Macaque Personality: Structure, Development, and Relationship to Social Behavior

Abstract

In this work, we use behavioral observations to quantify individual differences in infant and adult macaques. We demonstrate that these differences, which we refer to as temperament in infants and personality in adults, explain differences in behavior between individuals. Using infant pigtailed macaques (<italic>Macaca nemestrina</italic>), we show that temperament changes in predictable ways throughout the first 10 months of life. We also show that temperament scores are predictive of social behaviors in 4-month-old infants. Next, we show that adult personality also changes predictably with age, though individuals' personality scores are stable over shorter periods of up to a year. We measure adult personality in three closely related species of macaques - <italic>Macaca nemestrina</italic>, <italic>M. mulatta</italic>, and <italic>M. fascicularis</italic> - and demonstrate that mean personality scores differ between the three species. These personality differences reflect species differences in social behaviors. To further investigate the relationship between personality and social behavior, we observe social behaviors in pairs of adult macaques, as well as large breeding groups of adult macaques. We show that personality is a strong predictor of social behavior in pairs, but is a weaker predictor in larger groups. However, the variance in personalities present within a large social group does predict the group's rates of aggressive and affiliative behaviors. Moreover, the personality scores of adult female pigtailed macaques predict their behaviors within a social group. Overall, our findings suggest that temperament and personality in animals share many of the developmental features documented in humans. In addition, these results support the idea that differences in personality are an individual-level trait that predicts group differences in social behavior. We propose that personality is the target of selection that leads to such social behavior differences.