Anything for a cheerio: Brown capuchins (Sapajus [Cebus] apella) consistently coordinate in an Assurance Game for unequal payoffs.

Unequal outcomes disrupt cooperation in some situations, but this has not been tested in the context of coordination in economic games. To explore this, we tested brown capuchins (Sapajus [Cebus] apella) on a manual version of the Stag Hunt (or Assurance) Game, in which individuals sequentially chose between two options, Stag or Hare, and were rewarded according to their choices and that of their partner. Typically, coordination on Stag results in an equal highest payout, whereas coordinating on Hare results in a guaranteed equal but lower payoff and uncoordinated play results in the lowest payoff when playing Stag. We varied this structure such that one capuchin received double the rewards for the coordinated Stag outcome; thus, it was still both animals' best option, but no longer equally rewarding. Despite the inequality, capuchins coordinated on Stag in 78% of trials, and neither payoff structure nor their partner's choice impacted their decision. Additionally, there was no relationship between self-scratching, a measure of stress in capuchins, and choices. After completing the study, we discovered our reward, cheerios, was sufficiently valuable that in another study, capuchins never refused it, so post hoc we repeated the study using a lower value reward, banana flavored pellets. Capuchins completed only 26% of the pellet trials (compared to 98% with cheerios), constraining our ability to interpret the results, but nonetheless the monkeys showed a decrease in preference for Stag, particularly when they received fewer rewards for the coordinated Stag outcome. These results reinforce capuchins' ability to find coordinated outcomes in the Stag Hunt game, but more work is needed to determine whether the monkeys did not mind the inequality or were unwilling to sacrifice a highly preferred food to rectify it. In either case, researchers should carefully consider the impact of their chosen rewards on subjects' choices.

Capuchin monkeys (sometimes) go when they know: Confidence movements in Sapajus apella.

To test for evidence of metacognition in capuchin monkeys (Sapajus apella), we analyzed confidence movements using a paradigm adapted from research with chimpanzees. Capuchin monkeys provide an interesting model species for the comparative assessment of metacognition as they show limited evidence of such cognitive-monitoring processes in a variety of metacognition paradigms. Here, monkeys were presented with a computerized delayed matching-to-sample (DMTS) memory test in one location but were rewarded for correct responses in a separate location. Movements could be made from one location to the other at any time, but movements between a response and reward feedback may reflect confidence in the accuracy of the response. Critically, DMTS tests included occasional "no sample" trials where monkeys' performance was at chance when the trial started without a sample and a 1-s interval to the response options. We predicted that monkeys would (1) perform less accurately (and less confidently) at longer retention intervals, (2) move to the dispenser early more often on trials completed correctly than incorrectly, and (3) show a relation between faster response latency and early movements. Analyses of response times and "go" or "no go" confidence movements before feedback to the reward location suggested that the monkeys were capable of monitoring confidence in their responses. However, their confidence movements were less precise and less flexible than chimpanzees. Overall, this paradigm can reveal potential metacognitive abilities in nonhuman animals that otherwise demonstrate these abilities inconsistently.

The density bias: Capuchin monkeys (Sapajus apella) prefer densely arranged items in a food-choice task.

In the current work, we investigated whether capuchin monkeys preferred densely distributed resources to sparsely distributed resources in a 2-choice discrimination task with edible rewards. Capuchin monkeys were biased to select a denser food set over the same number of food items in a sparsely arranged set. Furthermore, increased density of the larger food set facilitated discrimination performance in quantity comparisons with a true difference in set size. These results align with previous studies demonstrating a preference for densely distributed food sets in infants and callitrichid primates, as well as previous evidence of a density bias among several rhesus macaques and capuchin monkeys in a computerized relative quantity discrimination task. Thus, the density bias appears to emerge across multiple domains and presentation formats for some primate species. The role of density in perceived numerosity by capuchin monkeys and other species as it pertains to the foraging domain is discussed. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Capuchin monkeys' use of human and conspecific cues to solve a hidden object-choice task.

