Gilloux, I., Gurnell, J., Shepherdson, D. 1992. An enrichment device for great apes. Animal Welfare 1(4), 279-289.
The influence of an environmental enrichment feeding device (puzzle feeder), on activity and behaviour patterns of captive orang-utans, gorillas and chimpanzees was studied at London Zoo. General activity levels and behaviours directed towards the feeder...