一个接受过别人帮助的人，往往会更愿意给予其他需要的人以同样的帮助，这种“投桃报李”的行为一直被认为是人类特有的美德。然而，科学家却在近日发现，这种美德并非只存在于人类身上：一只老鼠在接受其他同类的帮助后，也会变得更加“乐于助人”。
“一般性互惠”现象
据发布在PLoS Biology杂志上的最新文章表面，这种被称为“一般性互惠”的现象，以前只在人类身上发现过。为了解释“一般性互惠”现象，瑞士伯尔尼大学的迈克尔·塔波尔斯基举了一个例子：当一个人在电话亭里捡到别人留下的钱之后，他会更愿意帮助其他陌生人。
塔波尔斯基表示，人类社会中存在的这种“投桃报李”现象，虽然也可以用文化方面的原因来解释，但如果在其他动物身上也发现类似的行为，则说明“投桃报李”更有可能是一种进化现象。
隐藏进化机制
为了测试动物中的“投桃报李”行为，塔波尔斯基对一些老鼠进行了训练，让它们学会抬起一个杠杆。当这个杠杆被抬起来时，另一只老鼠就可以吃到食物，不过抬起杠杆的那只老鼠却吃不到。研究人员发现，和没有吃到这种免费食物的老鼠相比，那些受过别人恩惠的老鼠，会有20％的概率更愿意帮助其他陌生同类。
塔波尔斯基认为，这种行为并不只存在于老鼠和人类之中。他说：“我相信，‘一般性互惠’现象是广泛存在的，在许多不同的动物种类中都能发现这种现象，而我们的研究也表明，这种现象之下隐藏着一种进化机制。”
具有重要的进化意义
对于生物学家而言，解释动物之间的合作现象一直是个棘手问题。根据达尔文的进化理论，各种生物体总是最大限度地增加自己生存的机会，那么这种“投桃报李”的现象又该如何解释呢？
对此，一种解释是，这只动物帮助其他同类，是为了以后得到对方的帮助。不过，这样做无疑很容易受骗，因为它可能会遇到一个只愿索取、不愿回报的对象。
另一种解释是，在动物中也存在“一般性互惠”现象：一只动物会认为，自己与任何其他个体最近一次的互动，代表了它所生活的整个社会的一般行为。在这种情况下，动物不需要记住特定的帮助者。塔波尔斯基表示：“这种机制更加简单，因此可能具有更加重要的进化意义。”
英文原文：
Source: Public Library of Science
Date: July 5, 2007
Altruistic Rats: First Evidence For Generalized Reciprocal Cooperation In Non-humans
Science Daily — Cooperation in animals has long been a major focus in evolutionary biology. In particular, reciprocal altruism, where helpful acts are contingent upon the likelihood of getting help in return, is especially intriguing because it is open to cheaters. In a new study published this week in the open-access journal PLoS Biology, Claudia Rutte and Michael Taborsky demonstrate the first evidence for generalized reciprocal cooperation in non-humans. The authors show that rats who received help in the past were more likely to help another unknown partner.
Although many models have predicted reciprocal altruism, scientists had found evidence only of direct reciprocity ("if you help me, I'll help you") in non-human animals in previous studies. Direct reciprocity is intuitively appealing, yet requires that animals interact repeatedly with the same individuals and remember past interactions. By comparison, generalized reciprocity makes no such cognitive assumptions. In generalized reciprocity, animals are more likely to help a partner if they have been helped in the past, regardless of the past helper's identity.
For example, in humans, people who found a coin in the coin return of a telephone were more likely to help a stranger pick up dropped papers than control subjects who had not previously found money. In humans, this can be explained by cultural experience as well as by natural selection. But if similar reactions to anonymous experience can be found in non human animals, an evolutionary explanation would be far more likely.
In this study, rats received help gaining food from a partner who pulled a stick to produce the food. Rats could therefore be grouped into two classes: those that had previously received help and those that had not. The rats who had previously been helped were then more likely to help another unknown partner receive food. (Credit: iStockphoto/Leroy Dickson)
In this study, Norway rats received help gaining food from a partner who pulled a stick to produce the food. Rats could therefore be grouped into two classes: those that had previously received help and those that had not. The rats who had previously been helped were then more likely to help another unknown partner receive food. This simple mechanism may promote the evolution of cooperation among unfamiliar non-relatives in many other animals.
Citation: Rutte C, Taborsky M (2007) Generalized reciprocity in rats. PLoS Biol 5(7): e196. doi:10.1371/journal.pbio.0050196.
Note: This story has been adapted from a news release issued by Public Library of Science.
