Although capable of complex learning, monkeys have not displayed the kind of language skills seen in the great apes (Savage-Rumbaugh and others 1998). Gallup (1982) suggests that there is a large cognitive discontinuity between the great apes and monkeys as demonstrated by their consistent failure in such tasks as recognizing themselves in mirrors. The significance of mirror self-recognition in studies of cognitive capacity is controversial, and self-awareness might not be dependent on self-recognition in a mirror; moreover, not even all great apes demonstrate such a capacity. Great apes do, however, appear to show a greater diversity of learning skills than any monkey taxon, although most great apes' accomplishments might be matched in some particular monkey taxon.
The common practice of housing rhesus monkeys singly calls for special attention because it is well established that social deprivation can be counter to the goals of this report (Bayne and Novak 1998). Although many of these animals might become pair-housed as encouraged in this document, others will remain singly housed for reasons of research, incompatibility, or health. Addressing the welfare of these singly housed animals requires a concerted approach by investigators, veterinarians, and IACUCs. Every effort should be made to house these animals socially (in groups or pairs), but when this is not possible, the need for single housing should be documented by investigators and approved by the IACUC. A common approach is for the institution's environmental enrichment (or psychological well-being) plan to require all primates to be socially housed and to require justified exemptions for all others. There are reasons for single housing, that should not be accepted as the default situation. Institutions should provide social housing unless single housing is approved by the IACUC.
It is essential that personnel working with Old World monkeys be made aware of the various disease risks involved (Adams and others 1995) and the fact that many Old World monkeys are capable of inflicting serious bite wounds and have surprising strength for their body size. Macaques and some other cercopithecines also interpret the human stare as a challenge and might attack a person who is visually inspecting them, but most monkeys are not constantly hostile to people and will respond to considerate and consistent treatment. Personnel must take reasonable precautions to protect themselves from disease risks and attack (CDC 1990, 1993; CDC-NIH 1993; NRC 1996, 1997a). Even members of species that are more tolerant of humans can suddenly attack humans if an infant screams or if there is some other sudden disturbance. Rhesus monkeys are consistently more hostile than other macaques toward people, but personnel working with even the tamest of monkeys should be alert to the potential for attack.
Old World monkeys have been implicated in the transmission of several diseases to humans. (Adams and others 1995; CDC 1993; NRC 1997a). Most