Non-human primates use combined rules when deciding under ambiguity

Abstract

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’.

A causal role for the parietal cortex in ambiguity computations in humans

“…That is, for instance, simply dividing the hidden area of our task in half rather than making a more profound calculation. Heuristics are crucial in complex situations because they are simplified decision rules that help individuals deal with problems requiring high cognitive investment 38,39 . Compared with younger adults, older adults show different parietal activity when faced with a decision under uncertainty probability associated with another heuristic 40 .…”

“…Comparative studies indicate that nonhuman primates show ambiguity aversion as adult humans, revealing that this situation entails a high cognitive cost 3,4 . Interestingly, most primates species use simple heuristics to face ambiguous decisions, and only great apes consider the ambiguity of the information in the decision process 39 . In accordance, researchers in developmental neuroscience have shown that children do not show ambiguity aversion 41 as adults and adolescents 42 .…”

Exclusion by donkey’s ears: Donkeys (Equus asinus) use acoustic information to find hidden food in a two-way object-choice task.

“…Our first goal was to implement generalized linear mixed model (GLMM) trees to identify associations of variables that lead to different success rates. GLMM trees, described by Fokkema et al (2018), are particularly well fitted to animal cognitive studies involving decision-type data (for a detailed description and examples, see Danel et al, 2021; Romain et al, 2021). The GLMM tree algorithm identifies subgroups of observations (also called terminal nodes) that differ in success rate, taking into consideration the nested structure of the data set (subjects).…”

Reasoning

“…Our most recent study (Romain et al, 2021) used the same task, but also exposed individuals to ambiguous options. We expected individuals to make use of simple decision rules rather than attempting to evaluate the odds of winning or losing, thus expressing ambiguity aversion (with only partial information about the odds).…”