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Brosnan, S. F., Freeman, C., & De Waal, F. B. M. (2006). Partner's behavior, not reward distribution, determines success in an unequal cooperative task in capuchin monkeys. Am. J. Primatol., 68(7), 713–724.
Abstract: It was recently demonstrated that capuchin monkeys notice and respond to distributional inequity, a trait that has been proposed to support the evolution of cooperation in the human species. However, it is unknown how capuchins react to inequitable rewards in an unrestricted cooperative paradigm in which they may freely choose both whether to participate and, within the bounds of their partner's behavior, which reward they will receive for their participation. We tested capuchin monkeys with such a design, using a cooperative barpull, which has been used with great success in the past. Contrary to our expectations, the equity of the reward distribution did not affect success or pulling behavior. However, the behavior of the partner in an unequal situation did affect overall success rates: pairs that had a tendency to alternate which individual received the higher-value food in unequal reward situations were more than twice as successful in obtaining rewards than pairs in which one individual dominated the higher-value food. This ability to equitably distribute rewards in inherently biased cooperative situations has profound implications for activities such as group hunts, in which multiple individuals work together for a single, monopolizable reward.
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Brosnan, S. F., Schiff, H. C., & de Waal, F. B. M. (2005). Tolerance for inequity may increase with social closeness in chimpanzees. Proc Biol Sci, 272(1560), 253–258.
Abstract: Economic decision-making depends on our social environment. Humans tend to respond differently to inequity in close relationships, yet we know little about the potential for such variation in other species. We examine responses to inequity in several groups of chimpanzees (Pan troglodytes) in a paradigm similar to that used previously in capuchin monkeys (Cebus apella). We demonstrate that, like capuchin monkeys, chimpanzees show a response to inequity of rewards that is based upon the partner receiving the reward rather than the presence of the reward alone. However, we also found a great amount of variation between groups tested, indicating that chimpanzees, like people, respond to inequity in a variable manner, which we speculate could be caused by such variables as group size, the social closeness of the group (as reflected in length of time that the group has been together) and group-specific traditions.
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Brosnan, S. F., & De Waal, F. B. M. (2003). Monkeys reject unequal pay. Nature, 425(6955), 297–299.
Abstract: During the evolution of cooperation it may have become critical for individuals to compare their own efforts and pay-offs with those of others. Negative reactions may occur when expectations are violated. One theory proposes that aversion to inequity can explain human cooperation within the bounds of the rational choice model, and may in fact be more inclusive than previous explanations. Although there exists substantial cultural variation in its particulars, this 'sense of fairness' is probably a human universal that has been shown to prevail in a wide variety of circumstances. However, we are not the only cooperative animals, hence inequity aversion may not be uniquely human. Many highly cooperative nonhuman species seem guided by a set of expectations about the outcome of cooperation and the division of resources. Here we demonstrate that a nonhuman primate, the brown capuchin monkey (Cebus apella), responds negatively to unequal reward distribution in exchanges with a human experimenter. Monkeys refused to participate if they witnessed a conspecific obtain a more attractive reward for equal effort, an effect amplified if the partner received such a reward without any effort at all. These reactions support an early evolutionary origin of inequity aversion.
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de Waal, F. B., & Berger, M. L. (2000). Payment for labour in monkeys. Nature, 404(6778), 563.
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Brosnan, S. F., & de Waal, F. B. M. (2004). Socially learned preferences for differentially rewarded tokens in the brown capuchin monkey (Cebus apella). J Comp Psychol, 118(2), 133–139.
Abstract: Social learning is assumed to underlie traditions, yet evidence indicating social learning in capuchin monkeys (Cebus apella), which exhibit traditions, is sparse. The authors tested capuchins for their ability to learn the value of novel tokens using a previously familiar token-exchange economy. Capuchins change their preferences in favor of a token worth a high-value food reward after watching a conspecific model exchange 2 differentially rewarded tokens, yet they fail to develop a similar preference after watching tokens paired with foods in the absence of a conspecific model. They also fail to learn that the value of familiar tokens has changed. Information about token value is available in all situations, but capuchins seem to pay more attention in a social situation involving novel tokens.
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Dunbar, R. I. M., McAdam, M. R., & O'connell, S. (2005). Mental rehearsal in great apes (Pan troglodytes and Pongo pygmaeus) and children. Behav. Process., 69(3), 323–330.
