No systematic difference in obesity susceptibility between two groups of zoo primates with distinct digestive physiologies: hindgut-fermenting lemurs and foregut-fermenting colobines.

Obesity is an often-stated issue in zoo animals, including primates, and has historically been particularly emphasized for lemurs. By contrast, similar reports for colobine species-which are literally called 'slim monkeys' in at least one other language-are absent. This has been hypothsized to be linked to the colobine foregut fermentation system; diets high in easily digestible carbohydrates might disturb the microbiome in the foregut, similar to acidosis in domestic ruminants, leading to reduced intake and disease rather than obesity, but empirical data is lacking. We compared body mass (BM) data for 14 lemur species (7187 individuals) and 10 colobine species (1443 individuals) with literature data on free-ranging specimens to test this concept. In contrast to expectations, a similar percentage of species had a majority of individuals above the 'healthy' BM range in at least one sex (36% of lemur and 40% of colobine species). However, colobines had a higher percentage of individuals considered 'underweight', especially in the two rarely kept species with the more complex 'quadripartite' stomachs (Nasalis larvatus, Pygathrix nemaeus). Neither sample size, sexual size dimorphism, the degree of folivory, nor indications for a seasonal physiology were clearly associated with species' propensity for zoo obesity. There were no clear historical trends in the development of obesity in these species. We conclude that no general rules about species' susceptibility to obesity appear to apply. Body mass and condition of zoo primates should be monitored continuously, and feeding regimes possibly be adapted to contain diets of higher fibre levels more similar to those reported in natural diets.