When individuals exchange helpful acts reciprocally, increasing the benefit of the receiver can enhance its propensity to return a favour, as pay-offs are typically correlated in iterated interactions. Therefore, reciprocally cooperating animals should consider the relative benefit for the receiver when deciding to help a conspecific. Norway rats (Rattus norvegicus) exchange food reciprocally and thereby take into account both the cost of helping and the potential benefit to the receiver. By using a variant of the sequential iterated prisoner’s dilemma paradigm, we show that rats may determine the need of another individual by olfactory cues alone. In an experimental food-exchange task, test subjects were provided with odour cues from hungry or satiated conspecifics located in a different room. Our results show that wild-type Norway rats provide help to a stooge quicker when they receive odour cues from a hungry rather than from a satiated conspecific. Using chemical analysis by gas chromatography-mass spectrometry (GC-MS), we identify seven volatile organic compounds that differ in their abundance between hungry and satiated rats. Combined, this “smell of hunger” can apparently serve as a reliable cue of need in reciprocal cooperation, which supports the hypothesis of honest signalling.

当个人相互交换帮助行为时，增加接收者的利益可以增强其回馈偏好的倾向，因为收益通常与迭代的交互作用相关。因此，相互合作的动物在决定帮助同种动物时应考虑给接收者带来的相对利益。挪威大鼠（rattus norvegicus）相互交换食物，因此要同时考虑帮助的成本和给接收者带来的潜在利益。通过使用顺序迭代的囚徒困境范式的变体，我们证明了大鼠可能仅凭嗅觉提示就可以确定是否需要另一个人。在一项实验性的食物交换任务中，为测试对象提供了来自位于另一房间的饥饿或饱足的特定物种的气味提示。我们的结果表明，当野生型挪威大鼠从饥饿而不是从饱足的特定物种那里获得气味提示时，它们可以更快地帮助小偷。使用气相色谱-质谱法（GC-MS）进行化学分析，我们确定了7种挥发性有机化合物，它们在饥饿和饱腹的大鼠之间的丰度不同。结合起来