Maintenance of Self-Imposed Delay of Gratification by Four Chimpanzees (Pan troglodytes) and an Orangutan (Pongo pygmaeus)

Delay maintenance, which is the continuance over time of the choice to forgo an immediate, less preferred reward for a future, more preferred reward, was examined in 4 chimpanzees (Pan troglodytes) and 1 orangutan (Pongo pygmaeus). In the 1st experiment, the apes were presented with 20 chocolate pieces that were placed, one at a time, into a bowl that was within their reach. The apes could consume the available chocolate pieces at any time during a trial, but no additional pieces would be given. The total length of time taken to place the 20 items into the bowl ranged from 60 s to 180 s. All 5 apes delayed gratification on a majority of trials until all 20 chocolate pieces were presented. Unlike in most experiments with human children using this test situation, attention by the apes to the reward was not detrimental to delay maintenance. In a 2nd experiment with the chimpanzees only, 4 foods of differing incentive value were presented in different trials in the same manner as in Experiment 1. The chimpanzees were highly successful in obtaining all food pieces, and there was no difference in performance as a function of food type.     

Natural choice in chimpanzees (Pan troglodytes): Perceptual and temporal effects on selective value

In three experiments, four chimpanzees made choices between two visible food options to assess the validity of the selective value effect (the assignment of value to only the most preferred type of food presented in a comparison). In Experiment 1, we established that all chimpanzees preferred single banana pieces to single apple pieces before presenting the critical test. In this test two chimpanzees preferred a mix of one banana piece and one apple piece to a single banana piece when both banana pieces were approximately the same size, but two chimpanzees were indifferent between the two options, exhibiting the selective value effect. In Experiment 2, when the banana pieces in both options were more closely equated in size, the chimpanzees were biased to choose the single banana piece over the mixed array even though this was the smaller total amount of food. However, in Experiment 3, when we introduced longer intervals between each trial, the chimpanzees preferred the mixed set and thus the larger total amount of food. The results demonstrate that only some chimpanzees exhibit the choice pattern indicative of the selective value effect, and they do so only when item size is not carefully controlled and trials are presented quickly in succession. Thus, the behavior pattern originally labeled the selective value effect may actually be explained by a combination of chimpanzees’ sensitivity to small differences in preferred food amount and chimpanzees tendency to avoid less preferred foods that would delay the acquisition of further preferred food items.     

Choosing schedules of signaled appetitive events over schedules of unsignaled ones

Two experiments were completed allowing albino rats to choose between signaled and unsignaled reward conditions. These experiments examined the effects on preference of (1) response dependent versus response-independent reward and, (2) food pellets versus chocolate milk as the reward. All subjects preferred the signaled condition over the unsignaled condition, whether exposed to response-dependent, or to response-independent delivery of rewards. Preference was controlled most effectively by presenting both the signal itself and the correlated stimulus identifying the signaled condition. The signal presented alone (Extinction 3) controlled preference more effectively than did the stimulus correlated with the signaled condition (Extinction 2). The second experiment showed that the quality of the reinforcer (pellets and chocolate milk) did not affect preference for signaled reward since all subjects preferred the signaled condition at levels comparable to those observed in Experiment 1, with food pellets. These results, along with others, argue against species differences, response-dependency, and reinforcer quality as variables affecting the direction of preference.     

The magic cup: great apes and domestic dogs (Canis familiaris) individuate objects according to their properties

Despite current interest in dog (Canis familiaris) cognition, very little is known about how dogs represent objects and how they compare with other species, such as the great apes. Therefore, we investigated how dogs and great apes (chimpanzees [Pan troglodytes], bonobos [Pan paniscus], orangutans [Pongo pygmaeus], gorillas [Gorilla gorilla]) individuate objects in a classical violation of expectation paradigm. We used a container (magic cup) with a double bottom that allowed us to change the type of food that subjects had seen being placed in the container. Using a 2 × 2 design, we varied whether subjects received a generally preferred food and whether the food was substituted (surprise trials) or not (baseline trials). Apes showed increased begging and looking behaviors and dogs showed increased smelling behavior. Both species stayed near the experimenter more frequently in the surprise trials compared with baseline trials. Both species reacted to positive (i.e., good food substituted for bad food) and negative (i.e., bad food substituted for good food) surprises. These results suggest that apes and dogs were able to individuate objects according to their properties or type in comparable ways. In addition, we looked for frustration and elation effects, but subjects' behaviors were not influenced by the food they saw and which they received in previous trials.     

