External environmental conditions impact nocturnal activity levels in proboscis monkeys (Nasalis larvatus) living in Sabah, Malaysia

Abstract

Recently, several diurnal nonhuman anthropoids have been identified displaying varying degrees of nocturnal activity, which can be influenced by activity "masking effects"-external events or conditions that suppress or trigger activity, temporarily altering normal activity patterns. Environmental masking characteristics include nocturnal temperature, rainfall, cloud cover, and moon brightness. Similarly, other ecological characteristics, including proximity to humans and predators and daytime activity, may also trigger or suppress nocturnal activity. Understanding the effects of external conditions on activity patterns is pertinent to effective species conservation. We investigated the presence of nocturnal activity and the influence of masking effects on the level of nocturnal activity displayed by wild proboscis monkeys (Nasalis larvatus) in Sabah, Malaysian Borneo. Dual-axis accelerometers were attached by collar to six male proboscis monkeys from different one-male, multifemale groups to record activity continuously (165-401 days each). We measured the monkeys' nocturnal and diurnal activity levels and investigated the effects of seven potential masking effects. Nocturnal activity was much lower than diurnal activity. Still, proboscis monkeys did display varying levels of nocturnal activity. Generalized linear mixed models identified higher nocturnal activity in the study individuals during nights with cooler temperatures, higher rainfall, and after higher diurnal activity. These three masking effects affected nocturnal activity levels during the observation period that informed our model, although they did not predict nocturnal activity outside of this period. While the generalizability of these results

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

“…This means that sleeping site selection may have an important influence on their fitness (Feilen & Marshall, 2014), especially considering that primates reportedly spend up to half of their lives at sleeping sites (Anderson, 1998). Sleeping site selection in primates is strongly influenced by factors linked to safety from predators (Li et al ., 2010; Bernard et al ., 2011; Feilen & Marshall, 2014; Mochida & Nishikawa, 2014; Thiry et al ., 2016; Nunn & Samson, 2018; Reinhardt, 2020), weather conditions (Li et al ., 2010; Reyes et al ., 2021; Kooros et al ., 2022) and social behaviour (Anderson, 1998, 2000; Li et al ., 2010; Mochida & Nishikawa, 2014). Furthermore, it has already been described that water level (Matsuda et al ., 2010) and the presence of mosquitoes influence sleeping site selection (Feilen & Marshall, 2014, 2017).…”

“…Despite the fact that proboscis monkeys are assumed to be solely diurnal primates (Matsuda et al, 2009a), they show some activity throughout the night. For example, using accelerometery collars, Kooros et al (2022) showed that at night, proboscis monkeys engaged in between 3.5 and 8.5% of their daylight activity level, and that this activity depended on a variety of factors like diurnal activity, rainfall and temperature. Because our primary target behaviour, R/R, cannot be detected by measures that indicate whole-body animal movement, the present study relies on nighttime video footage recorded in the natural environment of the species.…”