Behavioral stereotypy in old and young rhesus monkeys

Four old (25 years) and six young (3 years) female rhesus monkeys (Macaca mulatta) were observed individually in a laboratory cage with either an old or a young monkey present as a social partner. The behavioral repertoire was coded into ten mutually exclusive and exhaustive categories. An information analysis of the individual behavior records is presented along with subject and partner age effects on the frequency of bouts and the amount of time spent in each category of behavior. Knowledge of the relative frequencies of the behaviors tells more about the behavior of young than old monkeys; and knowledge of the preceding behavior tells more about the behavior of old than young monkeys. Further, knowledge of the relative frequencies tells more about the behavior of both old and young monkeys with junior or elder partners than with age peers; whereas knowledge of the preceding behavior is more informative for old monkeys than for young ones regardless of the partner's age. Both old and young engaged in more affiliative behaviors with age peers than with junior or elder partners. An hypothesis is proposed which relates this social homophyly to the age difference in behavioral stereotypy.