Geophagy of Yunnan snub-nosed monkeys (Rhinopithecus bieti) at Xiangguqing in the Baimaxueshan Nature Reserve, China

Geophagy is common in extant nonhuman primate species, but the exact reasons for it across species remain unclear. Previous diet studies on Yunnan snub-nosed monkeys (Rhinopithecus bieti) were only focused on organic materials (plants and small animals). There are no reports on R. bieti exhibiting geophagy in the field before this study. This study was carried out at Xiangguqing in the Baimaxueshan Nature Reserve from June 2008 to May 2009. We recorded the behavior of geophagy and collected samples of soil consumed by the monkeys there and analyzed their content in a laboratory. We identified a total of eight sites where the monkeys consumed soil in the home range during the study period. The total time spent ingesting soil was 13,690 seconds. 20 adult males, 34 adult females except lactating mothers, and 12 immatures without infants were seen to eat soil throughout this study. Average time spent in soil-eating bouts differed significantly among age/sex classes. This study suggests that particular age/sex classes or individuals in certain states of society and health will predictably display a behavioral pattern of geophagy. Our data indicate that geophagy in R. bicti is best explained as a response to nutrient deficiency, as soil consumed by the monkeys was significantly higher in calcium, copper, zinc, iron, manganese, and sodium. Although R. bicti consumes iron-rich soil, control samples that were not consumed also had high levels of iron, suggesting that high altitude alone is not a sufficient explanation for geophagy in this species.