【2005年阅读Text 1 第3段】
Such characteristics make them perfect candidates for Dr. Brosnan’s and Dr. de Waal’s study. The researchers spent two years teaching their m&111nkeys to exchange tokens for food. Normally, the m&111nkeys were happy enough to exchange pieces of rock for slices of cucumber. However, when two m&111nkeys were placed in separate but adjoining chambers, so that each could observe what the other was getting in return for its rock, their behaviour became markedly different.