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‘Regurgitation and reingestion’ (R/R) in great apes: A review of current knowledgeResearch indicates that regurgitation and reingestion (R/R) is a relatively common behaviour in zoo-housed great apes, with most work to date carried out on Western lowland gorillas Gorilla gorilla gorilla and Chimpanzees Pan troglodytes. It is an abnormal behaviour because great apes are not anatomically adapted to regurgitate their food as part of their normal feeding processes, and because this behaviour is not seen in members of the species living freely in the wild, in conditions that would allow a full behavioural range. In this article, I give an overview of the published literature on R/R in great apes, which suggests that this behaviour is probably multifactorial and may be linked to inappropriate feeding environments (e.g. in terms of nutritional composition of the diet and/or presentation of food), and possibly also social and other factors as well. A similar behaviour to R/R, known as rumination disorder, can also occur in another great ape species, humans, in whom it is classified as a feeding and eating disorder, and there are potential consequences to people’s physical health as a result of oral acid. There have been no known studies to date to identify whether or not similar health consequences can occur in non-human great apes, but the regurgitant has been found to be significantly more acidic in gorillas than the food they ingested originally, meaning it is potentially injurious in non-human great apes. There is much that is not yet known about this behaviour and how to reduce or eliminate it when it does occur, as the research indicates that there are a range of factors involved, and these can vary by individual animal. More research into this behaviour is clearly needed to ensure that zoos and sanctuaries are providing the best possible care for these animals, and I make some suggestions for future research directions.