Selection of sleeping trees in pileated gibbons (Hylobates pileatus)

Selection and use patterns of sleeping sites in nonhuman primates are suggested to have multiple functions, such as predation avoidance, but they might be further affected by range defense as well as foraging constraints or other factors. Here, we investigate sleeping tree selection by the male and female members of one group of pileated gibbons (Hylobates pileatus) at Khao Ang Rue Nai Wildlife Sanctuary, Thailand. Data were collected on 113 nights, between September 2006 and January 2009, yielding data on 201 sleeping tree choices (107 by the female and 94 by the male) and on the characteristics of 71 individual sleeping trees. Each sleeping tree and all trees ≥40 cm diameter at breast height (DBH) in the home range were assessed (height, DBH, canopy structure, liana load) and mapped using a GPS. The gibbons preferentially selected tall (mean=38.5 m), emergent trees without lianas. The majority of the sleeping trees (53.5%) were used only once and consecutive reuse was rare (9.5%). Sleeping trees were closer to the last feeding tree of the evening than to the first feeding tree in the morning, and sleeping trees were located in the overlap areas with neighbors less often than expected based on time spent in these areas. These results suggest avoidance of predators as the main factor influencing sleeping tree selection in pileated gibbons. However, other non‐mutually exclusive factors may be involved as well. Am. J. Primatol. 72:617–625, 2010. © 2010 Wiley‐Liss, Inc.     

Use of sleeping trees by black and white Colobus monkeys (Colobus guereza) in the Kakamega Forest,Kenya

Groups of black and white colobus monkeys, or guerezas (Colobus guereza), observed in the Kakamega Forest, Kenya, had weak fidelity for sleeping sites. Groups often slept in trees near commonly used food sources, which might reduce the time and energetic costs of travel. Although the home range of each group overlapped with four to seven others, groups seemed to avoid sleeping near other groups, which would give them immediate and exclusive access to nearby food sources in the morning. The number of times a species of tree was slept in was positively correlated with its density. This may have occurred because so many suitable sites were available that proximity to feeding trees could be obtained whether or not groups slept in the feeding trees. Groups slept in tall trees, which provide stable sleeping sites and which may provide protection from both aerial and ground predators. Groups were more tightly clustered on nights with greater visibility, which might reduce the risk of predation. Am. J. Primatol. 45:281–290, 1998. © 1998 Wiley-Liss, Inc.     

Use of sleeping trees by ursine colobus monkeys (Colobus vellerosus) demonstrates the importance of nearby food

Examination of the characteristics and locations of sleeping sites helps to document the social and ecological pressures acting on animals. We investigated sleeping tree choice for four groups of Colobus vellerosus, an arboreal folivore, on 298 nights at the Boabeng-Fiema Monkey Sanctuary, Ghana using five non-mutually exclusive hypotheses: predation avoidance, access to food, range and resource defense, thermoregulation, and a null hypothesis of random selection. C. vellerosus utilized 31 tree species as sleeping sites and the species used differed per group depending on their availability. Groups used multiple sleeping sites and minimized their travel costs by selecting trees near feeding areas. The percentage that a food species was fed upon annually was correlated with the use of that species as a sleeping tree. Ninety percent of the sleeping trees were in a phenophase with colobus food items. Entire groups slept in non-food trees on only one night. These data strongly support the access to food hypothesis. Range and resource defense was also important to sleeping site choice. Groups slept in exclusively used areas of their home range more often than expected, but when other groups were spotted on the edge of the core area, focal groups approached the intruders, behaved aggressively, and slept close to them, seemingly to prevent an incursion into their core range. However, by sleeping high in the canopy, in large, emergent trees with dense foliage, positioning themselves away from the main trunk on medium-sized branches, and by showing low rates of site reuse, C. vellerosus also appeared to be avoiding predation in their sleeping site choices. Groups left their sleep sites later after cooler nights but did not show behavioral thermoregulation, such as huddling. This study suggests that access to food, range and resource defense, and predation avoidance were more important considerations in sleeping site selection than thermoregulation for ursine colobus.     