Intuitive optics: what great apes infer from mirrors and shadows

There is ongoing debate about the extent to which nonhuman animals, like humans, can go beyond first-order perceptual information to abstract structural information from their environment. To provide more empirical evidence regarding this question, we examined what type of information great apes (chimpanzees, bonobos, and orangutans) gain from optical effects such as shadows and mirror images. In an initial experiment, we investigated whether apes would use mirror images and shadows to locate hidden food. We found that all examined ape species used these cues to find the food. Follow-up experiments showed that apes neither confused these optical effects with the food rewards nor did they merely associate cues with food. First, naïve chimpanzees used the shadow of the hidden food to locate it but they did not learn within the same number of trials to use a perceptually similar rubber patch as indicator of the hidden food reward. Second, apes made use of the mirror images to estimate the distance of the hidden food from their own body. Depending on the distance, apes either pointed into the direction of the food or tried to access the hidden food directly. Third, apes showed some sensitivity to the geometrical relation between mirror orientation and mirrored objects when searching hidden food. Fourth, apes tended to interpret mirror images and pictures of these mirror images differently depending on their prior knowledge. Together, these findings suggest that apes are sensitive to the optical relation between mirror images and shadows and their physical referents.     

Behavioral cues that great apes use to forage for hidden food

We conducted three studies to examine whether the four great ape species (chimpanzees, bonobos, gorillas, and orangutans) are able to use behavioral experimenter-given cues in an object-choice task. In the subsequent experimental conditions subjects were presented with two eggs, one of which contained food and the other did not. In Study 1 the experimenter examined both eggs by smelling or shaking them, but only made a failed attempt to open (via biting) the egg containing food. In a control condition, the experimenter examined and attempted to open both eggs, but in reverse order to control for stimulus enhancement. The apes significantly preferred the egg that was first examined and then bitten, but had no preference in a baseline condition in which there were no cues. In Study 2, we investigated whether the apes could extend this ability to cues not observed in apes so far (i.e., attempting to pull apart the egg), as well as whether they made this discrimination based on the function of the action the experimenter performed. Subjects significantly preferred eggs presented with this novel cue, but did not prefer eggs presented with a novel but functionally irrelevant action. In Study 3, apes did not interpret human actions as cues to food-location when they already knew that the eggs were empty. Thus, great apes were able to use a variety of experimenter-given cues associated with foraging actions to locate hidden food and thereby were partially sensitive to the general purpose underlying these actions.     

Spatial memory and monitoring of hidden items through spatial displacements by chimpanzees (Pan troglodytes)

This study examined chimpanzee (Pan troglodytes) short-term memory for food location in near space. In Experiments 1 and 2, either 1 or 2 items (chocolate pieces) were hidden in an array of 3 or 5 containers that either remained stationary or were rotated 180 degrees or 360 degrees. When the array remained stationary, the chimpanzees remembered both item locations. When arrays were rotated, however, chimpanzees found only 1 item. In Experiment 3, 2 items were hidden in an array of 7 cups. Both items were found at levels significantly better than chance. Ninety percent of errors were made after the 1st item was found, and errors reflected memory failure rather than a failure of inhibitory control.     

