Lower-row caging in a two-tiered housing system does not affect the behaviour of young, singly housed rhesus macaques

It has been suggested that housing of laboratory primates in two-tiered rac ks adversely affects the psychological well-being of those primates housed on the lower row. Excessive darkness and its consequences are among the fac tors suggested to account for the supposed diminished well-being of lower-r ow inhabitants. Additionally two-tiered housing has been suggested to intro duce unacceptable variation into experimental designs, potentially necessit ating additional subjects and/or invalidating results. Only recently, have data been published to address these issues, but all studies have involved small numbers of subjects. In the present study, we compared the behaviour of 45 yearling rhesus macaques (Macaca mulatta) housed in upper-row cages w ith the behaviour of 48 yearling rhesus macaques housed in lower row cages during a year of single housing. There were no significant differences acro ss cage locations for time spent performing behaviours indicative of dimini shed psychological well-being (abnormal behaviour, inactivity, vocalisation , self-directed grooming) or for species-typical activities (feeding, playi ng). The difference in time spent exploring between macaques housed on the lower row and those housed on the upper row approached significance, with l ower-row-housed animals spending more time exploring.,es are significantly darker than zipper row cages at our facility, the Although lower-row cages at our facility, the data from the present study demonstrate that the dimin ished lighting and other supposed disadvantages experienced by lower-row-ho used monkeys have few behavioural consequences. Thus, there are now additio nal empirical data that suggest that lower-row-housed monkeys are not suffe ring in a "monkey cave", and that the findings of research projects using t wo-tiered housing systems are unlikely to be compromised.