Chamove, A. S., Hosey, G. R., Schaetzel, P. 1988. Visitors excite primates in zoos. Zoo Biology 7, 359-369.

Fifteen species of primate were observed to assess the effects of zoo visitors on their social behavior. When visitors were present primates were less affiliative, more active, but more aggressive. These changes were particularly marked in arboreal monkeys. It is likely that the stressful effects of visitors could be reduced by 1) setting viewing windows high, so that only the head and shoulders of visitors are visible to the animals; 2) increasing the heights of cages, so that the animals can become 'more arboreal'; and 3) lowering public walkways, so that visitors appear smaller to the animals.