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Multilevel societies (MLS), which are characterized by two or more levels of social organization, are among the most complex primate social systems. MLS have only been recorded in a limited number of primates, including humans. The aim of this study was to investigate whether proboscis monkeys (Nasalis larvatus) form MLS in Sabah, Malaysia, and to genetically characterize their dispersal patterns. Association data were obtained through direct observation (35 months) and kinship data through genetic analysis, based on feces collected from ~ 200 individuals. The results strongly suggest that proboscis monkeys exhibit a form of MLS, with several core reproductive units and a bachelor group woven together into a higher-level band. Genetic analysis revealed that the females migrated randomly over short and long distances; however, the males tended to migrate relatively shorter distances than females. Furthermore, male-male dyads showed a slightly higher average relatedness than female-female dyads. Combined with the results of direct observations, we conclude that proboscis monkeys form MLS with at least two layers and a patrilineal basis. Since patrilineal MLS have been identified as an important step in the evolution of human societies, their convergent appearance in proboscis monkeys may help us understand the drivers of human social evolution. Significance statement: The aim of this study was to determine the social organization of proboscis monkeys by direct observation and genetic analysis. The results revealed that their social system exhibited a form of multilevel society with a possible patrilineal basis. Since humans exhibit a similar constellation of social features, proboscis monkeys may offer insightful clues about human social evolution. [ABSTRACT FROM AUTHOR]

Proboscis monkeys, an iconic Bornean endemic species, are facing threats from habitat loss, degradation, and fragmentation due to human activities. Past surveys have covered only a limited number of proboscis monkey populations across their range, leaving the status of many populations largely unknown. We surveyed the previously unexplored Kota Belud region in northwestern Sabah, Malaysian Borneo, to determine the population and distribution of proboscis monkeys. Our assessment of the threats to their survival focused specifically on land-use and land-cover patterns, with the primary goal of evaluating the population status of proboscis monkeys in this region through an analysis of habitat change. Between January and September 2022, we conducted boat-based surveys in wetland forests—potential habitats of the proboscis monkeys. Of the six sites we surveyed, we directly observed proboscis monkeys at just one, although local residents reported their presence at other sites. Our observations identified 25 individuals in at least three one-male-multifemale groups and one all-male group, all along the Tempasuk River near the coastline. Overall, our study highlights the concerning state of the proboscis monkey population in Kota Belud, which is small, highly fragmented, and likely reaching its limits of adaptability to habitat changes due to human activities. While acknowledging the need for further studies, urgent conservation strategies are needed. For the long-term survival of the proboscis monkeys in Kota Belud, we recommend prioritizing the identification and protection of remaining habitats, rehabilitation, reconnection of isolated areas, and proactive measures to prevent further habitat fragmentation. [ABSTRACT FROM AUTHOR]

The Endangered proboscis monkey Nasalis larvatus is endemic to the island of Borneo. Habitat loss is a major threat to this species, and an understanding of long-term demographic trends is crucial for its conservation. We assessed the population trends and group sizes of proboscis monkeys over 10 years in the Lower Kinabatangan floodplain in Sabah, Malaysian Borneo. Comparisons of observed populations between 2004 and 2014 revealed significantly reduced group sizes, which is probably a result of forest fragmentation. Three long-term studies over 34–73 months in specific areas showed fluctuating estimated densities in each area, but no overall population increase or decrease. Riparian forests are the most important habitat for these monkeys, and one reason for the relatively stable population could be that there were only minor losses of forest along rivers during 2004–2014 because protected areas have been established in the region in 2005. However, proboscis monkey habitat remains under threat in areas allocated for oil palm, and protection of these areas is paramount to maintaining this population. [ABSTRACT FROM AUTHOR]

Abstract: Riverine refuging by non-human primates, with focus on proboscis monkeys, was studied in a forest along the Menanggul River, Sabah, Malaysia from May 2005 to 2006. The results of the primate census indicated that not only proboscis monkeys but also sympatric primates inhabiting the study site preferred to utilize the riverine habitat for night-time sleeping, though the frequency of riverine usage was different among these sympatric primates. Four predation-related events in the study site and two additional predation reports at other study sites involving clouded leopards suggest a relatively high predation pressure in proboscis monkeys relative to other sympatric primates. Riverine refuging, which represents a strategy of long-range visibility, may provide non-human primates including proboscis monkeys with the common benefit of an effective means of predation avoidance. In addition, a one-male group of proboscis monkeys was studied to clarify the effects of food availability and air temperature on riverine refuging. Proboscis monkeys spent more time in the inland habitat, though the food availability was not much different between riverine and inland habitats, indicating that food availability is not a fundamental factor in their preference for riverine habitat. Air temperature only had a small effect on their preference for the riverine habitat. However, to clarify the reasons why riverine refuging is more common in proboscis monkeys than in sympatric primates, further investigation is needed. [Copyright &y& Elsevier]

