Geophagy among nonhuman primates: A systematic review of current knowledge and suggestions for future directions

Objectives: Geophagy, the intentional consumption of earth, is widely practiced among humans and other mammals, but its causes are not well understood. Given the growing number of reports of geophagy among nonhuman primates (NHP), we sought to (1) advance and codify our understanding of the patterns and functional and evolutionary significance of geophagy among NHP and (2) provide a research agenda for a more unified approach to its study. Methods: We systematically reviewed all available literature on NHP geophagy, extracted available data on taxa, geography, climate, diet, sex, age-class, reproductive status, and the characteristics of the earth. We used these data to evaluate three major hypotheses about geophagy, that it is protective, provides mineral supplementation, and is nonadaptive. Results: We identified 287 accounts of geophagy among 136 species, adding 79 new primate species to the list of those considered in prior reviews. Nineteen percent of species were in the suborder Strepsirrhini, while 81% were in the suborder Haplorrhini. There were reports of geophagy from 9 of the 17 families and 39 of the 76 genera currently recognized by the International Union for Conservation of Nature. Discussion: The limited evidence suggests that geophagy is adaptive, and provides protection and mineral supplementation. We specify the behavioral, dietary, and soil data required to more rigorously investigate these hypotheses across representative species of all taxonomic groups, geographical regions, and dietary classification. Given the plausibility of geophagy for maintaining the health of both wild and captive populations, we urge further study and conservation of geophagy sites.