They chew by night? Night-time behaviour in a ‘ruminating’ primate, the proboscis monkey (Nasalis larvatus)

Studies of diurnal primates have long considered the nocturnal period to be a time spent merely sleeping and not effectively utilized for foraging or social behaviours. However, digestive activity should continue during the night. To explore the adaptive significance of the primate rumination-like behaviour, that is, regurgitation and re-mastication (R/R), observed in the proboscis monkey Nasalis larvatus (but only infrequently during the day), we tested the hypothesis that they frequently awaken and ‘ruminate’ at night through detailed nocturnal observations. We analysed infrared video recordings of nocturnal behaviours of 179 individuals over 35 nights, totalling over 251 h, of free-ranging proboscis monkeys in the lower Kinabatangan region of Sabah, Malaysian Borneo. The results showed that, as we expected, proboscis monkeys were frequently awake at night; they only slept about one third of the time observed at night (27.4 ± 24.6%), with juveniles sleeping the most and subadults sleeping the least. However, contrary to our expectations, R/R did not appear to occur more frequently than during daytime observations and accounted for a minor proportion of the total activity budget during the night. Whether frequent waking up during the night represents an adaptive strategy in relation to predation avoidance, or is a consequence of disturbance (e.g. due to moving branches or mosquitoes), requires further study, ideally in comparison with protected ex situ conditions. Our findings suggest that without detailed observations, primate sleeping times may easily be overestimated due to a high proportion of time spent awake but resting.