Primate species appear to cover the full variety of trophic niches—from nearly exclusive folivory to frugivory, insectivory, gummivory, omnivory [1
] and the nearly exclusive carnivory observed in some human cultures [2
]. Primates are also represented in nearly all major morphophysiological herbivore digestion types, where symbiotic microbes are hosted in a ‘fermentation chamber’ in the gastrointestinal tract to digest plant fibre—caecum fermenters (maybe even coupled with coprophagy [3
] as observed in non-primate caecum fermenters), colon fermenters and non-ruminant foregut fermenters [4
]. The only major strategy of herbivores not described in primates so far is the regurgitation and remastication of digesta. Such behaviour has been observed sporadically in macropods [8
] and the koala (Phascolarctos cinereus
], and is a physiological fixture of ruminant foregut fermentation.
Although non-ruminant foregut fermentation, including that found in primates, has been termed ‘ruminant-like’ and explicitly or implicitly equated with ruminant foregut fermentation [14
], there is a major difference between the two modes of foregut fermentation. The foregut of functional ruminants is equipped with a density-dependent sorting mechanism [16
], which not only ensures that large particles are regurgitated for rumination, but also that small digested particles leave the foregut at a faster rate, thus clearing the forestomach and facilitating high food intakes when compared with non-ruminant foregut fermenters [17
]. Non-ruminant foregut fermenters are constrained in their food intake level for the following reason [18
]. High food intake is generally associated with shorter digesta retention times in the gut, which may compromise the efficiency of microbial digestion of fibre. This is not a problem in hindgut fermenters, where easily digestible nutrients are first digested in the small intestine by the host's enzymes (a process that is not under a relevant time constraint), and fibre is subsequently digested in the hindgut by the microbes' enzymes; the latter part of digestion may either be more thorough (in a low intake-long retention strategy) or less thorough (in a high intake-short retention strategy). In foregut fermenters, the microbes will digest both, fibre and those nutrients that the host could potentially digest with its own enzymes, before the digesta reaches the size of auto-enzymatic digestion, the small intestine. Because the digestion of non-fibrous substrates by microbes is much faster than that of the fibre, yet energetically less efficient for the host than the auto-enzymatic digestion, a high intake-short retention strategy would leave the foregut fermenter with the worst of both ways: easily fermentable substrates are digested at reduced efficiency, but fibre is digested incompletely owing to insufficient retention. Clauss et al
] recently termed this predicament the ‘foregut fermentation trap’. It also appears to apply to primates, where hindgut fermenters cover the whole range of intake-retention strategies, whereas foregut fermenters are constrained to a low intake-long retention strategy [19
]. Reducing food particle size could be one strategy to alleviate this constraint, because smaller particles can be digested by microbes at a faster rate [20
Proboscis monkeys are the largest foregut-fermenting primates and ingest a diet consisting of various proportions of leaves and fruit [21
]. They are endemic to Borneo and inhabit mangroves, swamps and riverine forests. Here, we report regurgitation and remastication behaviours in this species that has, so far, to our knowledge, not been documented.