The fact that squirrel monkeys do not routinely cooperate in the wild has been proposed to explain their failure to show disadvantageous inequity aversion (i.e., negative reactions when receiving less than a partner) in an experimental exchange. Here we assessed whether the use of a tray-pulling paradigm, allowing for a larger variety of unequal testing situations, would bring additional insights into inequity aversion in this species. Squirrel monkeys were tested in pairs in which only the donor could pull a tray baited with food to within reach of itself and a recipient. Using pairs with different social relationships, we examined donors’ frequencies of pulling both in the presence and absence of a recipient, as well as across three different food distributions: equal, qualitative inequity (higher-value reward for the recipient), and quantitative inequity (no food reward for the donor). Results showed that female donors pulled the tray less often in the quantitative inequity condition with an out-group female recipient than when alone. However, such discrimination was not observed when females were with female in-group and male out-group recipients. By contrast, male donors did not adjust their pull frequencies according to the recipient’s presence or identity (female and male out-group recipients). These results point towards possible disadvantageous inequity aversion in female squirrel monkeys. However, alternative hypotheses such as increased arousal caused by out-group female recipients cannot be ruled out. We discuss the data in line with major theories of inequity aversion and cooperation in primates.

中文翻译：

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研究松鼠猴 (Saimiri sciureus) 对托盘拉动范式中食物分配不均的反应

松鼠猴在野外不经常合作的事实已被提出来解释它们未能在实验交流中表现出不利的不公平厌恶（即，在接受少于伴侣时的负面反应）。在这里，我们评估了使用托盘拉式范式，允许更多种类的不平等测试情况，是否会带来对该物种的不公平厌恶的更多见解。松鼠猴成对进行测试，其中只有捐赠者才能将装有食物的托盘拉到自己和接受者够得着的地方。使用具有不同社会关系的配对，我们检查了捐赠者在接受者在场和不在场以及三种不同食物分配中拉动的频率：平等、质量不公平（接受者获得更高价值的奖励）、和数量上的不平等（捐赠者没有食物奖励）。结果表明，在数量不公平的情况下，与单独的女性捐赠者相比，女性捐赠者在与外部女性接受者的情况下更少地拉托盘。然而，当女性与女性内群体和男性外群体接受者在一起时，没有观察到这种歧视。相比之下，男性捐赠者没有根据接受者的存在或身份（女性和男性外群接受者）调整他们的拉动频率。这些结果表明雌性松鼠猴可能存在不利的不平等厌恶。然而，不能排除其他假设，例如由外群女性接受者引起的性唤起增加。我们根据灵长类动物的不公平厌恶和合作的主要理论讨论数据。结果表明，在数量不公平的情况下，女性捐赠者在与外群女性接受者的情况下比单独行动时拉托盘的频率更低。然而，当女性与女性内群体和男性外群体接受者在一起时，没有观察到这种歧视。相比之下，男性捐赠者没有根据接受者的存在或身份（女性和男性外群接受者）调整他们的拉动频率。这些结果表明雌性松鼠猴可能存在不利的不平等厌恶。然而，不能排除其他假设，例如由外群女性接受者引起的性唤起增加。我们根据灵长类动物的不公平厌恶和合作的主要理论讨论数据。结果表明，在数量不公平的情况下，女性捐赠者在与外群女性接受者的情况下比单独行动时拉托盘的频率更低。然而，当女性与女性内群体和男性外群体接受者在一起时，没有观察到这种歧视。相比之下，男性捐赠者没有根据接受者的存在或身份（女性和男性外群接受者）调整他们的拉动频率。这些结果表明雌性松鼠猴可能存在不利的不平等厌恶。然而，不能排除其他假设，例如由外群女性接受者引起的性唤起增加。我们根据灵长类动物的不公平厌恶和合作的主要理论讨论数据。