Reinhardt, V. 1997. Refining the traditional housing and handling of laboratory rhesus macaques improves scientific methodology. Primate Report 49, 93-112.

A monkey housed in an empty cage, however, is literally a behavioral cripple because s/he is chronically deprived of appropriate stimuli for the expression of species-typical behavior patterns. It is difficult to know objectively if a monkey experiences boredom when being kept in an understimulating environment. However, many such animals show signs of depression and/or engage in gross behavioral disorders. Refinement techniques were developed for caged rhesus macaques in order to better foster the animals' behavioral health and well-being. Individuals were transferred from the traditional barren single-cages to compatible pair-housing arrangements in double-cages furnished with perches, gnawing sticks and custom-made food puzzles. ... Rather than applying the conventional forced restraint methods, the animals were trained to co-operate during common procedures. ... There is no scientific, veterinary or financial justification for not providing rhesus macaques a more species-appropriate environment than the barren single cage, and for not training them to co-operate during procedures. ... It cannot be overemphasized how critical the role of personnel is in determining the animals' well-being, and hence their 'quality' as research subjects. The macho-type person is out of place in the animal room because s/he triggers stress or even distress reactions. Typically, the animal will freak out and hide in a back corner of the cage when such a person comes into their room.