In 1977, Irene Pepperberg of Harvard University began studying what was on another creature's mind by talking to it. Her first experiments began with Alex. Alex was a one-year-old African grey parrot and Irene taught him to produce the sounds of the English language. "I thought if he learned to communicate, I could ask him questions about how he sees the world." At the time, most scientists didn't believe animals had any thoughts. They thought animals were more like robots but didn't have the ability to think or feel. Of course, if you own a pet you probably disagree. But it is the job of a scientist to prove this and nowadays more scientists accept that animals can think for themselves. "That's why I started my studies with Alex," Irene said, "Some people actually called me crazy for trying this." Nowadays, we have more and more evidence that animals have all sorts of mental abilities. Sheep can recognize faces. Chimpanzees   use a variety of tools and even use weapons to hunt. And Alex the parrot became a very good talker. Thirty years after the Alex studies began. Irene was still giving him English lessons up until his recent death. For example, if Alex was hungry he could say "want grape". Alex could count to six and was learning the sounds for seven and eight. "He has to hear the words over and over before he can correctly say them." Irene said, after pronouncing "seven" for Alex a few times in a row. Alex could also tell the difference between colors, shapes, sizes, and materials (e.g. wood and metal). Before he finally died, Alex managed to say "seven". Another famous pet that proved some animals have greater mental skills was a dog called Rico. He appeared on a German TV game show in 2011. Rico knew the names of 200 different toys and easily learned the names of new ones. When Rico became famous, many other dog owners wanted to show how clever their pets were. Another dog called Betsy could understand 300 words. One theory for dogs' ability to learn a language is that they have ben close companions to humans for many centuries and so their ability to understand us is constantly evolving  . While animals can't do what humans do yet, some scientists believe that examples like Alex and Rico prove that evolution develops intelligence, as well as physical appearance. The two dogs mentioned in the article could   _  .
understand some words recognize strange voices copy human gestures tell different colors