From frugivore to folivore: Altitudinal variations in the diet and feeding ecology of the Bioko Island drill (Mandrillus leucophaeus poensis)

Abstract

Variation in the quality and availability of food resources can greatly influence the ecology, behavior, and conservation of wild primates. We studied the influence of altitudinal differences in resource availability on diet in wild drill monkeys (Mandrillus leucophaeus poensis) on Bioko Island, Equatorial Guinea. We compared fecal samples (n=234) collected across three consecutive dry seasons for drills living in lowland (0-300m asl) forest with nearby (18km distance) drills living in montane forest (500-1000m asl) in the Gran Caldera Southern Highlands Scientific Reserve. Lowland forest drills had a frugivorous diet very similar to that reported from studies on nearby mainland drills (M. l. leucophaeus) and mandrills (M. sphinx), with fruits comprising 90% of their dried fecal samples. However drills living in montane forest had a more folivorous diet, with herbaceous pith, leaves and fungi comprising 74% of their dried fecal samples and fruit becoming a minor component (24%). Furthermore, a dietary preference index indicated that the differences in the proportion of fruit and fibrous vegetation in the diets of lowland compared to montane drills was not simply a result of relative availability. Montane drills were actively consuming a higher mass of the available fruits and fibrous vegetation, a condition reflected in the greater mass of their fresh feces. Our results demonstrate the unexpected flexibility and complexity of dietary choices of this endangered species in two adjacent habitat types, a comparison of considerable importance for many other limited-range species faced with habitat loss and climate change.