The monkey in the mirror: hardly a stranger.

It is widely assumed that monkeys see a stranger in the mirror, whereas apes and humans recognize themselves. In this study, we question the former assumption by using a detailed comparison of how monkeys respond to mirrors versus live individuals. Eight adult female and six adult male brown capuchin monkeys (Cebus apella) were exposed twice to three conditions: (i) a familiar same-sex partner, (ii) an unfamiliar same-sex partner, and (iii) a mirror. Females showed more eye contact and friendly behavior and fewer signs of anxiety in front of a mirror than they did when exposed to an unfamiliar partner. Males showed greater ambiguity, but they too reacted differently to mirrors and strangers. Discrimination between conditions was immediate, and blind coders were able to tell the difference between monkeys under the three conditions. Capuchins thus seem to recognize their reflection in the mirror as special, and they may not confuse it with an actual conspecific. Possibly, they reach a level of self-other distinction intermediate between seeing their mirror image as other and recognizing it as self.

Sex differences in age-related motor slowing in the rhesus monkey: behavioral and neuroimaging data.

The nigrostriatal system is critical for fine motor function and its deterioration during aging is thought to underlie the decline in fine manual ability of old persons. Because estrogen has a neuroprotective effect on this system, one might expect women's motor function to be less vulnerable to the detrimental effects of aging than that of men. We examined this hypothesis in the rhesus monkey, which has been established as an excellent model of human age-related motor impairment. We tested 28 young and old rhesus monkeys of both sexes in a task involving the retrieval of a Life Saver candy from rods of different complexity to determine whether fine motor ability (1) is sexually dimorphic, (2) declines with age and (3) declines differently in males and females. In addition, we measured the whole brain volume, the volumes of the caudate, putamen, hippocampal formation and the area of the corpus callosum in a subset of the monkeys (n=15) for which magnetic resonance images of the brain were available. All monkeys performed similarly in the test with the simplest rod. In the test with complex rods; however, age-related slowing of motor function was evident in males, but not in females. Age-related decreases in the normalized caudate and putamen volumes were similar in males and in females. In addition, motor speed was not significantly correlated to any of the neuroanatomical measures under study. Further studies will be necessary to uncover the neurohormonal bases of the differential age-related motor decline between males and females.