Hosey, G. R. 2000. Zoo Animals and their human audiences: What is the visitor effect? Animal Welfare 9, 343-357.

A review. We can conclude that social facilitation effects cannot explain the pattern of responses [increase in agonistic and decrease in affiliative behavior] of non-human primates to human observers. There is not sufficient evidence to support the hypothesis that human audience is enriching, but this does not necessarily imply that visitor presence is stressful.