Can nonhuman primates use tokens to represent and sum quantities?

Maze runners: monkeys show restricted Arabic numeral summation during computerized two-arm maze performance

Mazes have been used in many forms to provide compelling results showcasing nonhuman animals' capacities for spatial navigation, planning, and numerical competence. The current study presented computerized two-arm mazes to four rhesus macaques. Using these mazes, we assessed whether the monkeys could maximize rewards by overcoming mild delays in gratification and sum the values of Arabic numerals. Across four test phases, monkeys used a joystick controller to choose one of two maze arms on the screen. Each maze arm contained zero, one or two Arabic numerals, and any numerals in the chosen maze arm provided the monkeys with rewards equivalent to the value of those numerals. When deciding which arm to enter, monkeys had to consider distance to numerals and numeral value. In some tests, gaining the maximum reward required summing the value of two numerals within a given arm. All four monkeys successfully maximized reward when comparing single numerals and when comparing arms that each contained two numerals. However, some biases occurred that were suboptimal: the largest single numeral and the delay of reward (by placing numerals farther into an arm from the start location) sometimes interfered with the monkeys' abilities to optimize. These results indicate that monkeys experience difficulties with inhibition toward single, high valence stimuli in tasks where those stimuli must be considered in relation to overall value when represented by symbolic stimuli such as numerals.

Comparative economics: how studying other primates helps us better understand the evolution of our own economic decision making

The origins of evolutionary games are rooted in both economics and animal behaviour, but economics has, until recently, focused primarily on humans. Although historically, specific games were used in targeted circumstances with non-human species (i.e. the Prisoner's Dilemma), experimental economics has been increasingly recognized as a valuable method for directly comparing both the outcomes of economic decisions and their underlying mechanisms across species, particularly in comparison with humans, thanks to the structured procedures that allow for them to be instantiated across a variety of animals. So far, results in non-human primates suggest that even when outcomes are shared, underlying proximate mechanisms can vary substantially. Intriguingly, in some contexts non-human primates more easily find a Nash equilibrium than do humans, possibly owing to their greater willingness to explore the parameter space, but humans excel at more complex outcomes, such as alternating between two Nash equilibria, even when deprived of language or instruction, suggesting potential mechanisms that humans have evolved to allow us to solve complex social problems. We consider what these results suggest about the evolution of economic decision-making and suggest future directions, in particular the need to expand taxonomic diversity, to expand this promising approach. This article is part of the theme issue 'Half a century of evolutionary games: a synthesis of theory, application and future directions'.

Non-human primate token use shows possibilities but also limitations for establishing a form of currency

Non-human primates evaluate choices based on quantitative information and subjective valuation of options. Non-human primates can learn to value tokens as placeholders for primary rewards (such as food). With those tokens established as a potential form of 'currency', it is then possible to examine how they respond to opportunities to earn and use tokens in ways such as accumulating tokens or exchanging tokens with each other or with human experimenters to gain primary rewards. Sometimes, individuals make efficient and beneficial choices to obtain tokens and then exchange them at the right moments to gain optimal reward. Sometimes, they even accumulate such rewards through extended delay of gratification, or through other exchange-based interactions. Thus, non-human primates are capable of associating value to arbitrary tokens that may function as currency-like stimuli, but there also are strong limitations on how non-human primates can integrate such tokens into choice situations or use such tokens to fully 'symbolize' economic decision-making. These limitations are important to acknowledge when considering the evolutionary emergence of currency use in our species. This article is part of the theme issue 'Existence and prevalence of economic behaviours among non-human primates'.

Quantity-quality trade-off in the acquisition of token preference by capuchin monkeys (Sapajus spp.)

Money represents a cornerstone of human modern economies and how money emerged as a medium of exchange is a crucial question for social sciences. Although non-human primates have not developed monetary systems, they can estimate, combine and exchange tokens. Here, we evaluated quantity-quality trade-offs in token choices in tufted capuchin monkeys as a first step in the investigation of the generalizability of tokens as reinforcers, which is a potentially relevant factor underlying the emergence of money in humans. We measured capuchins' exchange preferences when they were repeatedly provided with 10 units of three token types yielding food combinations varying in quantity and quality. Overall, capuchins maximized their quantitative payoff, preferring tokens associated with a higher food amount, rather than showing violations of rationality. However, some individuals did not maximize their qualitative payoff, possibly because of conditional valuation effects or owing to the choice overload phenomenon, according to which too many options reduce the accuracy of choice. Our study supports the importance of comparative research to finely analyse the multiple components shaping the economic behaviours of other species, possibly to achieve a more comprehensive, evolutionary- and ecologically based understanding of human economic behaviour. This article is part of the theme issue 'Existence and prevalence of economic behaviours among non-human primates'.

Are the roots of human economic systems shared with non-human primates?

We review and analyze evidence for an evolutionary rooting of human economic behaviors and organization in non-human primates. Rather than focusing on the direct application of economic models that a priori account for animal decision behavior, we adopt an inductive definition of economic behavior in terms of the contribution of individual cognitive capacities to the provision of resources within an exchange structure. We spell out to what extent non-human primates' individual and strategic decision behaviors are shared with humans. We focus on the ability to trade, through barter or token-mediated exchanges, as a landmark of an economic system among members of the same species. It is an open question why only humans have reached a high level of economic sophistication. While primates have many of the necessary cognitive abilities (symbolic and computational) in isolation, one plausible issue we identify is the limits in exerting cognitive control to combine several sources of information. The difference between human and non-human primates' economies might well then be in degree rather than kind.

