This paper reports a study of the behaviour of captive orang utans (pongo pygmaeus) at Chester Zoo, UK. The study addressed two questions: what is the effect of the presence of fresh browse on the animals' behaviour; and what is the effect of the presence of visitors? The first
part of the study analysed the animals' time budgets. The results indicated that the provision of fresh browse led to a decrease in the time spent sitting inactive by both adults and infants; it also led to an increase in the time spent by adults foraging for small food items in the woodchip
floor-covering beneath the branches of browse. The time-budget data also showed differences in the animals' behaviour between periods when large groups of visitors arrived and other periods on the same days when visitors were fewer. Specifically, adults used available paper sacks to cover
their heads more during periods of high visitor density, and infants held onto adults more. The second part of the study comprised an experiment in which visitor behaviour was manipulated. Visitor groups of similar sizes were asked to behave either quietly or noisily (making vocal noise),
in order to determine whether the observed effects of visitors were attributable to group size or to the fact that larger groups tend to be noisier. The experiment indicated that the animals responded particularly to noise: when confronted with noisy groups, all animals looked more at the
visitors, and infants approached and held onto adults more. The findings suggest that zoo managers may need to take visitor behaviour into account in order to promote orang utan welfare.