英语听力：猛玛象进城记—7

...and may trace their ancestry all the way back to the first dogs to enter North America. The Carolina dogs are pack animals with a strick hierarchy topped by an alpha male. Like most wild dogs, they hunt in groups. But many of the kills they make are small. Rabbits and small rodents make up a large part of their diet in what can become a collective feeding frenzy. At the end of the ice age, these dogs' ancestors also had access to other sources of food. They may have hung around the camps of early native people, scavenging the scraps.

But although they lived on the fringes of human society, the early wild dogs were certainly not pets. Exactly when they arrived in North America is still uncertain. But it's likely that they slipped in after the extinction of the ice age beasts during a time of massive change.

Eventually the continent became more settled. Over the next few thousand years, people adapted to the many regions of the continent and their varying lifestyles were shaped by the landscape they lived in.

Then more than 14,000 years after the first people set foot in North America, another wave of immigrants brought changes that would have a dramatic impact on the landscape and wildlife of the new world.