Decision outcomes in unpredictable environments may not have exact known probabilities. Yet the predictability level of outcomes matters in decisions, and animals, including humans, generally avoid ambiguous options. Managing ambiguity may be more challenging and requires stronger cognitive skills than decision-making under risk, where decisions involve known probabilities. Here we compare decision-making in capuchins, macaques, orangutans, gorillas, chimpanzees and bonobos in risky and ambiguous contexts. Subjects were shown lotteries (a tray of potential rewards, some large, some small) and could gamble a medium-sized food item to obtain one of the displayed rewards. The odds of winning and losing varied and were accessible in the risky context (all rewards were visible) or partially available in the ambiguous context (some rewards were covered). In the latter case, the level of information varied from fully ambiguous (individuals could not guess what was under the covers) to predictable (individuals could guess). None of the species avoided gambling in ambiguous lotteries and gambling rates were high if at least two large rewards were visible. Capuchins and bonobos ignored the covered items and gorillas and macaques took the presence of potential rewards into account, but only chimpanzees and orangutans could consistently build correct expectations about the size of the covered rewards. Chimpanzees and orangutans combined decision rules according to the number of large visible rewards and the level of predictability, a process resembling conditional probabilities assessment in humans. Despite a low sample size, this is the first evidence in non-human primates that a combination of several rules can underlie choices made in an unpredictable environment. Our finding that non-human primates can deal with the uncertainty of an outcome when exchanging one food item for another is a key element to the understanding of the evolutionary origins of economic behaviour.

This article is part of the theme issue ‘Existence and prevalence of economic behaviours among non-human primates’.

不可预测环境中的决策结果可能没有确切的已知概率。然而，结果的可预测性水平在决策中很重要，包括人类在内的动物通常会避免模棱两可的选择。与风险下的决策相比，管理模糊性可能更具挑战性，并且需要更强的认知技能，因为决策涉及已知概率。在这里，我们比较了卷尾猴、猕猴、猩猩、大猩猩、黑猩猩和倭黑猩猩在危险和模棱两可的环境中的决策。受试者被展示了彩票（一盘潜在的奖励，一些大的，一些小的）并且可以赌博一个中等大小的食物以获得显示的奖励之一。获胜和失败的几率各不相同，并且在有风险的环境中可以访问（所有奖励都是可见的）或在模棱两可的环境中部分可用（包括一些奖励）。在后一种情况下，信息水平从完全模棱两可（个人无法猜测幕后内容）到可预测（个人可以猜测）不等。如果至少有两个大的奖励是可见的，那么没有一个物种避免在模棱两可的彩票中赌博，并且赌博率很高。卷尾猴和倭黑猩猩忽略了被覆盖的物品，大猩猩和猕猴考虑到了潜在奖励的存在，但只有黑猩猩和猩猩才能始终如一地对覆盖奖励的大小建立正确的预期。黑猩猩和猩猩根据大量可见奖励的数量和可预测性水平组合决策规则，这一过程类似于人类的条件概率评估。尽管样本量很小，这是非人类灵长类动物的第一个证据，表明在不可预知的环境中做出的选择可能是多个规则的组合。我们发现非人类灵长类动物在将一种食物换成另一种食物时可以处理结果的不确定性，这是理解经济行为进化起源的关键因素。

本文是主题“非人类灵长类动物经济行为的存在和流行”的一部分。