Learning by watching others can provide valuable information with adaptive consequences, such as identifying the presence of a predator or locating a food source. The extent to which nonhuman animals can gain information by reading the cues of others is often tested by evaluating responses to human gestures, such as a point, and less often evaluated by examining responses to conspecific cues. We tested whether ten brown capuchin monkeys (Cebus [Sapajus] apella) were able to use cues from monkeys and a pointing cue from a human to obtain hidden rewards. A monkey could gain access to a reward hidden in one of two locations by reading a cue from a conspecific (e.g., reaching) or a human pointing. We then tested whether they could transfer this skill from monkeys to humans, from humans to monkeys, and from one conspecific to another conspecific. One group of monkeys was trained and tested using a conspecific as the cue-giver and was then tested with a human cue-giver. The second group of monkeys was trained and tested with a human cue-giver and was then tested with a monkey cue-giver. Monkeys that were successful with a conspecific cue-giver were also tested with a novel conspecific cue-giver. Monkeys learned to use a human point and conspecific cues to obtain rewards. Monkeys that had learned to use the cues of a conspecific to obtain rewards performed significantly better than expected by chance when they were transferred to the cues of a novel conspecific. Monkeys that learned to use a human point to obtain rewards performed significantly better than expected by chance when tested while observing conspecific cues. Some evidence suggested that transferring between conspecific cue-givers occurred with more facility than transferring across species. Results may be explained by simple rules of association learning and stimulus generalization; however, spontaneous flexible use of gestures across conspecifics and between different species may indicate capuchins can generalize learned social cues within and partially across species.

Testing the Glucose Hypothesis among Capuchin Monkeys: Does Glucose Boost Self-Control?

The ego-depletion hypothesis states that self-control diminishes over time and with exertion. Accordingly, the glucose hypothesis attributes this depletion of self-control resources to decreases in blood glucose levels. Research has led to mixed findings among humans and nonhuman animals, with limited evidence for such a link between glucose and self-control among closely-related nonhuman primate species, but some evidence from more distantly related species (e.g., honeybees and dogs). We tested this hypothesis in capuchin monkeys by manipulating the sugar content of a calorie-matched breakfast meal following a nocturnal fast, and then presenting each monkey with the accumulation self-control task. Monkeys were presented with food items one-by-one until the subject retrieved and ate the accumulating items, which required continual inhibition of food retrieval in the face of an increasingly desirable reward. Results indicated no relationship between self-control performance on the accumulation task and glucose ingestion levels following a fast. These results do not provide support for the glucose hypothesis of self-control among capuchin monkeys within the presented paradigm. Further research assessing self-control and its physiological correlates among closely- and distantly-related species is warranted to shed light on the mechanisms underlying self-control behavior.

Self-control assessments of capuchin monkeys with the rotating tray task and the accumulation task.

Recent studies of delay of gratification in capuchin monkeys using a rotating tray (RT) task have shown improved self-control performance in these animals in comparison to the accumulation (AC) task. In this study, we investigated whether this improvement resulted from the difference in methods between the rotating tray task and previous tests, or whether it was the result of greater overall experience with delay of gratification tasks. Experiment 1 produced similar performance levels by capuchins monkeys in the RT and AC tasks when identical reward and temporal parameters were used. Experiment 2 demonstrated a similar result using reward amounts that were more similar to previous AC experiments with these monkeys. In Experiment 3, monkeys performed multiple versions of the AC task with varied reward and temporal parameters. Their self-control behavior was found to be dependent on the overall delay to reward consumption, rather than the overall reward amount ultimately consumed. These findings indicate that these capuchin monkeys' self-control capacities were more likely to have improved across studies because of the greater experience they had with delay of gratification tasks. Experiment 4 and Experiment 5 tested new, task-naïve monkeys on both tasks, finding more limited evidence of self-control, and no evidence that one task was more beneficial than the other in promoting self-control. The results of this study suggest that future testing of this kind should focus on temporal parameters and reward magnitude parameters to establish accurate measures of delay of gratification capacity and development in this species and perhaps others.

Trading up: chimpanzees (Pan troglodytes) show self-control through their exchange behavior.

Self-control is defined as the ability or capacity to obtain an objectively more valuable outcome rather than an objectively less valuable outcome though tolerating a longer delay or a greater effort requirement (or both) in obtaining that more valuable outcome. A number of tests have been devised to assess self-control in non-human animals, including exchange tasks. In this study, three chimpanzees (Pan troglodytes) participated in a delay of gratification task that required food exchange as the behavioral response that reflected self-control. The chimpanzees were offered opportunities to inhibit eating and instead exchange a currently possessed food item for a different (and sometimes better) item, often needing to exchange several food items before obtaining the highest valued reward. We manipulated reward type, reward size, reward visibility, delay to exchange, and location of the highest valued reward in the sequence of exchange events to compare performance within the same individuals. The chimpanzees successfully traded until obtaining the best item in most cases, although there were individual differences among participants in some variations of the test. These results support the idea that self-control is robust in chimpanzees even in contexts in which they perhaps anticipate future rewards and sustain delay of gratification until they can obtain the ultimately most valuable item.