LESSON 101
1
perfectly choice situation
responded feature
2
Baluchitherium powerful interesting
positions creatures thousands full-size
strange-looking grazing survived
opossum predators developed attempted
porpoises tongue weighing kingdom
horsepower lizards adequate whales
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1. occupied
2. complicated
3. avoid
4. marvelous
5. beneficial
6. incredible
4
The Spread of Mammals
Some scientists tell us that when the great reptiles ruled the earth, they were in the sky,
in the sea, and on the land. They came in all forms, from tiny lizards to animals that
could easily look into third-story windows of today's buildings.1
For reasons that are not perfectly clear, the great reptiles of the Mesozoic era died off,
leaving the sky, the sea, and the land open to new animals. Thousands of new birds
and mammals appeared. We would consider many of the early mammals strangelooking. They were as different from each other as the reptiles * they replaced.
Some were huge, much larger than the African elephant, which is the largest living
land animal. Baluchitherium, the largest land mammal that ever lived, stood more
than five meters tall at the shoulder and weighed almost twenty metric tons. That's
more than three full-sized elephants weigh.2
Like the great reptiles, not all mammals survived. Some types died off. Some changed.
And some live today in much the same way they lived during the Mesozoic era.
The opossum, for example, lives at night, does not use its eyes very much, and has
a fairly small brain. The reason * it has a small brain is that the opossum doesn't
need a larger brain. The brain that it had in the Mesozoic era was adequate.
The brains changed most in animals that changed the way they lived. The horse became
a daytime animal, changing in size from an animal no bigger than a fox to the fullsized horse of today.3 Its brain also changed so that it could survive in the open
among predators that hunted it. As grazing animals like the horse developed larger
brains, they learned more tricks to avoid their hunters.
One theory is that predators * or hunters needed even bigger and more complicated
brains than the hunted. Unless predators were smarter than grazing animals, they
would not be able to catch their prey, and they would starve to death. Therefore,
predators developed larger and more powerful brains, even larger than those of
grazing animals.4
Among the most interesting predators are those land mammals that became sea animals.
Many of them live today as whales and porpoises. Their legs became flippers and
tails.
Their bodies took the shape of fish. Although they changed in many ways, they kept the
mammal brain.
A killer whale's brain weighs * more than the brain of a human being. This does not
mean that the killer whale is smarter than a person. The killer whale is a very large
animal, weighing as much as an elephant. To handle messages in a body this large,
the killer whale's brain must be quite large.5
Whales are among the most amazing creatures in the animal kingdom. In addition to
being smart, some are incredibly large. The largest animal that has ever existed on
Earth is the blue whale. To get an idea of its size, let's compare the blue whale to an
African elephant. * A blue whale is about as large at birth as a full-grown African
elephant! A full-grown whale may be over thirty meters long and may weigh 175
metric tons. Its heart weighs more than half a metric ton, and its tongue weighs
almost four metric tons. When it travels at top speed, which is over thirty-five
kilometers an hour, it develops 520 horsepower. Indeed, the blue whale is one of
the most marvelous creatures in the animal kingdom.6