Applying wet sieving fecal particle size measurement to frugivores: A case study of the eastern chimpanzee (Pan troglodytes schweinfurthii)

Objectives: Fecal particle size (FPS) as quantified by wet sieving analysis is a measure of chewing efficiency relevant for the understanding of physiological adaptations and constraints in herbivores. FPS has not been investigated systematically in frugivores, and important methodological problems remain. In particular, food items that are not chewed may skew estimates of FPS. We address such methodological issues and also assess the influence of diet type and age on FPS in wild chimpanzees. Materials and Methods: About 130 fecal samples of 38 individual chimpanzees (aged from 1.3 to similar to 55 years) from the Kanyawara community of Kibale National Park (Uganda) were collected during three fruit seasons and analyzed using standardized wet sieves (pores from 16 to 0.025 mm). The effects of using different sieve series and excluding large seeds were investigated. We also assessed the relationship between FPS and sex, age, and fruit season. Results: The treatment of seeds during the sieving process had a large influence on the results. FPS was not influenced by chimpanzee sex or age, but was smaller during a fig season (0.88 +/- 0.31 mm) than during two drupe-fruit seasons (1.68 +/- 0.37 mm) (0.025-4 mm sieves, excluding seeds). Discussion: The absence of an age effect on FPS suggests that dental senescence might be less critical in chimpanzees, or in frugivores in general, than in folivorous herbivores. To increase the value of FPS studies for understanding frugivore and hominoid dietary evolution we propose modifications to prior herbivore protocols.