阅读下面材料，在空白处填入适当的内容（1个单词）或括号内单词的正确形式。
People can often see a talking parrot on a TV show, in a movie, or even in someone's home. The parrot has learned ____ (copy) sounds that people make. Dolphins, bats, and some other animals also copy sounds. Now we can add elephants ____this list of copycats (盲目模仿者).
Dr. Joyce Poole is an expert,____studies the sounds of elephants. While she was in Kenya, she would hear strange noises ____ (make) by Mlaika after sunset. Mlaika was____ 8-year-old African elephant and it lived near a highway.
Dr. Poole says that she couldn't tell the ____ (different) between Mlaika's call and the ____ (distance) truck noise. Why did it copy the sounds of the trucks driving by? Animals that are able to copy sounds may enjoy ____(practice) new sounds. When they ____ (keep) outside of their natural environment, they may copy unusual sounds.
So far Dr. Poole ____(spend) 18 years with two female Asian elephants. Asian elephants make sounds like birds to talk with one another.
Parrots, dolphins, humans, and elephants show that being a copycat is one way that animals and people make new friends and keep old ones.
胚胎干细胞 ④造血干细胞 ⑤神经细胞