Antipredation Sleeping Behavior of Skywalker Hoolock Gibbons (<Emphasis Type="Italic">Hoolock tianxing</Emphasis>) in Mt. Gaoligong,Yunnan, China

Studying sleeping behavior can provide key information for understanding the ecology of a species. Antipredation is an important factor that affects primate sleeping behavior. We studied antipredation sleeping behavior in skywalker hoolock gibbons (Hoolock tianxing). We studied one group (NA) and a solitary female (NB) at Nankang from July 2010 to September 2011, and another group (BB) at Banchang from May 2013 to December 2014 in Mt. Gaoligong, Yunnan, China. Over the study period, we recorded 67 sleeping trees for members of group NA over 92 days, 17 trees for the solitary female NB over 22 days, and 159 trees for members of group BB over 186 days. Skywalker hoolock gibbons at both sites rarely used the same tree on consecutive days (N = 3 at both sites). They traveled fast to enter sleeping tree a mean of 160 ± SD 43 min before sunset at Nankang, and a mean of 192 ± SD 40 min before sunset at Banchang. They seldom (Nankang: 14%, N = 183 observations; Banchang: 25%, N = 548 observations) defecated in sleeping trees. They slept at sites with more tall and large trees and preferred to sleep on tall trees in the site. They slept on branches of small diameter and closer to tree tops. Our study suggests that antipredation plays an important role in skywalker hoolock gibbons’ sleeping tree selection and sleeping behavior. In addition, our data suggest potential effects of habitat degradation on gibbons’ sleeping behavior. Tall trees are especially important for gibbons in degraded forest and should be protected.     

Sleeping site selection and presleep behavior in wild pigtailed macaques

Several factors are likely to control sleeping site selection and presleep behavior in nonhuman primates, including predation risk and location of food resources. We examined the effects of these factors on the sleeping behavior of northern pigtailed macaques (Macaca leonina). While following a troop living in the surroundings of the Visitor Center of Khao Yai National Park (Thailand), we recorded the physical characteristics and location of each sleeping site, tree, the individuals' place in the tree, posture, and behavior. We collected data for 154 nights between April 2009 and November 2010. The monkeys preferred tall sleeping trees (20.9 ± SD 4.9 m) and high sleeping places (15.8 ± SD 4.3 m), which may be an antipredator strategy. The choice of sleeping trees close to the last (146.7 ± SD 167.9 m) or to the first (150.4 ± SD 113.0 m) feeding tree of the day may save energy and decrease predation risk when monkeys are searching for food. Similarly, the choice of sleeping sites close to human settlements eases the access to human food during periods of fruit scarcity. Finally, the temporal pattern of use of sleeping sites, with a preference for four of the sleeping sites but few reuses during consecutive nights, may be a trade‐off between the need to have several sleeping sites (decreasing detection by predators and travel costs to feeding sites), and the need to sleep in well‐known sites (guaranteeing a faster escape in case of predator attack). Am. J. Primatol. 73:1222–1230, 2011. © 2011 Wiley Periodicals, Inc.     

白马雪山自然保护区格花箐滇金丝猴夜宿地的季节性选择

2005年9月至2006年9月,在白马雪山自然保护区南端的格花箐,对一群(约250只)滇金丝猴的夜宿地选择与利用情况进行调查.整个调查历时13个月,确认研究群的夜宿点54个,共记录夜宿地的利用次数137次,提示研究群在其中一些地点多次夜宿.滇金丝猴夜宿地在平均海拔分布上存在明显的季节性差异:夏季猴群夜宿地的平均海拔为3 352 m,为一年中夜宿地分布最高的时期;春季猴群则多选择在低海拔地区夜宿,平均海拔在一年中最低(3 082 m).猴群夜宿地集中分布于3 200～3 400 m的海拔范围,随季节变化,不同海拔梯度上的夜宿地数量和利用频次差异明显.虽然猴群夜宿地主要位于针阔混交林中,但是不同季节猴群夜宿地植被的组成明显不同.猴群明显偏好在位于南坡面和西坡面的地点过夜,在夏季和秋季尚未发现北坡向的夜宿地.研究还表明,不同季节,猴群都会不同程度地在少数几个地点多次过夜,但大多数夜宿地在一年中仅利用一次.滇金丝猴为什么会对某些地点多次利用尚需进一步研究证实.     

