Abstract: Reconciliation in primates, a post-conflict affiliative interaction between former opponents, appears to have two functions: (1) to repair relationship damaged by aggression such that animals who share more valuable relationships are more likely to reconcile, and (2) to reduce the post-conflict uncertainty and stress of former combatants. The 'integrated hypothesis' of reconciliation links these functions by arguing that the disturbance of a valuable relationship by aggression should result in particularly high levels of stress, which in turn should facilitate efforts to reconcile and thus gain relief from post-conflict stress. A key prediction of the integrated hypothesis is that victims of aggression suffer more stress following conflicts with individuals with whom they share a valuable relationship. In this article, we test the integrated hypothesis by observing the post-conflict behaviour of victims among a free-ranging provisioned troop of Japanese macaques ( Macaca fuscata fuscata) living in Shiga Heights, Nagano, Japan. In this troop, monkeys reconciled roughly one in seven conflicts. The only factor that we could significantly relate to the occurrence of reconciliation was kinship; kin reconciled more frequently than non-kin did. Receiving aggression increased and reconciliation reduced the probability of being re-attacked after aggressive interactions, supporting the hypothesis that reconciliation repairs relationships. Victims' self-directed behaviour (SDB) – a behavioural index of stress comprising increases in scratching, self-grooming, and body-shaking – was elevated following aggression but decreased rapidly following reconciliation, supporting the idea that reconciliation functions to reduce post-conflict stress. Post-conflict SDB varied as follows: (1) victims showed a higher level of stress following aggression with kin than with non-kin, and (2) juvenile victims were less distressed than adults. The level of post-conflict SDB performed by juveniles following conflicts with kin was indistinguishable from that performed by adults but was greatly reduced following attacks from non-kin. These results indicate that post-conflict SDB keenly reflects the value of relationships between opponents, and that the post-conflict behaviour of free-ranging Japanese macaques fits the predictions of the integrated hypothesis.

Abstract: An adult female chimpanzee with previous training in the use of Arabic numerals 1–9 was introduced to the meaning of “zero” in the context of three different numerical tasks. The first two were cardinal tasks where the subject was required either to select numerals corresponding to the number of items presented on a computer screen (productive use of numerals) or to match sets of the appropriate size to numerals presented as samples (receptive use). The third task addressed the ordinal meaning of the same symbols where the subject was required to respond to numerals sequentially, arranging them into an ascending series. The subject mastered the recognition of the meaning of zero in all three tasks. However, details of her usage of the symbol revealed that transfer of the meaning between different kinds of tasks was incomplete, suggesting that the level of ion characteristic of human numerical ability was not attained in the chimpanzee. Over the course of acquisition leading to the high levels of accuracy eventually observed, the newly introduced zero appeared to shift along the length of a continuous numerical scale toward the lower end, while confusions with 1 remained the most frequently encountered mistakes. Such patterns of error thus suggest that Ai's understanding of the meaning of zero in relation to the rest of the number symbols was not consistent with an “absence of items versus presence of items” scheme.

Abstract: Two experiments assessed the ability of four adult female chimpanzees to categorize natural objects. Chimpanzees were initially trained to match different color photographs of familiar objects from four possible categories. In training, all the comparison stimuli were from the same category in one condition, and from different categories in another condition. For all subjects, training performance was consistently better for the “different category” than for the “same category” trials. Probe trials were shown after training. In probe trials, the sample and positive comparison stimuli were different items from the same category, and the foils were selected from among the three other test categories. Individual performance was above chance in probe trials, suggesting that categorization by chimpanzees may transcend perceptual resemblance. These results were later replicated with novel stimulus items from the same four categories (experiment 2). Altogether, this research demonstrates that chimpanzees grouped perceptually different exemplars within the same category, and further suggests that these animals formed conceptual representations of the categories.

Abstract: The use of leaves for drinking water by wild chimpanzees ( Pan troglodytes verus) at Bossou, Guinea, was observed intensively. The natural hollow of a tree, used by chimpanzees, was filled up with fresh water every morning. Seventy episodes of leaf-using behavior by 14 chimpanzees were directly observed and video-recorded. The chimpanzees at Bossou most frequently (70.3%) used a particular kind of leaf, Hybophrynium braunianum as tool material. The chimpanzees folded one or more leaves in the mouth. This technique, “leaf folding”, was observed more frequently (57.9 %) than “leaf sponge” or “leaf spoon”. Chimpanzees began to perform this behavior at about 2.5 years old. Infant chimpanzees showed more frequent observations of others (especially their mothers) using leaves before trying to drink water with leaves. Both observation and trial and error might be necessary for the acquisition of this tool-use behavior.