Alternatives to single caging of rhesus monkeys (Macaca mulatta) used in research

Alternatives to single caging of laboratory rhesus monkeys were explored. 1. Surplus infants (12-18 months old) from breeding troops were paired with 48 adult females (6-28 years old) and 12 adult males (8-28 years old) that had lived alone for several years. Adults and infants were not related. The partners were compatible in 87% (42/48) and 83% (10/12) of cases, respectively, throughout follow-up periods of 16-25 months. The young companions reached 28-44 months of age during that time. 2. Forty-six unrelated adult female rhesus monkeys (7-23 years old) and 10 unrelated adult male rhesus monkeys (8-16 years old) that had lived alone for several years were familiarized in isosexual dyads and then paired in different cages. Paired companions were compatible in 83% (19/23) and 80% (4/5) of cases, respectively, throughout follow-up periods of 9-24 months. Of all animals tested, 2% (3/176) received a bleeding but not life-threatening wound by an incompatible partner. Pairing previously individually caged rhesus monkeys did not interfere with the following research protocols: tethering of one partner for remote collection; head-cap implantation on one or both partners; chair-restraining of one partner while the other is kept close by for psychological support; in-cage venipuncture of both partners (attempted with females only); temporary (1-7 d) partner separation for timed breeding or for experimental surgery. Pairing singly caged adult rhesus monkeys with infant or with other adult companions is recommended as an effective way to promote the animals' wellbeing by enabling them to express their inherent need for social contact.