This past year we expanded our study of rhesus monkey infants capacity to imitate specific facial expressions directed toward them by a human model throughout their initial week of life. Such early imitative capabilities have been reported for human neonates, and they are thought to be initially reflexively mediated by mirror neurons. We found that approximately 60% of nursery-reared (NR) newborns tested were able to mimic specific facial expressions involving differential mouth and tongue movements, whereas over 90% of mother-reared (MR) neonates were successful imitators. Follow-up behavioral observations revealed that NR infants who exhibited imitative behavior during their first week of life subsequently exhibited significantly greater sensory-motor coordination throughout their first month and higher levels of social play during peer interactions sessions from 4 months onward than infants who failed to imitate during their first week. Moreover, when NR monkeys were subsequently permanently moved into large groups of same-age peers, those individuals who had failed to imitate during their initial days of life developed much higher rates of self-directed and autistic-like repetitive stereotypic behavior than NR monkeys who had demonstrated imitative capabilities as neonates. This past year, in collaboration with colleagues from the University of Maryland, we also monitored EEG activity in the monkey neonates throughout their imitative test sessions, as well as during appropriate non-imitative control periods, during their first postnatal week, Preliminary analyses have revealed specific patterns of slow-wave EEG alpha concomitant with imitative behavior, but not under other conditions and that are not seen in infants who fail to imitate in the same setting. This past year we also intensified our study of rhesus monkey mother-infant interactions during the initial postnatal days. In marked contrast to previous reports concerning the normative development of attachment relationships in this species, we found that rhesus monkey mother- infant dyads engaged in frequent and intensive face-to-face interactions throughout their first 3 weeks of life, after which those patterns largely disappear. We demonstrated the rhesus monkey infants can differentiate pictures of monkey faces from nonsocial stimuli from Day 1 onward and that by Day 10 they show a significant preference for pictures of adult female monkey faces over those of adult males in the absence of any postnatal exposure to adults of either gender Another study involved collaborators from UCSF who had previously demonstrated differences in telomere length, thought to be a marker of relative cellular age, in adult humans as a function of differences in social status and cumulative social stress. These researchers hypothesized that such differences may have their origin in differential experiences with social stress during the childhood years, a period when telomere length shortens rapidly. Accordingly we have been providing them with DNA extracted from rhesus monkeys living outdoors in a physically and socially naturalistic setting. Initial analyses revealed both age and status differences in telomere length, with younger monkeys having longer telomeres than older ones and members of high-ranking families having longer telomeres than members of low-ranking families. In collaboration with colleagues from the University of Massachusetts, Amherst, we also collected multiple samples of hair from the same monkeys in order to assay for cortisol concentrations as a potential index of chronic stress, and we found significant age, gender, and social status differences: younger, female, and low-ranking monkeys had higher hair cortisol concentrations than older, male and high-ranking individuals. We also compared these values with hair cortisol samples obtained from a captive colony of Tonkean macaques living in a similar outdoor enclosure. The Tonkean macaques exbibited that same pattern of age, gender, and status differences, but overall their cortisol levels were significantly higher than those of their rhesus counterparts. Finally, this past year we completed several studies utilizing the CBGS's colony of tufted capuchin monkeys. One study demonstrated that capuchin monkeys are capable of recognizing when they are being imitated by a human observer and such imitation influences their behavioral preferences. Another study, in contrast, found that capuchin monkeys fail to exhibit the same pattern of memory awareness previously demonstrated in rhesus monkeys, apes, and humans. A third study investigated the use of different sensory information by capuchin monkeys when locating hidden food items and found that they consistently utilize visual but not auditory information in such endeavors. A fourth study demonstrated that when faced with choices having differential potential reward, capuchin monkeys do not demonstrate loss aversion but instead choose on the basis of alternatives in delay of reinforcement. A final study showed that fur-rubbing behavior, previously thought to have largely medicinal functions, instead serves social purposes, especially with respect to affiliative and aggressive activities.