Everybody is happy as his pay rises. Yet pleasure at your own can disappear if you learn that a fellow worker has been given a bigger one. Indeed, if he is known as being lazy, you might even be quite cross. Such behavior is regarded as “all too human”, with the underlying belief that other animals would not be able to have this finely developed sense of sadness. But a study by Sarah Brosnan of Emory University in Atlanta, Georgia, which has just been published in Nature, suggests that it is all too monkey, as well.
    The researchers studied the behaviors of some kind of female brown monkeys. They look smart. They are good-natured, co-operative creatures, and they share their food happily. Above all, like female human beings, they tend to pay much closer attention to the value of “goods and services” than males.
    Such characteristics make them perfect subjects for Doctor Brosnan’s study. The researchers spent two years teaching their monkeys to exchange tokens (奖券) for food. Normally, the monkeys were happy enough to exchange pieces of rock for pieces of cucumber. However, when two monkeys were placed in separate and connected rooms, so that each other could observe what the other is getting in return for its rock, they became quite different.
    In the world of monkeys，grapes are excellent goods (and much preferable to cucumbers). So when one monkey was handed a grape in exchange for her token, the second was not willing to hand hers over for a mere piece of cucumber. And if one received a grape without having to provide her token in exchange at all, the other either shook her own token at the researcher, or refused to accept the cucumber. Indeed, the mere presence of a grape in the other room (without an actual monkey to eat it) was enough to bring about dissatisfaction in a female monkey.