Reinhardt, V. 1992. Space utilization by captive rhesus macaques. Animal Technology 43, 11-17.

The area covered by the floor was 3 times larger than that covered by elevated structures; nonetheless the animals were located significantly more often (89.8% of 108 scan samples) on elevated structures than on the floor (8.6% of 108 scan samples). ... The higher an animal's rank position, the more pronounced was its habit to utilize high-level (>130 cm above floor) structures of the pen, while low ranking animals had to be content with low-level structures (40 cm above floor) and the floor. ... All members of the group would inevitably take to elevated sites whenever they heard or saw fear-inducing personnel. ... The animals huddled together with regularity on high-level structures but never on low-level structures or on the floor. ... It was concluded that [group-housed] laboratory rhesus macaques prefer the vertical dimension over the horizontal dimension as primary living space.