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 _ , 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   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. The researches suggest that these monkeys, like humans, are guided by social senses. In the wild, they are co-operative and group-living. Such co-operation is likely to be firm only when each animal feels it is not being cheated. Feelings of anger when unfairly treated, it seems, are not the nature of human beings alone. Refusing a smaller reward completely makes these feelings clear to other animals of the group. However, whether such a sense of fairness developed independently in monkeys and humans, or whether it comes from the common roots that they had 35 million years ago, is, as yet, an unanswered question. We can learn   _  according to the passage?
Human beings' feelings of anger are developed from the monkeys. Cooperation between monkeys stays firm before the realization of being cheated. In the research, male monkeys are less likely to exchange food with others. Only monkeys and humans have the sense of fairness dating back to 35 million years ago.