Abstract: The ability to rehearse possible future courses of action in the mind is an important feature of advanced social cognition in humans, and the “social brain” hypothesis implies that it might also be a feature of primate social cognition. We tested two chimpanzees, six orangutans and 63 children aged 3-7 years on a set of four puzzle boxes, half of which were presented with an opportunity to observe the box before being allowed to open it (“prior view”), the others being given without an opportunity to examine the boxes before handling them (“no prior view”). When learning effects are partialled out, puzzle boxes in the “prior view” condition were opened significantly faster than boxes given in the “no prior view” condition by the children, but not by either of the great apes. The three species differ significantly in the speed with which they opened boxes in the “no prior view” condition. The three species' performance on this task was a function of relative frontal lobe volume, suggesting that it may be possible to identify quantitative neuropsychological differences between species.
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Friedrich, A. M., & Zentall, T. R. (2004). Pigeons shift their preference toward locations of food that take more effort to obtain. Behav. Process., 67(3), 405–415.
Abstract: Although animals typically prefer to exert less effort rather than more effort to obtain food, the present research shows that requiring greater effort to obtain food at a particular location appears to increase the value of that location. In Experiment 1, pigeons' initial preference for one feeder was significantly reduced by requiring 1 peck to obtain food from that feeder and requiring 30 pecks to obtain food from the other feeder. In Experiment 2, a similar decrease in preference was not found when pigeons received reinforcement from both feeders independently of the amount of effort required. These results are consistent with the within-trial contrast effect proposed by in which the relative hedonic value of a reward depends on the state of the animal immediately prior to the reward. The greater the improvement from that prior state the greater the value of the reinforcer.
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Brosnan, S. F., & de Waal, F. B. M. (2005). Responses to a simple barter task in chimpanzees, Pan troglodytes. Primates, 46(3), 173–182.
Abstract: Chimpanzees (Pan troglodytes) frequently participate in social exchange involving multiple goods and services of variable value, yet they have not been tested in a formalized situation to see whether they can barter using multiple tokens and rewards. We set up a simple barter economy with two tokens and two associated rewards and tested chimpanzees on their ability to obtain rewards by returning the matching token in situations in which their access to tokens was unlimited or limited. Chimpanzees easily learned to associate value with the tokens, as expected, and did barter, but followed a simple strategy of favoring the higher-value token, regardless of the reward proffered, instead of a more complex but more effective strategy of returning the token that matched the reward. This response is similar to that shown by capuchin monkeys in our previous study. We speculate that this response, while not ideal, may be sufficient to allow for stability of the social exchange system in these primates, and that the importance of social barter to both species may have led to this convergence of strategies.
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Henderson, J., Hurly, T. A., & Healy, S. D. (2006). Spatial relational learning in rufous hummingbirds (Selasphorus rufus). Anim. Cogn., 9(3), 201–205.
Abstract: There is increasing evidence that animals can learn abstract spatial relationships, and successfully transfer this knowledge to novel situations. In this study, rufous hummingbirds (Selasphorus rufus) were trained to feed from either the lower or the higher of two flowers. When presented with a test pair of flowers, one of which was at a novel height, they chose the flower in the appropriate spatial position rather than the flower at the correct height. This response may also have been influenced by a preference for taller flowers as acquisition of the task during experimental training occurred more readily when the reward flower was the taller of the pair. Thus, it appears that although learning abstract relationships may be a general phenomenon across contexts, and perhaps across species, the ease with which they are learned and the context in which they are subsequently used may not be the same.
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Church, D. L., & Plowright, C. M. S. (2006). Spatial encoding by bumblebees (Bombus impatiens) of a reward within an artificial flower array. Anim. Cogn., 9(2), 131–140.
Abstract: We presented bumblebees a spatial memory task similar to that used with other species (e.g., cats, dogs, and pigeons). In some conditions we allowed for presence of scent marks in addition to placing local and global spatial cues in conflict. Bumblebees (Bombus impatiens) were presented an array of artificial flowers within a flight cage, one flower offering reward (S+), while the others were empty (S-). Bees were tested with empty flowers. In Experiment 1, flowers were either moved at the time of testing or not. Bees returned to the flower in the same absolute position of the S+ (the flower-array-independent (FAI) position), even if it was in the wrong position relative to the S- and even when new flower covers prevented the use of possible scent marks. New flower covers (i.e., without possible scent marks) had the effect of lowering the frequency of probing behavior. In Experiment 2, the colony was moved between training and testing. Again, bees chose the flower in the FAI position of the S+, and not the flower that would be chosen using strictly memory for a flight vector. Together, these experiments show that to locate the S+ bees did not rely on scent marks nor the positions of the S-, though the S- were prominent objects close to the goal. Also, bees selected environmental features to remember the position of the S+ instead of relying upon a purely egocentric point of view. Similarities with honeybees and vertebrates are discussed, as well as possible encoding mechanisms.
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