Harper, P. 1992. Behavioural enrichment of Adelaide Zoo's sole male orangutan. Australian Primatology 6(4), 2-9.

Certainly, the results show that his activity levels of foraging [at the probe feeder] and eating are being raised; he is being stimulated to make and use tools, and by the same token, the levels of his unnatural behaviours of banging, begging, and pirouetting have been decreased with his feeding routines.