Oral Processing Behaviors of Golden Monkeys (Cercopithecus mitis kandti) From Volcanoes National Park in Rwanda

Primate foraging behaviors are influenced by a variety of factors including the physical and mechanical properties of food items. Golden monkeys (Cercopithecus mitis kandti) inhabiting Volcanoes National Park (VNP), Rwanda, rely heavily on mechanically challenging bamboo shoots and bamboo leaves for the bulk of their diet. We examined the oral processing behaviors of golden monkeys and predicted that bamboo shoots would be characterized by more incising and post-canine crushing behaviors needed to remove tough outer casings and process culms while leaves would require more mastications compared to other food items. We also predicted that juveniles would utilize more incisions and post-canine crushing behaviors to process bamboo shoots and more mastications to process leaves due to the reduced foraging efficiency characteristic of juveniles. We opportunistically filmed habituated groups of golden monkeys foraging during November-December 2022 and January-March 2024 (n = 328 videos). Oral processing behaviors (including incision, canine puncture, post-canine crushing, and mastication scaled to ingestive action) were scored from video footage using Behavioral Observation Research Interactive Software (BORIS). We found that bamboo shoots required more post-canine crushing behaviors compared to other food categories and that non-bamboo leaves required more mastications than bamboo leaves and shoots, respectively. Juveniles did not use more oral processing behaviors per ingestive action, although the inability to control for the size or amount of food ingested may obfuscate these results. Our findings show that golden monkeys adjust their oral processing behaviors to different food items and the routine use of post-canine crushing behaviors may help explain the pronounced tooth wear characteristic of golden monkeys in VNP.