“We will work for you” – Social influence may suppress individual food preferences in a communicative situation in dogs

The level of motivation (i.e. incentive power) is thought to be one of the most important factors affecting performance and learning in various tasks. We investigated whether reward quality has an effect on the performance of family dogs in a two-way object choice test in which they can find the hidden food by relying on distal momentary human pointing cues. In three experiments we varied (1) the type of food reward according to the subjects’ own preference; (2) the quality of the reward offered at the same time in the indicated and not-indicated locations; and (3) the order of the high or low quality rewards in consecutive sessions. In Experiment 1, we first tested whether dogs prefer one kind of reward over another. Then one group was tested with the ‘preferred’ food as reward in the indicated bowl, while dogs in the other group received the ‘non-preferred’ food as reward. We did not find any difference between the performance and choice latencies of the two groups. In Experiment 2 for the first group, the indicated bowl contained a piece of carrot and the not-indicated bowl was empty. In the second group the indicated bowl contained carrot, but the not-indicated bowl contained sausage. According to a preliminary preference test, most dogs prefer sausage over carrot invariably. After 20 trials, the two groups performed surprisingly similarly. There was no difference found between groups in the number of correct choices, incorrect choices and non-choices. However, the comparison between the first and last five trials revealed that subjects who found sausage when they chose the not-indicated bowl (did not follow the pointing) chose the non-indicated bowl significantly more often toward the end of their test session. In Experiment 3, each dog received two sessions with 12 pointing trials in each. For the first session, one group was rewarded with sausage and the other with carrot upon choosing the indicated bowl. In the second session, the indicated bowl contained dry dog food for both groups. We found that correct choices and response latencies did not change over two sessions in the ‘sausage’ group. In the ‘carrot’ group, the dogs chose faster in the second session, but their performance did not improve; in fact, they chose the not-indicated bowl more often than the indicated bowl. As a conclusion, we can say that reward quality had some effect on dogs’ choice behavior in these experiments. The drop in their performance was not drastic, taking into account the general refusal to eat one of the ‘rewards’ (carrot) during the preference tests and also during the test trials. It seems that incentive contrast may play a relatively minor role in dog-human social interactions. Appropriate reward quality can be very important in asocial problem solving tasks, but, when interacting with humans, following human signals may override the effect of changed incentive power.     

Time preferences in long-tailed macaques (Macaca fascicularis) and humans (Homo sapiens)

Rosati et al. (Curr Biol 17(19):1663–1668, 2007) found in a self-control test in which choice was between a smaller, immediately delivered food and a larger, delayed food, that chimpanzees preferred the larger reward (self-control); humans, however, preferred the smaller reward (impulsivity). They attributed their results to a species difference in self-control. In Experiment 1, monkeys (long-tailed macaques) were exposed to a self-control task in two conditions: where the food was hidden under differently colored bowls and where it was visible. When these two conditions were compared, choice shifted from greater preference for the impulsive alternative in the hidden condition to greater preference for the self-control alternative in the visible condition. Additionally, in both conditions, preference shifted from self-control to impulsivity over sessions. These results were explained in terms of the reversed-contingency effect (a propensity to reach for more over less when rewards are visible) and not to a capacity for self-control. In Experiment 2, humans that demonstrated preference for more over less in choice preferred the impulsive alternative when choice to either alternative was followed by the same intertrial interval—a preference that accelerates trial rates relative to preference of the self-control alternative. When trial rates were equated so that neither choice accelerated session’s end, humans demonstrated self-control. These results suggest that Rosati et al.’s demonstration of impulsivity in humans was due to participants’ desire to minimize session time.     