Sleeping Tree Selection of Cao Vit Gibbon (Nomascus nasutus) Living in Degraded Karst Forest in Bangliang,Jingxi, China

We studied the sleep‐related behavior of two Cao Vit gibbon (Nomascus nasutus) groups in Bangliang Nature Reserve in Jingxi County, China between January 2008 and December 2009 to test four hypotheses related to sleeping tree selection (predation avoidance, thermoregulation, food access, and range defense). Gibbons entered sleeping trees 88 ± SD 37 min before sunset before their main potential nocturnal predator become active. They usually moved rapidly and straight to sleeping trees and kept silent once settled. Over the course of the study, gibbon groups used many (87 and 57 per group) sleeping trees and reused them irregularly. They also tended to sleep in relatively tall trees without lianas, choosing small branches close to the treetop. These behaviors would make it difficult for potential terrestrial predators to detect and approach the gibbons. Therefore, these results strongly support the predation avoidance hypothesis. Gibbons tended to sleep closer to ridges than to valley bottoms and they did not sleep at lower elevations in colder months. They thus appeared not to select sleeping trees to minimize thermoregulatory stress. Gibbons very rarely slept in feeding trees, instead generally sleeping more than 100 m away from the last feeding trees of the day or the first feeding tree of the next morning. These patterns led us to reject the food access hypothesis. Lastly, we did not find evidence to support the range defense hypothesis because gibbons did not sleep in overlap areas with neighbors more often than expected based on the proportion of overlap and exclusively used areas. Am. J. Primatol. 74:998‐1005, 2012. © 2012 Wiley Periodicals, Inc.     

Multiple Ecological Factors Influence the Location of Proboscis Monkey (<Emphasis Type="Italic">Nasalis larvatus</Emphasis>) Sleeping Sites in West Kalimantan,Indonesia

Multiple ecological factors have been hypothesized to influence primate sleeping site selection. Testing multiple hypotheses about sleeping site selection permits examination of the relative strength of distinct ecological factors and expands our ability to understand how selection pressures influence primate sleeping behavior. Here we examine how avoidance of biting insects, thermoregulation, foraging efficiency, tree stability, and interspecific competition influence selection of sleeping sites by proboscis monkeys (Nasalis larvatus) in Indonesian Borneo. We collected data on relative insect abundance, temperature, rainfall, food availability, group size, sleeping site location, and presence of other primates for 12 mo. Using formal model comparison and information criteria, we analyzed the relative importance of these ecological factors in determining one aspect of sleeping site location: distance from the river. Our models supported the avoidance of biting insects and the foraging efficiency hypotheses. Proboscis monkeys slept further inland on nights when the abundance of sandflies was high along the river, and when less food was available along the river. Many studies suggest that primates select sleeping trees and locations to reduce predation risk; our study indicates that additional factors may also be important in determining sleeping site selection.     

Sleeping site selection by golden-backed uacaris, Cacajao melanocephalus ouakary (Pitheciidae), in Amazonian flooded forests

In Amazonian seasonally flooded forest (igapó), golden-backed uacaris, Cacajao melanocephalus ouakary, show high selectivity for sleeping trees. Of 89 tree species in igapó, only 16 were used for sleeping (18%). Hydrochorea marginata (Fabaceae) and Ormosia paraensis (Fabaceae) were used most frequently (41% of records) despite being uncommon (Ivlev electivity ratios were 0.76, and 0.84, respectively), though the third most commonly used species (11%), Amanoa oblongifolia (Euphorbiaceae), was selected at near parity. All three species have broad, open canopies with large horizontal limbs and uncluttered interiors. Compared with random trees, sleeping trees had above average diameter at breast height (DBH) and height, lacked lianas and wasp nests, and were more frequently within 5 m of open water. Uacaris generally slept one adult per tree or widely separated in the same canopy and on the outer third of the branch. These behaviours are interpreted as maximising detection of both aerial and arboreal predators.     

Effects of Habitat Degradation on Sleeping Site Choice and Use in Sahamalaza Sportive Lemurs (Lepilemur sahamalazensis)

Sleeping sites may be beneficial for animals in terms of thermoregulation, proximity to foraging sites, and protection from predators and infectious diseases. The abundance of adequate sleeping sites is thus essential for the survival of primates. We investigated microhabitats around sleeping sites, and the influence of habitat degradation on sleeping site choice and usage, in the nocturnal Sahamalaza sportive lemur, Lepilemur sahamalazensis. We used quarter point sampling (N?=?315) to describe five forest fragments and 57 sleeping sites and continuous focal animal sampling (N?=?45) to determine the diurnal activity budget, to determine whether individuals inhabiting different fragments or sleeping site types showed different levels of vigilance. Our results suggest that tall trees with large crowns, a high density of small trees, and dense canopy are particularly important for sleeping site choice. Microhabitat structure around sleeping sites did not differ between forest fragments or sleeping site types. Diameter at breast height, crown diameter, canopy cover, and bole height were similar for all sleeping trees, as were the number of lianas in trees with tree-tangle sleeping sites, and the volume of tree holes. Tree holes used as sleeping sites were most often found in dead trees of Bridelia pervilleana (50–62.5 %), whereas tree tangle sites were most often located in Sorindeia madagascariensis (20–62.5 %). Lemurs were active 5–14 % of the daytime, although they never left their sleeping sites or fed. Individuals occupying tree holes had higher levels of activity than those in tree tangles, and those in more degraded fragments were more active. Our results suggest that Sahamalaza sportive lemurs choose their sleeping sites according to specific habitat characteristics, and that factors associated with old and intact forest are likely to be crucial for their survival.     

