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Faecal particle size in free-ranging primates supports a 'rumination' strategy in the proboscis monkey (Nasalis larvatus)

Ikki Matsuda, Augustine Tuuga, Chie Hashimoto, Henry Bernard, Juichi Yamagiwa, Julia Fritz, Keiko Tsubokawa, Masato Yayota, Tadahiro Murai, Yuji Iwata, Marcus Clauss

In mammalian herbivores, faecal particle size indicates chewing efficiency. Proboscis monkeys (Nasalis larvatus) are foregut fermenters in which regurgitation and remastication (i.e. rumination) was observed in the wild, but not with the same consistency as found in ruminants and camelids. To test whether this species has exceptional chewing efficiency among primates, as ruminants have among mammals, we compared faecal particle size in free-ranging specimens with those of 12 other primate species. The discrete mean faecal particle size (dMEAN) increased with body mass (M) as dMEAN (mm) = 0.65 (95 % confidence interval 0.49-0.87) M 0.33 (0.23-0.43) in simple-stomached species. At 0.53 ± 0.09 mm, dMEAN of proboscis monkeys was particularly small for their average M (15 kg) and significantly smaller than values of two other foregut fermenting primate species. While we cannot exclude other reasons for the exceptional chewing efficiency in proboscis monkeys, this represents circumstantial evidence for regular use of rumination in this species. Thus, proboscis monkeys might be a model for convergent evolution towards rumination in a non-ungulate taxon. 


Oecologia (DOI 10.1007/s00442-013-2863-9:online first)


Relationship between body mass and mean faecal particle size in a the primate data set of this study, comprising simple-stomached species (SS), foregut fermenters that do not practice rumination (NRUM F), and the proboscis monkey (Nasalis larvatus), b the same data set as compared to the data set from Fritz et al. (2009) for captive herbivorous mammals [including NRUM F and ruminants (RUM)], and c compared to only the primates from that same dataset. Note the similarity of the position of the proboscis monkey compared to other primates with the position of RUM compared to other mammals


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