadvantage, because while they eat they are vulnerable to attack. The hunters also have a weakness, however, and it is one that allows the grazers to survive. Hunters can attack only one prey animal at a time. This applies even to the predators that hunt as a team, such as lionesses, wolves, and hunting dogs. Their hunt involves running down or ambushing an individual. Teamwork allows them to hunt animals much bigger and stronger than themselves and to hunt more successfully, but it does not allow them to attack more than one individual at a time.
[#paragraph3]The grazers exploit this weakness by making it as difficult as they can for the predators to choose an individual as a target. They do not graze alone, scattered widely across the landscape, but together, as a herd. The approaching hunter sees not a solitary animal, but a crowd of animals, all of them moving, so they are constantly crossing and recrossing each other’s paths. No sooner does the hunter choose an individual than another animal has crossed in front of it and the target has disappeared into the herd.
[#paragraph4]There is another advantage to the grazers: A herd is much more alert than a solitary animal. An animal has to relax its guard while it is taking food, but in a herd there are at any time some animals with their heads down, biting, and others, with their heads up, watching. What is more, those with their heads up are looking in different directions so that together they are alert to any movement anywhere on the landscape around them. There is no way for a hunter to approach a herd unobserved. When a member of the herd spots trouble, it starts to move away. Other members of the herd move with it and the entire herd starts to move. If the trouble is serious and close, the herd will run. The individual raising the alarm is simply protecting itself, but in doing so it is warning all of the others.
[#paragraph5]Herding is highly successful, [#highlight6]provided[/highlight6] members of the herd stay together in a tight bunch. The hunter moves with the herd, watching for an individual to wander away from the others. When that happens, it tries to move between that individual and the rest of the herd, preventing it from rejoining. Once it has done that the hunter has a good chance of making a kill. If the herd starts to run, a solitary hunter may abandon the chase, but a pack of wolves or hunting dogs will regard the running herd of animals as an opportunity and set off in pursuit. As the herd runs, one or two old or sick animals, or young animals that become separated from their mothers, may fall behind.
[#paragraph6][#highlight8]As soon as the hunter or hunters seize their prey, they lose interest in all other grazers since then they, too, must concentrate on eating, at which point the herd stops running, those who were left behind rejoin the group, and they all resume grazing.[/highlight8]