Challenge of the night-time observation on diurnal primates: Investigation on the first observed rumination behaviour and anti-predator strategy in proboscis monkeys

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Colobine monkeys including proboscis monkeys tended to prefer leaves containing higher protein levels, although more abundant plant species were chosen within the preferred species, suggesting that there is no specific food preferences between the primates with rumination (i.e. proboscis monkey) and non-rumination (other primates) behaviours. Based on the comparisons of the discrete mean faecal particle size among non-human primates, we represented circumstantial evidence for regular use of rumination in proboscis monkeys probably during the night-time. Indeed, their rumination behaviours were more frequently recorded during the night-time than during the day-time.