Reverse-reward learning in squirrel monkeys (Saimiri sciureus): retesting after 5 years, and assessment on qualitative transfer

Seven squirrel monkeys (Saimiri sciureus) previously trained on reverse-reward tasks were presented with the original "1-versus-4" task after a 5-year interval without reverse-reward experience (Experiment 1). None of them reliably selected the smaller food array; however, at around chance level, their performance was superior to when they were first exposed to the task almost 6 years previously, suggesting some long-term memory retention. One naive monkey consistently selected the larger array, as expected. In Experiment 2, trials consisting of 1 versus 1 piece of two qualitatively different types of food were interspersed among familiar 1-versus-4 trials. None of five monkeys tested reliably selected the less-preferred food to get the more preferred food as the reward, and one monkey scored below chance. However, when one piece of low-preference food was paired with four pieces of high-preference food (Experiment 3), all four monkeys tested avoided reaching for the latter and thereby obtained it as the reward; two monkeys obtained perfect scores on these trials. These two monkeys were trained on a specific qualitative reverse-reward pairing and then again tested on new pairings (Experiment 4), but transfer was incomplete. Compound trials that pit quantity against quality in novel ways appear taxing for squirrel monkeys, despite competence in reverse-reward on both dimensions separately.     

How the great apes (Pan troglodytes, Pongo pygmaeus, Pan paniscus, Gorilla gorilla) perform on the reversed reward contingency task II: transfer to new quantities, long-term retention, and the impact of quantity ratios

We tested 6 chimpanzees (Pan troglodytes), 3 orangutans (Pongo pygmaeus), 4 bonobos (Pan paniscus), and 2 gorillas (Gorilla gorilla) in the reversed reward contingency task. Individuals were presented with pairs of quantities ranging between 0 and 6 food items. Prior to testing, some experienced apes had solved this task using 2 quantities while others were totally na?ve. Experienced apes transferred their ability to multiple-novel pairs after 6 to 19 months had elapsed since their initial testing. Two out of 6 na?ve apes (1 chimpanzee, 1 bonobo) solved the task--a proportion comparable to that of a previous study using 2 pairs of quantities. Their acquisition speed was also comparable to the successful subjects from that study. The ratio between quantities explained a large portion of the variance but affected na?ve and experienced individuals differently. For smaller ratios, na?ve individuals were well below 50% correct and experienced ones were well above 50%, yet both groups tended to converge toward 50% for larger ratios. Thus, some apes require no procedural modifications to overcome their strong bias for selecting the larger of 2 quantities.     

Skill mastery inhibits adoption of observed alternative solutions among chimpanzees (Pan troglodytes)

Geographic variation in socially transmitted skills and signals, similar to human culture, has been well documented for great apes. The rules governing the adoption of novel behaviours, however, are still largely unknown. We conducted an innovation-and-transmission experiment with two groups of chimpanzees living at hopE Primate Sanctuary Gänserndorf, Austria, presenting a board on which food had to be manoeuvred around obstacles to be acquired. Most chimpanzees used sticks to acquire the food, but five adults independently invented a novel technique, rattling, which was subsequently tested by almost all group members. However, individuals who had become proficient with sticks were reluctant to switch to rattling, despite it being more efficient. Similarly, after rattling was prevented, rattle specialists kept trying to rattle and made no attempt to use the stick technique, despite their knowledge about its existence. We conclude that innovators stimulate others to experiment with the solutions they display, but that chimpanzees are nevertheless conservative; mastery of a skill inhibits further exploration, and hence adoption of alternative techniques even if these are more efficient. Consequently, conformity among group members should not be expected in great apes when individuals develop proficiency at different techniques. Conservatism thus joins conformity as a mechanism to bring about cultural uniformity and stability.     

Do orangutans (<Emphasis Type="Italic">Pongo pygmaeus</Emphasis>) know when they do not remember?

Metacognition refers to the ability to monitor and control one’s own cognitive activities such as memory. Although recent studies have raised an interesting possibility that some species of nonhuman animals might possess such skills, subjects often required a numerous number of training trials to acquire the effective use of metacognitive responses. Here, five orangutans (Pongo pygmaeus) were tested whether they were able to escape spatial memory tests when they did not remember the location of preferred reward in a relatively small number of trials. The apes were presented with two identical cups, under one of which the experimenter hid a preferred reward (e.g., two grapes). The subjects were then presented with a third container, “escape response”, with which they could receive a less preferred but secure reward (e.g., one grape). The orangutans as a group significantly more likely selected the escape response when the baiting of the preferred reward was invisible (as compared to when it was visible) and when the hiding locations of the preferred reward were switched (as compared to when they remained unchanged). Even when the escape response was presented before the final presentation of the memory test, one orangutan successfully avoided the test in which she would likely err. These findings indicate that some orangutans appear to tell when they do not remember correct answers in memory tests.     

