The relationship between diet change and regurgitation and reingestion in captive chimpanzees

Captive chimpanzees regularly show abnormal behaviour, including regurgitation and reingestion (R/R). R/R may have several causes, among them a suboptimal diet. For this reason, the effect of a diet change towards a more fibre-rich diet on R/R was studied in the Amersfoort Zoo chimpanzee group comprising 15 individuals. In addition, the relationship with self-directed behaviour, inactivity level and temperature, proxies of the alternative factors stress, boredom and crowding, were studied. Measures of stress and boredom did not show any correlation to R/R behaviour. The new diet did result in less R/R. However, the data could not conclusively dismiss temperature as a factor or confound in the reduction of R/R. Still, the R/R rate in three individuals that showed most R/R did not concur with temperature, countering an effect of temperature on R/R. Individuals that showed R/R in the study period still showed this behaviour several years later, suggesting that R/R may not be related to current welfare but possibly become a habit, where stress and R/R become dissociated and the behaviour persists in improved conditions and over time. This study shows that diet change can contribute to a reduction in R/R, yet this may not be due to the change of diet quality, but to the change in diet as such. The change in diet was not able to abolish R/R behaviour entirely for these chimpanzees.