Abstract: The idea that consistencies in behavior exist over time and across situations underlies human personality research. Although several studies have examined personality in nonhuman primates, there are very few data showing the predictive power of personality factors. The goal of the present study was to determine whether personality dimensions, identified in adult male rhesus monkeys living in half-acre cages, predicted behavior in situations different from the one from which the dimensions were originally derived and at time points of up to 4.5 years after the original assessments. Four personality dimensions (Sociability, Confidence, Excitability, and Equability) were identified using psychometric procedures and were correlated with behaviors recorded in several situations: the animals' natal groups, during tests of behavioral responsiveness while in individual cages, in small stable and unstable social groups, while viewing stimulus videotapes, and during stable social dyads. Results indicated substantial predictability. Sociability reflected a greater tendency to engage in affiliative interactions. Confidence correlated with more aggressive behaviors and with behaviors that suggest less attractiveness. Animals high in Excitability were somewhat inconsistent in their social behavior, perhaps reflecting hyper-responsiveness to novel circumstances and thwarted opportunities for escape. Equability appeared to be related to a less aggressive, more passive, style of interaction. Excitability and Equability appear to reflect more stylistic components of social behavior, whereas Sociability and Confidence may be more content-based dimensions. Sociability was strongly related to size of kin network in the animals' natal groups, suggesting an important role for ontogeny in this dimension. These data suggest that a limited number of personality dimensions exist in adult male rhesus macaques, and that these dimensions have predictive power that is both long-term and cross situational.

Abstract: A fundamental question in cognitive science is whether animals can represent numerosity (a property of a stimulus that is defined by the number of discriminable elements it contains) and use numerical representations computationally. Here, it was shown that rhesus monkeys represent the numerosity of visual stimuli and detect their ordinal disparity. Two monkeys were first trained to respond to exemplars of the numerosities 1 to 4 in an ascending numerical order (1 --> 2 --> 3 --> 4). As a control for non-numerical cues, exemplars were varied with respect to size, shape, and color. The monkeys were later tested, without reward, on their ability to order stimulus pairs composed of the novel numerosities 5 to 9. Both monkeys responded in an ascending order to the novel numerosities. These results show that rhesus monkeys represent the numerosities 1 to 9 on an ordinal scale.

Abstract: The use of nonhuman primates, particularly rhesus macaques (Macaca mulatta), as the best model for human physiological and cognitive aging is broadly accepted. Studies employing nonhuman primates to investigate behavioral changes that may occur with increasing age, however, are not common mostly because of the unavailability of appropriate subjects. Recent longitudinal human studies suggest that individual personality might play a large role in aging “successfully” and in the retention of high levels of cognition into old age. As a result of the demographic trend of increasing numbers of aged monkeys and apes in captivity, an opportunity exists to further investigate behavioral aging using the monkey model.

Abstract: Reconciliation, defined as a friendly reunion between former opponents shortly after an aggressive encounter, is common in the stumptail macaque (Macaca arctoides) but rare in the rhesus macaque (M. mulatta). Juveniles of the two species were cohoused for 5 months, after which they were observed with conspecifics only. Control rhesus monkeys, matched in age and sex to the experimental subjects, went through the same procedure without exposure to the other species. A threefold increase in the proportion of reconciled fights was measured in the rhesus subjects. The difference emerged gradually during cohousing with the tutor species and was sustained following removal of this species. Other behavior, such as grooming and aggression, decreased over time. It is suggested that the social attitude of the subjects was affected through contact with a species characterized by a more relaxed dominance style.

Abstract: We tested the ability of number-trained rhesus monkeys to use Arabic numeral cues to discriminate between different series of maze trials and anticipate the final trial in each series. The monkeys' prior experience with numerals also allowed us to investigate spontaneous transfer between series. A total of four monkeys were tested in two experiments. In both experiments, the monkeys were trained on a computerized task consisting of three reinforced maze trials followed by one nonreinforced trial. The goal of the maze was an Arabic numeral 3, which corresponded to the number of reinforced maze trials in the series. In experiment 1 (n=2), the monkeys were given probe trials of the numerals 2 and 4 and in experiment 2 (n=2), they were given probe trials of the numerals 2-8. The monkeys receiving the probe trials 2 and 4 showed some generalization to the new numerals and developed a pattern of performing more slowly on the nonreinforced trial than the reinforced trial before it for most series, indicating the use of the changing numeral cues to anticipate the nonreinforced trial. The monkeys receiving probe trials of the numerals 2-8 did not predict precisely when the nonreinforced trial would occur in each series, but they did incorporate the changing numerals into their strategy for performing the task. This study provides the first evidence that number-trained monkeys can use Arabic numerals to perform a task involving sequential presentations.