Macaque monkeys can learn token values from human models through vicarious reward

Monkeys can learn the symbolic meaning of tokens, and exchange them to get a reward. Monkeys can also learn the symbolic value of a token by observing conspecifics but it is not clear if they can learn passively by observing other actors, e.g., humans. To answer this question, we tested two monkeys in a token exchange paradigm in three experiments. Monkeys learned token values through observation of human models exchanging them. We used, after a phase of object familiarization, different sets of tokens. One token of each set was rewarded with a bit of apple. Other tokens had zero value (neutral tokens). Each token was presented only in one set. During the observation phase, monkeys watched the human model exchange tokens and watched them consume rewards (vicarious rewards). In the test phase, the monkeys were asked to exchange one of the tokens for food reward. Sets of three tokens were used in the first experiment and sets of two tokens were used in the second and third experiments. The valuable token was presented with different probabilities in the observation phase during the first and second experiments in which the monkeys exchanged the valuable token more frequently than any of the neutral tokens. The third experiments examined the effect of unequal probabilities. Our results support the view that monkeys can learn from non-conspecific actors through vicarious reward, even a symbolic task like the token-exchange task.

Chimpanzees (Pan troglodytes) transfer tokens repeatedly with a partner to accumulate rewards in a self-control task

There has been extensive research investigating self-control in humans and nonhuman animals, yet we know surprisingly little about how one's social environment influences self-control. The present study examined the self-control of chimpanzees in a task that required active engagement with conspecifics. The task consisted of transferring a token back and forth with a partner animal in order to accumulate food rewards, one item per token transfer. Self-control was required because at any point in the trial, either chimpanzee could obtain their accumulated rewards, but doing so discontinued the food accumulation and ended the trial for both individuals. Chimpanzees readily engaged the task and accumulated the majority of available rewards before ending each trial, and they did so across a number of conditions that varied the identity of the partner, the presence/absence of the experimenter, and the means by which they could obtain rewards. A second experiment examined chimpanzees' self-control when given the choice between immediately available food items and a potentially larger amount of rewards that could be obtained by engaging the token transfer task with a partner. Chimpanzees were flexible in their decision-making in this test, typically choosing the option representing the largest amount of food, even if it involved delayed accumulation of the rewards via the token transfer task. These results demonstrate that chimpanzees can exhibit self-control in situations involving social interactions, and they encourage further research into this important aspect of the self-control scenario.

Adult cleaner wrasse outperform capuchin monkeys, chimpanzees and orang-utans in a complex foraging task derived from cleaner--client reef fish cooperation

The insight that animals' cognitive abilities are linked to their evolutionary history, and hence their ecology, provides the framework for the comparative approach. Despite primates renowned dietary complexity and social cognition, including cooperative abilities, we here demonstrate that cleaner wrasse outperform three primate species, capuchin monkeys, chimpanzees and orang-utans, in a foraging task involving a choice between two actions, both of which yield identical immediate rewards, but only one of which yields an additional delayed reward. The foraging task decisions involve partner choice in cleaners: they must service visiting client reef fish before resident clients to access both; otherwise the former switch to a different cleaner. Wild caught adult, but not juvenile, cleaners learned to solve the task quickly and relearned the task when it was reversed. The majority of primates failed to perform above chance after 100 trials, which is in sharp contrast to previous studies showing that primates easily learn to choose an action that yields immediate double rewards compared to an alternative action. In conclusion, the adult cleaners' ability to choose a superior action with initially neutral consequences is likely due to repeated exposure in nature, which leads to specific learned optimal foraging decision rules.

Sequential responding and planning in capuchin monkeys (Cebus apella)

Previous experiments have assessed planning during sequential responding to computer generated stimuli by Old World nonhuman primates including chimpanzees and rhesus macaques. However, no such assessment has been made with a New World primate species. Capuchin monkeys (Cebus apella) are an interesting test case for assessing the distribution of cognitive processes in the Order Primates because they sometimes show proficiency in tasks also mastered by apes and Old World monkeys, but in other cases fail to match the proficiency of those other species. In two experiments, eight capuchin monkeys selected five arbitrary stimuli in distinct locations on a computer monitor in a learned sequence. In Experiment 1, shift trials occurred in which the second and third stimuli were transposed when the first stimulus was selected by the animal. In Experiment 2, mask trials occurred in which all remaining stimuli were masked after the monkey selected the first stimulus. Monkeys made more mistakes on trials in which the locations of the second and third stimuli were interchanged than on trials in which locations were not interchanged, suggesting they had already planned to select a location that no longer contained the correct stimulus. When mask trials occurred, monkeys performed at levels significantly better than chance, but their performance exceeded chance levels only for the first and the second selections on a trial. These data indicate that capuchin monkeys performed very similarly to chimpanzees and rhesus monkeys and appeared to plan their selection sequences during the computerized task, but only to a limited degree.

Language-trained chimpanzees (Pan troglodytes) delay gratification by choosing token exchange over immediate reward consumption

Token exchange inherently introduces an element of delay between behavior and reward and so token studies may help us better understand delay of gratification and self-control. To examine this possibility, we presented three language-trained chimpanzees with repeated choices involving different foods that could be eaten immediately or lexigram (graphic symbol) tokens that represented (and could be traded for) foods later. When both options were foods, chimpanzees always chose more preferred foods over less preferred foods. When both options were lexigram tokens representing those same foods, performance remained the same as chimpanzees selected the higher value token and then traded it for food. Then, when faced with choosing a token that could be traded later or choosing a food item that could be eaten immediately, most chimpanzees learned to make whatever response led to the more preferred food. They did this even when that meant selecting a high value lexigram token that could be traded only 2 to 3 min later instead of a medium value, but immediately available, food item. Thus, chimpanzees flexibly selected tokens even though such selections necessarily delayed gratification and required forgoing immediately available food. This finding illustrates the utility of symbolic token exchange for assessing self-control in nonhuman animals.