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

Abstract

Reducing the size of food particles is crucial for herbivores. Seasonal dietary changes are known to influence animals’ chewing efficiency. Proboscis monkeys (Nasalis larvatus) are foregut fermenters, with a high chewing efficiency allowing them to achieve very fine faecal particles. In this study, we investigated how proboscis monkeys’ chewing efficiency varies between wet and dry seasons, hypothesising differences possibly related to diet change. Faecal particle size analysis is an established approach to estimate chewing efficiency in mammalian herbivores. We analysed 113 proboscis monkey faecal samples collected in the Lower Kinabatangan Wildlife Sanctuary, between 2015 and 2017. By following standard sieve analysis protocols, we measured a mean particle size MPS_0.025–8 of 0.45 ± 0.14 mm, and confirmed a previous result that proboscis monkeys have a very low faecal MPS. This study highlights a seasonal influence on proboscis monkeys’ chewing efficiency, with smaller MPS (better chewing efficiency) during the wet season. During that time of the year, individuals may potentially change their diet, as all faecal samples contained intact seeds. Whether the seasonal MPS difference in proboscis monkeys is smaller than in other colobines due to their “rumination” strategy remains to be investigated.

Dust affects chewing efficiency and tooth wear in forest dwelling Western chimpanzees (Pan troglodytesverus)

“…These authors hypothesized that the mainly frugivorous diet may be mechanically less challenging compared to the more folivorous diet of geladas. Likewise, Milette, Sauther, Cuozzo, and Ness () suggested for ring‐tailed lemurs ( Lemur catta ) that fruits are easier to process and digest than leaves, and Thiry et al () found that proboscis monkeys ( Nasalis larvatus ) had smaller FPS during the wet season when their feces contained higher amounts of seeds, indicating frugivory.…”

Effects of diet and age–sex class on the fecal particle size of wild Japanese macaques (Macaca fuscata yakui)

“…Base on the hypothesis that dietary toughness impedes chewing (Pérez-Barbería & Gordon, 1998;Venkataraman et al, 2014), we predicted that fecal particle size increases when dietary toughness increases. A previous study found smaller fecal particle sizes when proboscis monkeys consumed preferred food (Thiry et al, 2018). We predicted that higher fruit consumption leads to smaller fecal particle size, while higher mature leaf consumption results in larger fecal particle size.…”

“…Since the digestion processes other than mastication have little influence on digesta particle size in terrestrial mammalian herbivores (McLeod & Minson, 1988b;Poppi et al, 1980;Spalinger & Robbins, 1992), fecal particle size has been repeatedly used as a proxy for chewing efficiency (Dunbar & Bose, 1991;Fritz et al, 2009;Matsuda et al, 2014;Venkataraman et al, 2014;Weary et al, 2017). Previous studies have found that several factors affect fecal particle sizes in different primate species, such as diet composition (Thiry et al, 2018;Weary et al, 2017), dietary toughness and age (Venkataraman et al, 2014), and re-mastication (Matsuda et al, 2014). However, some of these factors only pertained to species with a highly specialized lifestyle (Matsuda et al, 2014;Venkataraman et al, 2014), while the determinants of fecal particle size remain unclear in other species (He et al, 2020).…”

“…Proboscis monkeys (Nasalis larvatus) had smaller fecal particle sizes during the wet season, when a higher percentage of fecal samples containing intact seeds, than in the dry season (Thiry et al, 2018).…”

A preliminary study on semen collection, its evaluation, and testicular and sperm morphometries in the wild proboscis monkey (Nasalis larvatus)

“…In the wild, the breeding season for proboscis monkeys could be affected by the rainfall pattern altering the abundance of leaves which give nutrients, consequently affecting chewing efficiency and rumination ( 27 ). It is logical to investigate the environmental impact on the breeding season of the proboscis monkey.…”