Susan P. Lambeth

Chimpanzees are indifferent to the welfare of unrelated group members.

Humans are an unusually prosocial species—we vote, give blood, recycle, give tithes and punish violators of social norms. Experimental evidence indicates that people willingly incur costs to help strangers in anonymous one-shot interactions, and that altruistic behaviour is motivated, at least in part, by empathy and concern for the welfare of others (hereafter referred to as other-regarding preferences). In contrast, cooperative behaviour in non-human primates is mainly limited to kin and reciprocating partners, and is virtually never extended to unfamiliar individuals. Here we present experimental tests of the existence of other-regarding preferences in non-human primates, and show that chimpanzees (Pan troglodytes) do not take advantage of opportunities to deliver benefits to familiar individuals at no material cost to themselves, suggesting that chimpanzee behaviour is not motivated by other-regarding preferences. Chimpanzees are among the primates most likely to demonstrate prosocial behaviours. They participate in a variety of collective activities, including territorial patrols, coalitionary aggression, cooperative hunting, food sharing and joint mate guarding. Consolation of victims of aggression and anecdotal accounts of solicitous treatment of injured individuals suggest that chimpanzees may feel empathy. Chimpanzees sometimes reject exchanges in which they receive less valuable rewards than others, which may be one element of a ‘sense of fairness’, but there is no evidence that they are averse to interactions in which they benefit more than others.

Endowment Effects in Chimpanzees

Human behavior is not always consistent with standard rational choice predictions. Apparent deviations from rational choice predictions provide a promising arena for the merger of economics and biology [1-6]. Although little is known about the extent to which other species exhibit these seemingly irrational patterns [7-9], similarities across species would suggest a common evolutionary root to the phenomena. The present study investigated whether chimpanzees exhibit an endowment effect, a seemingly paradoxical behavior in which humans tend to value a good they have just come to possess more than they would have only a moment before [10-13]. We show the first evidence that chimpanzees do exhibit an endowment effect, by favoring items they just received more than their preferred items that could be acquired through exchange. Moreover, the effect is stronger for food than for less evolutionarily salient objects, perhaps because of historically greater risks associated with keeping a valuable item versus attempting to exchange it for another [14, 15]. These findings suggest that many seeming deviations from rational choice predictions may be common to humans and chimpanzees and that the evaluation of these through a lens of evolutionary relevance may yield further insights in humans and other species.

Chimpanzees (Pan troglodytes) do not develop contingent reciprocity in an experimental task

Chimpanzees provide help to unrelated individuals in a broad range of situations. The pattern of helping within pairs suggests that contingent reciprocity may have been an important mechanism in the evolution of altruism in chimpanzees. However, correlational analyses of the cumulative pattern of interactions over time do not demonstrate that helping is contingent upon previous acts of altruism, as required by the theory of reciprocal altruism. Experimental studies provide a controlled approach to examine the importance of contingency in helping interactions. In this study, we evaluated whether chimpanzees would be more likely to provide food to a social partner from their home group if their partner had previously provided food for them. The chimpanzees manipulated a barpull apparatus in which actors could deliver rewards either to themselves and their partners or only to themselves. Our findings indicate that the chimpanzees’ responses were not consistently influenced by the behavior of their partners in previous rounds. Only one of the 11 dyads that we tested demonstrated positive reciprocity. We conclude that contingent reciprocity does not spontaneously arise in experimental settings, despite the fact that patterns of behavior in the field indicate that individuals cooperate preferentially with reciprocating partners.

Effects of predictable versus unpredictable feeding schedules on chimpanzee behavior

Abstract One factor not often studied in the design of feeding enrichment programs is the timetable according to which enrichments are offered. This study was performed to lend a quantitative basis to primate colony management decisions concerning feeding schedules. The objective was to determine whether feeding fresh produce meals on a predictable schedule was preferable to doing so on an unpredictable schedule from the perspective of promoting psychological well-being of chimpanzees. The subjects were 30 adult or adolescent chimpanzees ( Pan troglodytes ) living in four different social groups. Feeding practices were changed to a more unpredictable schedule for two of these groups, 5 or 6 months prior to the initiation of data collection; feeding of the other two groups continued on a more predictable schedule. The predictable meal was always fed within a 30 min period and the unpredictable meal was fed within a 150 min period. Observational data were collected during prefeeding and baseline periods. A mixed model multivariate analysis of variance for repeated measures revealed that inactivity was more prevalent in the prefeeding period for the chimpanzees fed on the predictable schedule than in the other study phases, and abnormal behavior showed a similar trend. These findings indicate that feeding on a more unpredictable schedule may lead to increased species-appropriate behavior. The scheduling of animal care routines deserves closer attention from those conducting research on improving care and well-being of captive primates.

A potent effect of observational learning on chimpanzee tool construction

Although tool use occurs in diverse species, its complexity may mark an important distinction between humans and other animals. Chimpanzee tool use has many similarities to that seen in humans, yet evidence of the cumulatively complex and constructive technologies common in human populations remains absent in free-ranging chimpanzees. Here we provide the first evidence that chimpanzees have a latent capacity to socially learn to construct a composite tool. Fifty chimpanzees were assigned to one of five demonstration conditions that varied in the amount and type of information available in video footage of a conspecific. Chimpanzees exposed to complete footage of a chimpanzee combining the two components to retrieve a reward learned to combine the tools significantly more than those exposed to more restricted information. In a follow-up test, chimpanzees that constructed tools after watching the complete demonstration tended to do so even when the reward was within reach of the unmodified components, whereas those that spontaneously solved the task (without seeing the modification process) combined only when necessary. Social learning, therefore, had a powerful effect in instilling a marked persistence in the use of a complex technique at the cost of efficiency, inhibiting insightful tool use.

