1. (2020高二下·上海月考) Directions: After reading the passages below, fill in the blanks to make the passages coherent and grammatically correct For the blanks with a given word, fill in each blank with the proper form of the given word; for the other blanks, use one word that best fits each blank.

Are Bamboo-Eating Pandas Really Herbivores?

On the outside, giant pandas look like herbivores (食草动物). They spend nearly all of their waking hours  (eat) bamboo. But on the inside, they're built like carnivores (食肉动物). About half of the calories they eat come from protein, according to a new study.

The ancestor of giant pandas were omnivorous (杂食的). They ate both animals and plants, and had the digestive system and gut bacteria to metabolize (使发生新陈代谢) them. They had "umami taste receptors," to appreciate the flavors of meat.

However, about 2. 4 million years ago, things began to change. The gene for their "umami taste receptors" became  (active). Their jaw and teeth evolved to help them crush bamboo, and their wrist bone became capable of grasping the stalks (秆) of their favorite plant. Scientists think pandas switched to eating bamboo partly  they didn't have to fight with other animals to get it. Bamboo is high in fiber but has a low concentration of nutrients, so pandas  eat 20 to 40 pounds of the plant every day just to get by.

David Raubenheimer, a nutritional ecologist at the University of Sydney, and his colleagues put GPS trackers on two giant pandas and followed their movements throughout the year. They discovered that the pandas followed the protein. Between August and April, they (seek) food in low elevations (海拔) on China's Qinling Mountains. At the start of the cycle, they ate Bashania fargesii leaves  they got the chance to feast on young shoots, which contained more protein.

The more the shoots grew, the more their protein was diluted (冲淡) by fiber. That caused the pandas to move to higher ground,  Fargesia qinlingensis grew. First, they ate the shoots, but these, too, went from being protein-rich to fiber-rich as they grew. The pandas responded by switching to the leaves. They fed  them until they went back down the mountain and started eating Bashania fargesii leaves again. The researchers found that about half of the calories the pandas ate were in the form of protein.

Scientists think the research shows that pandas are very clever. "They can know exactly where to go, and when to go, so they can get the most of the nutrients  their ecosystem can provide," said Silvia Pineda-Munoz, who was not involved in the study.

The work also shows that classifying an animal as herbivore or carnivore is more complex than one might assume. "It's not  you're eating plants but what part of the plants you're eating," said Pineda-Munoz.