CARACTERISTICAS TECNICAS 1 ADOQUINES Y BALDOSAS DE PIEDRA

February, when the plains are lush with green grass, is the time of greatest danger for a wildebeest herd. Thousands of female wildebeest give birth to calves in synchrony with one another. The birthing period lasts for about three weeks. Females lie down, give birth in about 10 minutes, and are on their feet as soon as possible. The calves are up and running within minutes of being born. Quick births and strong-legged newborns are

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essential to wildebeest survival because predators have been fol- lowing the herd. Lions, leopards, hyenas, cheetahs, and jackals wait for birthing season. Every female on the ground is vulnera- ble to a predator. Thousands of newborn calves are taken as prey, even before they can get to their feet. On the open plains of the Serengeti, there are no hiding places. Only the calves and moth- ers that can run with the herd will survive the predator attacks. By taking wildebeest females and calves as prey, the predators help ensure their own survival and the survival of their young.

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the herd AS An AdvAntAge

Predators take so many calves that it does not seem as if a herd helps with survival. The large, noisy herd is obvious to preda- tors. The thousands of daily births make it easy for predators to fi nd calves. Nevertheless, scientists have discovered that wilde- beest are safer in a herd than they are alone.

Wildebeest are large antelopes. They can grow to be 4.5 feet (1.4 meters) tall and weigh up to 600 pounds (272 kilograms). Such a big animal cannot hide from predators, especially on a treeless plain. In a large group, however, each animal can “hide” by being surrounded by thousands of others. Hundreds of thou- sands of animals are visually confusing. Usually, only the strag- glers are in danger.

Adult wildebeest are powerful. They have large, dangerous horns. As a group, wildebeest can fi ght off single predators or groups of smaller ones, such as wild dogs. When faced by large predators, such as lions, some wildebeest will group together and approach, behaving aggressively and staring. There is no way for the predator to surprise a wildebeest. This group behavior gives the entire herd a better chance of survival.

The synchronous birthing is also a survival strategy. So many calves are born at once that predators can’t eat them all. Calves born outside of the birthing season have a much smaller chance of survival. They are too easy for predators to see.

Once a calf survives its fi rst month of life, it has a good chance of making it to adulthood. The calf has plenty of rich grass to eat, and the grass is available in the same areas at the same times. Those calves that get to their feet the quickest and learn to run the fastest are likely to make it through the fi rst month. They will then probably fi nd enough to eat, grow to adulthood, and pass on their genes to the next generation.

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in A BuffAlo herd

African buffalo herds are similar to wildebeest herds, but their behavior is quite different. Buffalo also live in selfi sh herds, but each herd numbers only in the hundreds. Instead of migrating long distances, a herd lives in a home territory. It moves from area to area within this territory while searching for grass to eat. Like wildebeest, buffalo have no leader, but they do something that amazes scientists: They vote on where to go.

Researcher H.H.T. Prins discovered this communication among buffalo. At fi rst, he did not understand what the buffalo were doing. He would observe the herd during the day, while the

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buffalo rested. He noticed that some females (called cows) would get up, move around, and then lie down again. After a while he realized that this was a form of communication. The cows stand in a certain way, look in one direction, and raise their heads slightly.

Prins put his observations together after many hours of watching the buffalo. He saw them getting up, moving their legs a bit, and then lying down again. Eventually, he realized that the cows held their heads in a unique position while they were staring. He noticed that their heads were higher than a resting position but lower than an alert (or startled) position. The head position and the shuffl ing of their feet seemed to have a special meaning. Prins discovered that the cows performed this pattern of standing, staring, and lying back down over and over for about an hour at a time. Then suddenly, at the end of the hour, the whole herd seemed energized and started moving.

“The exciting thing is that they start trekking [traveling] at the beginning and independently of each other in the same direction,” Prins wrote in his 1996 book, Ecology and Behav-

iour of the African Buffalo. “Within seconds, the animals that

[start] these movements are followed by other individuals . . . and within about three to fi ve minutes the whole herd of hun- dreds of individuals moves as if conducted by one master. They totally give the impression that they know where they are going to: Apparently, some decision has been taken by the group.”

Prins realized that multiple cows looking in the same direc- tion becomes a majority vote that seems to decide where the herd will go. No cow ever makes the decision alone, and the same cows do not always decide when to move. Sometimes, however, the cows do disagree. If some cows look one way and too many others look in an opposite direction, the herd might split up. Two groups then might travel to different grazing patches and spend the night. Somehow, cows decide which group to follow. Prins

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and other scientists have no idea how this behavior started in Af- rican buffalo. They are not sure how it helps the herd to survive, but it seems to work for the African buffalo, and scientists assume it is a valuable survival tool for every individual in the herd.