Jennifer Vonk

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.

Do chimpanzees learn reputation by observation? Evidence from direct and indirect experience with generous and selfish strangers.

Can chimpanzees learn the reputation of strangers indirectly by observation? Or are such stable behavioral attributions made exclusively by first-person interactions? To address this question, we let seven chimpanzees observe unfamiliar humans either consistently give (generous donor) or refuse to give (selfish donor) food to a familiar human recipient (Experiments 1 and 2) and a conspecific (Experiment 3). While chimpanzees did not initially prefer to beg for food from the generous donor (Experiment 1), after continued opportunities to observe the same behavioral exchanges, four chimpanzees developed a preference for gesturing to the generous donor (Experiment 2), and transferred this preference to novel unfamiliar donor pairs, significantly preferring to beg from the novel generous donors on the first opportunity to do so. In Experiment 3, four chimpanzees observed novel selfish and generous acts directed toward other chimpanzees by human experimenters. During the first half of testing, three chimpanzees exhibited a preference for the novel generous donor on the first trial. These results demonstrate that chimpanzees can infer the reputation of strangers by eavesdropping on third-party interactions.

Parenting and potency : alternative routes to reproductive success in male Mongolian gerbils

Adult male Mongolian gerbils, Meriones unguiculatusgestated in intrauterine positions between two female fetuses (2F males) are less likely than are adult males gestated between two male fetuses (2M males) to impregnate strange female gerbils with whom they are paired. The reduced copulatory success of 2F males is correlated with both lower circulating levels of and reduced sensitivity to testosterone. We asked whether 2F male gerbils compensated for their reduced copulatory success by increasing their parental effort. 2F male gerbils engaged in less sexual activity with their mates, but were more frequently in contact with pups than were 2M males, huddling over the young when their mates were absent from the nest. Although there were no differences in rates of survival or growth of pups reared by pairs consisting of a female and either a 2M or 2F male, mates of 2F males delivered significantly more pups as a consequence of copulations occurring during postpartum oestrus than did either mates of 2M males or females rearing young alone. We interpreted these results as consistent with Ketterson & Nolans (1992, Am. Nat. (Supplement)140, 533-562) hypothesis of a testosterone-mediated trade-off between investment in sexual and parental behaviours.1997The Association for the Study of Animal Behaviour

Levels of Abstraction in Orangutan (Pongo abelii) Categorization

Levels of abstraction have rarely been manipulated in studies of natural concept formation in nonhumans. Isolated examples have indicated that animals, relative to humans, may learn concepts at varying levels of abstraction with differential ease. The ability of 6 orangutans (Pongo abelii) of various ages to make natural concept discriminations at 3 levels of abstraction was therefore investigated. The orangutans were rewarded for selecting photos of orangutans instead of humans and other primates (concrete level), primates instead of other animals (intermediate level), and animals instead of nonanimals (abstract level) in a 2-choice touch screen procedure. The results suggest that, like a gorilla (Gorilla gorilla gorilla) tested previously (Vonk & MacDonald, 2002), orangutans can learn concepts at each level of abstraction, and unlike other nonhumans, most of these subjects rapidly learned the intermediate level discrimination.

Intrauterine position, parenting, and nest-site attachment in male Mongolian gerbils.

We housed male Mongolian gerbils, their mates, and foster litters of standardized size and sex ratio in enclosures that provided cover in two locations. Males had been gestated in known intrauterine positions: either between two females (2F males) or between two males (2M males). From Days 1 to 20 postpartum, we examined the frequency with which both males and females were in contact with the pups they were rearing. We found that 2F males spent more time with pups than did 2M males both during entire observation periods and when females were away from the nest. Further, when pups were moved from the nest site. 2M males spent more time than did 2F males in the vacated nest site. We concluded that 2F male gerbils spent more time with pups than 2M males not because of a greater attachment of 2F than 2M males to places of concealment, their male, or their nest site. Rather, 2F males were more attracted to pups than were 2M males.

Quantity estimation and comparison in western lowland gorillas ( Gorilla gorilla gorilla )

Abstract We investigated the quantity judgment abilities of two adult male western lowland gorillas (Gorilla gorilla gorilla) by presenting discrimination tasks on a touch-screen computer. Both gorillas chose the larger quantity of two arrays of dot stimuli. On some trials, the relative number of dots was congruent with the relative total area of the two arrays. On other trials, number of dots was incongruent with area. The gorillas were first tested with static dots, then with dots that moved within the arrays, and finally on a task where they were required to discriminate numerically larger subsets within arrays of moving dots. Both gorillas achieved above-chance performance on both congruent and incongruent trials with all tasks, indicating that they were able to use number as a cue even though ratio of number and area significantly controlled responding, suggesting that number was not the only relevant dimension that the gorillas used. The pattern of performance was similar to that found previously with monkeys and chimpanzees but had not previously been demonstrated in gorillas within a computerized test format, and with these kinds of visual stimuli.

