Abstract: The behaviour of members of a group of rhesus monkeys was observed in experimentally created, competitive feeding situations. Socially dominant members of the group tended to start eating before lower-ranking subjects, and generally ate more. Dominants sometimes used aggression to control access to food, but overall, intermediate-ranking monkeys were involved in most agonistic episodes. Non-dominant subjects improved their feeding performance when food was presented in three piles rather than one pile, often by snatching food and consuming it away from the pile. These general patterns were less evident when realistic snake models were placed on some of the food piles. Feeding was disrupted by the presence of snakes, but notably, subordinates risked feeding in these conditions. Piles containing preferred foods and snakes were eaten from, but a low-preference food (carrot) under snakes went untouched by all subjects. The results suggest that group-members evaluate potential risks and benefits of competing for a restricted resource, and that dominance status, while an important factor, is only one element in the equation.

Abstract: A horizontal plexiglas tube containing a food-reward was presented to four naive tufted capuchins and suitable sticks were provided to push the reward out. Three monkeys out of four spontaneously used the tools and showed very different styles of solving the task. In more complex conditions, in which the sticks needed to be combined or actively modified in order to become effective, the monkeys were always successful; however, their performance was loaded with errors which did not disappear throughout the trials. Evidence of a difference between success in solving the problem and its understanding was found. This suggests that although capuchins can discover new means through active experimentation, they do not mentally represent the characteristics necessary for a tool to be effective, nor do they modify the tool appropriately beforehand. At this level, a major difference with chimpanzees emerges.

Abstract: Abstract  Based on previous research in captivity, bonobos, Pan paniscus, have been called a female-bonded species. However, genetic and behavioural data indicate that wild females migrate. Bonding between these unrelated females would then be in contradiction with socio-ecological models. It has been argued that female bonding has been overemphasized in captive bonobos. We examine patterns of proximity, grooming and support behaviour in six well established captive groups of bonobos. We find that female bonding was not a typical characteristic of all captive bonobo groups. In only two groups there was a trend for females to prefer proximity with other females over association with males. We found no evidence that following or grooming between females was more frequent than between males and unrelated females or between males. Only in coalitions, females supported each other more than male-female or male-male dyads. We also investigated five mother-son pairs. Grooming was more frequent among mothers and sons than in any other dyad, but sons did not groom their mothers more than males groomed unrelated females. Mothers groomed their sons, or provided more support to them than females groomed or supported unrelated males. Thus, while bonds between females were clearly present, intersexual relations between males and either unrelated females or their mothers are of more, or equal importance.

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.

Abstract: Observations of play behavior were made on a troop of Japanese monkeys for five months. The troop consisted of 125 animals during the study period. Only 104 animals were observed playing with the troop members while the other 21 animals were never observed playing with other individuals. Two-member play was the most frequent. On the average, a monkey played with 20.7 individuals. A total of 6,068 play bouts were observed. The frequency of play appeared to be affected by age, sex, and degree of relatedness. One-year-old infant males played most with other members and the frequency of play decreased with age. Between monkeys whose disparity of age was less than two years, 5,763 bouts (95.0% of the total) were observed. Moreover, among sameaged monkeys who comprised 10.6% of the possible pair combinations, 2,739 play bouts (45.1%) were observed. Juvenile males played with same-sexed peers more than with opposite-sexed peers, whereas older juvenile females appeared to play with infants of both sexes. Individuals who were related and similarly-ranked tended to play together. There was no apparent preference for animals to play with the offspring of the highest-ranking female. Dominance rank of infnats and juveniles was primarily affected by rank of their mothers and to a lesser extent by play partners. Dominance rank of older juvenile males is more likely to be affected by play partners than females. It may be a critical time for males when they leave their natal troop and join a new troop. The timing of troop shifting by males seemed to be affected by the presence or absence of play-mates. For male Japanese monkeys, play is very important in developing social bonds. Play may act to perpetuate social bonds, enhance the chance of survival, and may contribute to their future reproductive success.