Effects of dietary fracture toughness and dental wear on chewing efficiency in geladas (Theropithecus gelada)

Abstract

Chewing efficiency has been associated with fitness in mammals, yet little is known about the behavioral, ecological, and morphological factors that influence chewing efficiency in wild animals. Although research has established that dental wear and food material properties independently affect chewing efficiency, few studies have addressed the interaction among these factors. We examined chewing efficiency, measured as mean fecal particle size, as a function of seasonal shifts in diet (and corresponding changes in food fracture toughness) in a single breeding population of a grazing primate, the gelada monkey, at Guassa, Ethiopia. We also measured dental topographic traits (slope, angularity, and relief index) and relative two- and three-dimensional shearing crest lengths in a cross-sectional wear series of gelada molars. Chewing efficiency decreased during the dry season, a pattern corresponding to the consumption of foods with higher fracture toughness. Older individuals experienced the most pronounced decreases in chewing efficiency between seasons, implicating dental wear as a causal factor. This pattern is consistent with our finding that dental topographic metrics and three-dimensional relative shearing crest lengths were lowest at the last stage of wear. Integrating these lines of behavioral, ecological, and morphological evidence provides some of the first empirical support for the hypothesis that food fracture toughness and dental wear together contribute to chewing efficiency. Geladas have the highest chewing efficiencies measured thus far in primates, and may be analogous to equids in their emphasis on dental design as a means of particle size reduction in the absence of highly specialized digestive physiology.

Faecal Particle Size in Free-Ranging Proboscis Monkeys, Nasalis larvatus: Variation between Seasons

“…The same pattern has been observed in folivorous gelada baboons [Venkataraman et al, 2014]. The geladas showed a lower MPS in the wet season when consuming less tough food items.…”

“…As in gelada baboons, we suggest here that fallback food consumption during some parts of the year leads to a reduction of chewing efficiency which might potentially negatively impact the animals' fitness [Venkataraman et al, 2014]. There is preliminary evidence that "rumination" activity in proboscis monkeys is higher during times of increased leaf consumption [Matsuda et al, 2014], which could potentially attenuate the change in MPS associated with leaves.…”

“…Measuring faecal particle size by wet sieving analysis is an established non-invasive approach to determine the chewing efficiency of mammals [Fritz et al, 2009]. Several studies have focused on faecal particle size in mammals [Fritz et al, 2009;Clauss et al, 2015], or more specifically in ruminants [Renecker and Hudson, 1990;Clauss et al, 2002] and primates [Dunbar and Bose, 1991;Matsuda et al, 2014;Venkataraman et al, 2014;Weary et al, 2017]. Across mammals, the size of faecal particles usually increases with animal body mass [Fritz et al, 2009].…”

“…Seasonal dietary change and dental wear are known to influence animals' chewing efficiency [Venkataraman et al, 2014]. Within a species, faecal particle size can vary in relation to diet [Renecker and Hudson, 1990] or seasons [Nygren and Hofmann, 1990].…”

“…Within a species, faecal particle size can vary in relation to diet [Renecker and Hudson, 1990] or seasons [Nygren and Hofmann, 1990]. For instance, in gelada baboons (Theropithecus gelada), chewing efficiency decreases during the dry season when individuals feed on tougher non-preferred food items, with a more distinct effect in older individuals [Venkataraman et al, 2014]. The opposite is observed in frugivorous chimpanzees (Pan troglodytes schweinfurthii): MPS is higher when chimpanzees feed on drupe fruits (preferred foods) than on figs (non-preferred foods) [Weary et al, 2017].…”