Primates spend about half of their lives at sleeping sites, and their choice of sleeping sites may affect individual survival. We identified a total of 88 trees used by proboscis monkeys ( Nasalis larvatus) as night sleeping sites on 16 nights from June to September 2008 in riverine, mangrove, and mixed mangrove-riverine forests along the Garama River, a tributary of the Klias River, in the west of Sabah, Malaysia. We recorded 11 variables for each tree, including the species, physical structure, distance from the riverbank, and connectivity with surrounding trees. We compared sleeping trees with 114 trees with ≥30 cm girth at breast height (GBH) located ≤50 m of the riverbank in 8 botanical plots (total 1 ha). Trees in the plots represented the general vegetation patterns of the study area. Choice of sleeping trees did not depend on the tree species. Although sleeping trees included trees ≤46 m from the river, those closer to riverbanks (5-35 m, n = 76) were more likely to be used as sleeping sites. Compared to the available trees, sleeping trees had larger trunks (mean±SD = 143.6 ± 56.9 cm GBH), and were taller (mean±SD = 34.3 ± 8.1 m), with greater number (median = 6; range = 12) and larger (mean±SD = 24.1 ± 15.2 cm circumference) main branches. They were also located near to other trees, with overlapping branches, creating good arboreal connectivity. Choice of sleeping trees by proboscis monkeys is likely to be related to risks of predation and injury from falling, as well as ease of social interaction and efficiency of locomotion. [ABSTRACT FROM AUTHOR]

A group of proboscis monkeys (Nasalis larvatus) consisting of an alpha-male, six adult females, and several immatures was observed from May 2005–2006. We collected over 1,968 hr of focal data on the adult male and 1,539 hr of focal data on the six females in a forest along the Menanggul River, Sabah, Malaysia. Availability and seasonal changes in plant species consumed by the focal monkeys were determined by vegetation surveys carried out across an area of 2.15 ha along 200–500 m trails in riverine forest. A total of 188 plant species were consumed by the focal monkeys. The activity budget of members of our study group was 76.5% resting, 19.5% feeding, and 3.5% moving. Young leaves (65.9%) and fruits (25.9%) accounted for the majority of feeding time. Over 90% of fruit feeding involved the consumption of unripe fruits and in the majority of case both the fruit flesh and seeds were eaten. Although fruit eating was rare in some months, during other times of the year time fruit feeding exceeded the time devoted to young leaves. We found that monthly fruit availability was positively related to monthly fruit eating and feeding activity, and seasonal fluctuations in dietary diversity were significantly affected by fruit eating. These results suggest that fruit availability and fruit-eating behaviors are key factors that influence the activity budget of proboscis monkeys. Earlier assumptions that colobine monkeys are obligate folivores do not apply well to proboscis monkeys and certain other colobines. Our findings may help contribute to a better understanding of the dietary adaptations and feeding ecology of Asian colobines. Am. J. Primatol. 71:478–492, 2009. © 2009 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

From May 2005–2006, selections of river crossing locations and sleeping sites used by a one-male group (BE-Group) of proboscis monkeys (Nasalis larvatus) were investigated along the Menanggul River, tributary of the Kinabatangan River, Sabah, Malaysia. The frequency of river crossings for focal monkeys in the BE-Group was significantly higher at locations with narrow branch-to-bank distances. Branch-to-bank distances were defined as the distances between the longest tree branches extending over the river and the bank of river on each side. This was measured in areas crossed by the monkeys. The focal monkeys used locations with a higher probability of successful river crossings that did not require jumping into the water and swimming across than those that did. The frequency of sleeping site usage by the BE-Group was positively correlated with the frequency of using river crossing locations by the focal monkeys. Previous reports on predation of proboscis monkeys indicate that clouded leopards (Neofelis diardi) and crocodilians (Tomistoma schlegeli and Crocodylus porosus) may be the major terrestrial and aquatic predators of these monkeys. The selection of river crossing locations by proboscis monkeys may be influenced both by the threat of these predators and the location of suitable and protected sleeping sites. Finally, sleeping sites locations that offer arboreal escape routes may protect proboscis monkeys from leopard attack. Am. J. Primatol. 70:1097–1101, 2008. © 2008 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]