Developing a Comprehensive and Comparative Questionnaire for Measuring Personality in Chimpanzees Using a Simultaneous Top-Down/Bottom-Up Design

One effective method for measuring personality in primates is to use personality trait ratings to distill the experience of people familiar with the individual animals. Previous rating instruments were created using either top‐down or bottom‐up approaches. Top‐down approaches, which essentially adapt instruments originally designed for use with another species, can unfortunately lead to the inclusion of traits irrelevant to chimpanzees or fail to include all relevant aspects of chimpanzee personality. Conversely, because bottom‐up approaches derive traits specifically for chimpanzees, their unique items may impede comparisons with findings in other studies and other species. To address the limitations of each approach, we developed a new personality rating scale using a combined top‐down/bottom‐up design. Seventeen raters rated 99 chimpanzees on the new 41‐item scale, with all but one item being rated reliably. Principal components analysis, using both varimax and direct oblimin rotations, identified six broad factors. Strong evidence was found for five of the factors (Reactivity/Undependability, Dominance, Openness, Extraversion, and Agreeableness). A sixth factor (Methodical) was offered provisionally until more data are collected. We validated the factors against behavioral data collected independently on the chimpanzees. The five factors demonstrated good evidence for convergent and predictive validity, thereby underscoring the robustness of the factors. Our combined top‐down/bottom‐up approach provides the most extensive data to date to support the universal existence of these five personality factors in chimpanzees. This framework, which facilitates cross‐species comparisons, can also play a vital role in understanding the evolution of personality and can assist with husbandry and welfare efforts. Am. J. Primatol. 75:1042–1053, 2013.

Bipedal tool use strengthens chimpanzee hand preferences

The degree to which non-human primate behavior is lateralized, at either individual or population levels, remains controversial. We investigated the relationship between hand preference and posture during tool use in chimpanzees (Pan troglodytes) during bipedal tool use. We experimentally induced tool use in a supported bipedal posture, an unsupported bipedal posture, and a seated posture. Neither bipedal tool use nor these supported conditions have been previously evaluated in apes. The hypotheses tested were 1) bipedal posture will increase the strength of hand preference, and 2) a bipedal stance, without the use of one hand for support, will elicit a right hand preference. Results supported the first, but not the second hypothesis: bipedalism induced the subjects to become more lateralized, but not in any particular direction. Instead, it appears that subtle pre-existing lateral biases, to either the right or left, were emphasized with increasing postural demands. This result has interesting implications for theories of the evolution of tool use and bipedalism, as the combination of bipedalism and tool use may have helped drive extreme lateralization in modern humans, but cannot alone account for the preponderance of right-handedness.

Effects of human activity on chimpanzee wounding

Reducing the frequency and/or severity of aggression and wounding is a major concern of people managing socially housed chimpanzees. One factor that has not been investigated intensively for its effect on captive chimpanzee agonism is the presence of humans. Therefore, we examined an archival database of wounding incidents among 88 adult and adolescent members of 8 social groups of chimpanzees (42 females, 46 males) to determine whether variable levels of human activity (higher weekday versus lower weekend-day activity levels) in the colony was associated with changes in chimpanzee wounding patterns. Wounding was tabulated for each group for periods of 38 - 118 months. A series of Chi-square tests indicated that there were a greater than expected number of wounding episodes on weekdays but that day of the week did not affect the age or sex distribution of wounding. Together, these results suggest that the presence of personnel completing routine activities is associated with chimpanzee agonism. Zoo Biol 16:327–333, 1997.

Responses to the Assurance game in monkeys, apes, and humans using equivalent procedures

There is great interest in the evolution of economic behavior. In typical studies, species are asked to play one of a series of economic games, derived from game theory, and their responses are compared. The advantage of this approach is the relative level of consistency and control that emerges from the games themselves; however, in the typical experiment, procedures and conditions differ widely, particularly between humans and other species. Thus, in the current study, we investigated how three primate species, capuchin monkeys, chimpanzees, and humans, played the Assurance (or Stag Hunt) game using procedures that were, to the best of our ability, the same across species, particularly with respect to training and pretesting. Our goal was to determine what, if any, differences existed in the ways in which these species made decisions in this game. We hypothesized differences along phylogenetic lines, which we found. However, the species were more similar than might be expected. In particular, humans who played using “nonhuman primate-friendly” rules did not behave as is typical. Thus, we find evidence for similarity in decision-making processes across the order Primates. These results indicate that such comparative studies are possible and, moreover, that in any comparison rating species’ relative abilities, extreme care must be taken in ensuring that one species does not have an advantage over the others due to methodological procedures.

Training nonhuman primates to perform behaviors useful in biomedical research.

Data collected from NHPs that are trained to participate voluntarily in husbandry, veterinary, and research procedures are likely to have particular value. The authors present the results of a series of studies that examined the effects of PRT on the performance by chimpanzees of a variety of biomedically relevant behaviors: presenting their perineum for pinworm testing, providing a semen sample, presenting for an s.c. injection, and presenting for an i.m. injection. The overall trends across studies indicate that PRT techniques have significant value in the handling and management of NHPs in many laboratory research settings, including less variability in the data collected and fewer potential confounding variables, which should lead to important refinements in the definition of NHPs as biomedical research models.