Numerical judgments by chimpanzees (Pan troglodytes) in a token economy

We presented four chimpanzees with a series of tasks that involved comparing two token sets or comparing a token set to a quantity of food. Selected tokens could be exchanged for food items on a one-to-one basis. Chimpanzees successfully selected the larger numerical set for comparisons of 1 to 5 items when both sets were visible and when sets were presented through one-by-one addition of tokens into two opaque containers. Two of four chimpanzees used the number of tokens and food items to guide responding in all conditions, rather than relying on token color, size, total amount, or duration of set presentation. These results demonstrate that judgments of simultaneous and sequential sets of stimuli are made by some chimpanzees on the basis of the numerousness of sets rather than other non-numerical dimensions. The tokens were treated as equivalent to food items on the basis of their numerousness, and the chimpanzees maximized reward by choosing the larger number of items in all situations.     

Delaying gratification for food and tokens in capuchin monkeys (Cebus apella) and chimpanzees (Pan troglodytes): when quantity is salient, symbolic stimuli do not improve performance

Capuchin monkeys have been tested for the capacity to delay gratification for accumulating rewards in recent studies and have exhibited variable results. Meanwhile, chimpanzees have consistently excelled at this task. However, neither species have ever been tested at accumulating symbolic tokens instead of food items, even though previous reports indicate that tokens sometimes facilitate performance in other self-control tasks. Thus, in the present study, we tested capuchin monkeys and chimpanzees for their capacity to delay gratification in a delay maintenance task, in which an experimenter presented items, one at a time, to within reach of an animal for as long as the animal refrained from taking them. In Experiment 1, we assessed how long capuchin monkeys could accumulate items in the delay maintenance task when items were food rewards or tokens exchangeable for food rewards. Monkeys accumulated more food rewards than they did tokens. In Experiment 2, we tested capuchin monkeys and chimpanzees in a similar accumulation test. Whereas capuchins again accumulated more food than tokens, all chimpanzees but one showed no difference in performance in the two conditions. These findings provide additional evidence that chimpanzees exhibit greater self-control capacity in this task than do capuchin monkeys and indicate that symbolic stimuli fail to facilitate delay maintenance when they do not abstract away from the quantitative dimension of the task. This is consistent with previous findings on the effects of symbols on self-control and illuminates what makes accumulation a particularly challenging task.     

Multimodal communication by captive chimpanzees (Pan troglodytes)

Many studies have shown that apes and monkeys are adept at cross-modal matching tasks requiring the subject to identify objects in one modality when information regarding those objects has been presented in a different modality. However, much less is known about non-human primates’ production of multimodal signaling in communicative contexts. Here, we present evidence from a study of 110 chimpanzees demonstrating that they select the modality of communication in accordance with variations in the attentional focus of a human interactant, which is consistent with previous research. In each trial, we presented desirable food to one of two chimpanzees, turning mid-way through the trial from facing one chimpanzee to facing the other chimpanzee, and documented their communicative displays, as the experimenter turned towards or away from the subjects. These chimpanzees varied their signals within a context-appropriate modality, displaying a range of different visual signals when a human experimenter was facing them and a range of different auditory or tactile (attention-getting) signals when the human was facing away from them; this finding extends previous research on multimodal signaling in this species. Thus, in the impoverished circumstances characteristic of captivity, complex signaling tactics are nevertheless exhibited by chimpanzees, suggesting continuity in intersubjective psychological processes in humans and apes.     

