英语听力：《恐龙的战争》3 顽强防御—17

Individually, their huge size is a powerful defense, but sticking together makes these Leviathans almost indestructible. Keeping a herd of 50-ton animals filled up, however, means traveling vast distances, grazing territory covering thousands of square kilometers. And moving a mountain of flesh requires sophisticated coordination.

One of the curious things about sauropods is that they did have a swelling in the spinal cord in the neighborhood of their pelvis. It is sort of like a second brain to help control the back half of the body.

It’s called the sacro-lumbar expansion, a bundle of nerves which acts like a primitive brain controlling the movements of the legs and tail. The brain in a Sauroposeidon’s head is simply too small and far away to coordinate its front and back legs. So a second brain operates the back end of the animal.

The reason why animals like Sauroposeidon have an enlargement in their hip is because there’s actually a lot going on back there. There are lots of muscles that are in the legs, and also muscles that are going through the tail.

The primitive brain in its pelvis keeps Sauroposeidon’s legs and tail in sync with its body. Without it, they could never have grazed over such large territories or possibly, they wouldn’t have even been able to walk at all. This tiny bundle of nerves weighing just a few grams allows herds of Sauroposeidon to rule the great plains of the southern Midwest of America for almost 15 million years.

Another plant eater couldn’t appear more different. It’s small, weak, and highly intelligent. But like Sauroposeidon, Parasaurolophus live in herds. They stick together and are constantly on the lookout for threats. But for Parasaurolophus, there’s a problem. Their eyes …