英語聽力：猛瑪象進(jìn)城記—9

By the mid 1800s, the horse and the cowboy had come to symbolize the wild west. And of course, with the cowboys came cattle. As the bison quickly vanished from the landscape, cattle filled their place.

Meanwhile increasing numbers of the tame horses escaped into the wild. These feral horses became known by the Spanish name, mustang. They put extra pressure on the dwindling bison numbers by competing for their grazing sites and drinking holes.

Mustangs form social groups led by a dominant stallion, echoing their prehistoric relatives that lived here during the Ice Age.

Bred to carry the weight of a rider, mustangs are larger than those early wild horses, but they still display the same kind of behavior.

By the early 1800s, the wild horse was well and truly back in North America, but how did other wildlife on the continent survive alongside growing human populations?

Some wild creatures did the reverse of mustangs by becoming tame and choosing to live close to people. The purple martin became an unofficial mascot for native inhabitants in the eastern half of the continent. Here people erected special nesting sites to encourage the birds to stay.