Great apes’ strategies to map spatial relations

We investigated reasoning about spatial relational similarity in three great ape species: chimpanzees, bonobos, and orangutans. Apes were presented with three spatial mapping tasks in which they were required to find a reward in an array of three cups, after observing a reward being hidden in a different array of three cups. To obtain a food reward, apes needed to choose the cup that was in the same relative position (i.e., on the left) as the baited cup in the other array. The three tasks differed in the constellation of the two arrays. In Experiment 1, the arrays were placed next to each other, forming a line. In Experiment 2, the positioning of the two arrays varied each trial, being placed either one behind the other in two rows, or next to each other, forming a line. Finally, in Experiment 3, the two arrays were always positioned one behind the other in two rows, but misaligned. Results suggested that apes compared the two arrays and recognized that they were similar in some way. However, we believe that instead of mapping the left–left, middle–middle, and right–right cups from each array, they mapped the cups that shared the most similar relations to nearby landmarks (table’s visual boundaries).     

Can brown lemurs (<Emphasis Type="Italic">Eulemur fulvus</Emphasis>) learn to deceive a human competitor?

In the present study we asked whether lemurs could learn to manipulate information in order to deceive a human competitive trainer. Four brown lemurs were trained to communicate about the location of a hidden reward to a cooperative trainer, who rewarded the subject if he indicated the baited bowl. Next, a competitive trainer was introduced who kept the reward for himself if the subject indicated the baited bowl. In a first experiment, sessions were randomly assigned to be with either the cooperative or competitive trainer. No subject was able to show an efficient tactic with both trainers. In a second experiment, the participation of the two trainers was randomized across the trials for each session. When trials were mixed, one subject significantly chose baited location when interacting with the cooperative trainer, and reliably increased his choices of the unbaited location when presented with the competitive trainer. As with most other primate species tested under the same paradigm, associative learning may explain deceptive pointing by lemurs in this study.     

A competitive nonverbal false belief task for children and apes

A nonverbal false belief task was administered to children (mean age 5 years) and two great ape species: chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). Because apes typically perform poorly in cooperative contexts, our task was competitive. Two versions were run: in both, a human competitor witnessed an experimenter hide a reward in one of two containers. When the competitor then left the room (version A) or turned around (version B), the experimenter switched the locations of the containers. The competitor returned and reached with effort, but unsuccessfully, towards the incorrect container. Children displayed an understanding of the competitor's false belief by correctly choosing the other container to find the reward. Apes did not. However, in version A (but not version B), apes looked more often at the unchosen container in false belief trials than in true belief control trials, possibly indicating some implicit or uncertain understanding that needs to be investigated further.     

Gambling primates: reactions to a modified Iowa Gambling Task in humans,chimpanzees and capuchin monkeys

Humans will, at times, act against their own economic self-interest, for example, in gambling situations. To explore the evolutionary roots of this behavior, we modified a traditional human gambling task, the Iowa Gambling Task (IGT), for use with chimpanzees, capuchin monkeys and humans. We expanded the traditional task to include two additional payoff structures to fully elucidate the ways in which these primate species respond to differing reward distributions versus overall quantities of rewards, a component often missing in the existing literature. We found that while all three species respond as typical humans do in the standard IGT payoff structure, species and individual differences emerge in our new payoff structures. Specifically, when variance avoidance and reward maximization conflicted, roughly equivalent numbers of apes maximized their rewards and avoided variance, indicating that the traditional payoff structure of the IGT is insufficient to disentangle these competing strategies. Capuchin monkeys showed little consistency in their choices. To determine whether this was a true species difference or an effect of task presentation, we replicated the experiment but increased the intertrial interval. In this case, several capuchin monkeys followed a reward maximization strategy, while chimpanzees retained the same strategy they had used previously. This suggests that individual differences in strategies for interacting with variance and reward maximization are present in apes, but not in capuchin monkeys. The primate gambling task presented here is a useful methodology for disentangling strategies of variance avoidance and reward maximization.