Do chimpanzees know what others can and cannot do? Reasoning about ‘capability’

Much recent comparative work has been devoted to exploring what nonhuman primates understand about physical causality. However, few laboratory experiments have attempted to test what nonhumans understand about what physical acts others are capable of performing. We tested seven chimpanzees’ ability to predict which of two human experimenters could deliver a tray containing a food reward. In the ‘floor’ condition, legs were required to push the tray toward the subject. In the ‘lap’ condition, arms were required to hand the tray to the subject. In Exp. 1, chimpanzees begged (by gesturing) to either an experimenter whose legs were not visible (LNV) or whose arms were not visible (ANV). Rather than flexibly altering their preferences between conditions, the chimpanzees preferred the ANV experimenter regardless of the task. In subsequent experiments, we manipulated various factors that might have controlled the chimpanzees’ preferences, such as (a) distance between experimenter and subject (Experiment 2), (b) amount of occlusion of experimenters’ body (Experiments 2 and 3), (c) contact with the food tray (Experiments 3 and 4) and (d) positioning of barriers that either impeded the movement of the limbs or not (Experiment 5). The chimpanzees’ performance was best explained by attention to cues such as perceived proximity, contact, and maximal occlusion of body that although highly predictive in certain tasks, were irrelevant in others. When the discriminative role of such cues was eliminated, performance fell to chance levels, indicating that chimpanzees do not spontaneously (or after considerable training) use limb visibility as a cue to predict the ability of a human to perform particular physical tasks. Thus, the current findings suggest a possible failure of causal reasoning in the context of reasoning about the use of the limbs to perform physical acts.

Quantity matching by an orangutan (Pongo abelii)

An adult male orangutan (Pongo abelii) was presented with a series of delayed matching-to-sample (DMTS) tasks in which he was to match images based on (a) the number of individual animals depicted in the photograph (from 1 to 4), (b) the number of abstract shapes presented in the stimulus (from 1 to 4), or (c) the number of dots presented in the stimulus (from 1 to 4, 4–7, or 7–10). The spatial arrangement of the dots and the background color of the stimuli varied, and the size of the dots was manipulated to control for overall ratio of foreground to background. The subject’s performance was not affected by these perceptual features, but was affected by the absolute difference and ratio between number of elements in the comparison stimuli. However, the relationship between these variables and his performance was not always linear as predicted by the analog magnitude model. In addition, the subject showed a high degree of transfer to novel numerosities up to ten, indicating that orangutans are capable of estimating quantity for a greater number of items than can presumably be subtilized by humans.

Picture object recognition in an American black bear ( Ursus americanus )

Many animals have been tested for conceptual discriminations using two-dimensional images as stimuli, and many of these species appear to transfer knowledge from 2D images to analogous real life objects. We tested an American black bear for picture-object recognition using a two alternative forced choice task. She was presented with four unique sets of objects and corresponding pictures. The bear showed generalization from both objects to pictures and pictures to objects; however, her transfer was superior when transferring from real objects to pictures, suggesting that bears can recognize visual features from real objects within photographic images during discriminations.

The Phylogenetic Roots of Cognitive Dissonance

We presented 7 Old World monkeys (Japanese macaques [Macaca fuscata], gray-cheeked mangabey [Lophocebus albigena], rhesus macaques [Macaca mulatta], bonnet macaque [Macaca radiate], and olive baboon [Papio anubis]), 3 chimpanzees (Pan troglodytes), 6 members of the parrot (Psittacinae) family, and 4 American black bears (Ursus americanus) with a cognitive dissonance paradigm modeled after Egan, Santos, and Bloom (2007). In experimental trials, subjects were given choices between 2 equally preferred food items and then presented with the unchosen option and a novel, equally preferred food item. In control trials, subjects were presented with 1 accessible and 1 inaccessible option from another triad of equally preferred food items. They were then presented with the previously inaccessible item and a novel member of that triad. Subjects, as a whole, did not prefer the novel item in experimental or control trials. However, there was a tendency toward a subject by condition interaction. When analyzed by primate versus nonprimate categories, only primates preferred the novel item in experimental but not control trials, indicating that they resolved cognitive dissonance by devaluing the unchosen option only when an option was derogated by their own free choice. This finding suggests that this phenomenon might exist within but not outside of the primate order.