Masticatory and ingestive effort in Procolobus verus, a small-bodied African colobine

The olive colobus (Procolobus verus) is the smallest extant colobine. Based on the axiom that folivory is associated with larger body mass, the olive colobus is expected to be less folivorous than its sister taxon Piliocolobus badius, but previous studies show that the opposite is true. Here we test the hypothesis that masticatory and ingestive effort in the olive colobus is greater due to allometric factors related to bite force scaling and throughput of ingested foods. We analyzed oral processing data collected on olive colobus in the Taï Forest, Ivory Coast, between May 2016 and May 2018. We compare these with previously published data on P. badius and Colobus polykomos from Taï. In terms of overall feeding effort, olive colobus invest more effort (i.e., chewing cycles) than the larger colobines. When contrasts are restricted to commonly consumed foods, this greater energetic investment is not consistently observed. Ingestion of young leaves is associated with a reduced number of masticatory cycles in all three colobine species. A slightly elevated average effort in the olive colobus during young leaf feeding suggests this food source is more challenging in smaller monkeys, but mature leaf processing effort is generally the same among Taï colobines. Thus, for olive colobus, leaf ductility may be more problematic than leaf toughness in terms of masticatory effort. While there may be an allometric cost to being a small colobine, food selectivity is an important mitigating factor.