Craniodental mechanics and diet in Asian colobines: morphological evidence of mature seed predation and sclerocarpy

Folivory has been accepted as the general dietary pattern for colobines. However, recent ecological studies have revealed that extensive seed eating is found in some colobine species. The ripeness of foraged seeds is also reported to differ between seed eaters. As seeds are generally stress-limited and may pose greater mechanical demands, seed-eating species are predicted to exhibit morphological features adaptive for seed predation. In addition, species that feeds on seeds from unripe fruits with hard pericarp is predicted to exhibit increased leverage for anterior dentition. To test these hypotheses, we compared the craniodental morphology of seed-eating Asian colobines (Presbytis rubicunda and Trachypithecus phayrei) with those of species that rarely exploit seeds (Presbytis comata, Trachypithecus obscurus, and Semnopithecus vetulus). The results show that the seed-eating colobines possess a masticatory system with enhanced leverage at postcanine bite points. The sclerocarpic forager P. rubicunda also exhibits markedly greater masticatory leverage at anterior dental bite points, while the mature-seed-eating T. phayrei shows no such advantage for canine and incisor use. These observations suggest that P. rubicunda is well adapted to husking the resistant pericarps of unripe fruits, using the anterior dentition and to gain access to the immature seeds, whereas such sclerocarpic feeding behavior may be less important for T. phayrei. Our findings indicate that the distinctive craniodental variations of colobines may be linked to mature and/or immature seed eating and suggest the significance of seed predation for the evolution of colobine monkeys.