NARRATOR：

Now listen to part of a lecture on the topic you just read about.

FEMALE PROFESSOR：

The hypothesis that the edmontosaur migrated every winter is not convincing.

First, the edmontosaur did not have to migrate to find food.

One hundred million years ago, the summer temperatures in the North Slope area were warmer than they are today.

And remember, in arctic regions like the North Slope, the Sun shines twenty four hours a day at the peak of the summer.

The warm temperatures and extensive daylight created incredibly good growing conditions for plants.

So much vegetation was produced during the summer that when the vegetation died as the winter came, there was a lot of nutritious dead vegetation around in the winter.

The edmontosaur could have easily lived on the dead plant matter during the winter.

Second, just because edmontosaurs lived in herds doesn’t mean they migrated.

Animals live in herds for many other reasons. Living in herds, for example, provides animals with extra protection from predators.

Having extra protection is useful even for the animals that live in the same area the whole year round.

A modern example of this is the Roosevelt elk, a large plant eater.

Roosevelt elk live in the forests of the western United States.

They live in herds but they do not migrate.

Third, although adult edmontosaurs were capable of migrating long distances, what about edmontosaurs that were not yet adults?

Juvenile edmontosaurs were not physically capable of traveling the great distances required to reach warmer territories and would have slowed the herd so much that the herd never would have made it to its destination.

The herd could not have left the juveniles behind, because the juveniles would not have survived on their own.

So the whole herd had to stay where they were and survive on the cold North Slope.