Tree and forest characteristics influence sleeping site choice by golden lion tamarins

Lion tamarin monkeys are among a small number of primates that repeatedly use a few tree holes for the majority of their sleeping sites. To better understand why lion tamarins rely on tree holes as sleeping sites, we compared the physical characteristics of frequently used sleeping sites, infrequently used sleeping sites, and randomly selected forest locations at multiple spatial scales. From 1990 to 2004, we recorded 5,235 occurrences of sleeping site use by 10 groups of golden lion tamarins (Leontopithecus rosalia) in Poço das Antas Reserve, Rio de Janeiro State, Brazil. Of those, 63.6% were tree holes. Bamboo accounted for an additional 17.5% of observations. Frequently used tree holes were more likely to be found in living trees and their entrances were at lower canopy heights than infrequently used tree holes. We also found that frequently used sleeping sites, in comparison to random sites, were more likely to be found on hillsides, be close to other large trees, have a lower percent of canopy cover, and have larger diameter at breast height. Topography and small‐scale variables were more accurate than were habitat‐level classifications in predicting frequently used sleeping sites. There are ample tree holes available to these lion tamarins but few preferred sites to which they return repeatedly. The lion tamarins find these preferred sites wherever they occur including in mature forest and in relics of older forest embedded in a matrix of secondary forest. Am. J. Primatol. 69:976–988, 2007. © 2007 Wiley‐Liss, Inc.     

The influence of snowfall,temperature and social relationships on sleeping clusters of Japanese monkeys during winter in Shiga Heights

We studied Japanese monkeys (Macaca fuscata) of the Shiga A₁ troop at their sleeping sites in Shiga Heights, Japan, for 41 nights during 3 winters. Monkeys chose their sleeping sites in Japanese cedars and in deciduous broad-leaved forests on non-snowing nights and in Japanese cedar forests on snowing nights. We counted 399 sleeping clusters in which 2 or more monkeys remained in physical contact through the night and 43 solitary sleeping monkeys, though monkeys did not maintain physical contact with others in the daytime. We found 397 clusters on tree branches and 2 clusters on rocks. The mean size of huddling clusters was 3.06±1.22 SD. The cluster size (3.17±1.26 SD) at lower ambient temperatures between −7 and −4°C was larger than that at higher temperatures between −2 and 4°C (cluster size 2.88±1.13 SD). Most clusters were composed of kin. Females kept close to related females in the daytime and huddled with them at night. The highest-ranking male mainly huddled with his kin and his familiar females. Other males kept farther apart from each other in the daytime, probably to avoid social conflicts. Through cold winter nights, however, such males reduced inter-individual distances and huddled with other males. Japanese monkeys appear to recognize three types of inter-individual distances: an intimate distance less than 1 m, a personal distance of 1–3 m and a social distance of 3–20 m; they change their inter-individual distances according to social and ecological circumstances.     

Characteristics of Night Sleeping Trees of Proboscis Monkeys (<Emphasis Type="BoldItalic">Nasalis larvatus</Emphasis>) in Sabah,Malaysia

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.     

Spider monkey sleeping sites: Use and availability

The behavior of spider monkeys (Ateles geoffroyi) at sleeping sites and the characteristics of these sites were studied in Santa Rosa National Park, Costa Rica. The spider monkeys tended to congregate just prior to dusk at a number of sleeping sites which were repeatedly used (81.6%), but occasionally they slept in trees which were only used once (18.4%). All of the regularly used sleeping trees were not used concurrently, but rather, there was a rotation between sites. In general, males were not encountered at regularly used sleeping sites as often as other age/sex classes, and when they were in all male subgroups, they did not sleep in repeatedly used sites. The trees used as regular sleeping sites tended to be large, but such trees were common in the group's home range. The size of the subgroups attending repeatedly used sleeping trees was large when food was abundant and small when food was scarce. It is suggested that this relationship reflects that the costs of travelling to the sleeping site would be more easily recovered when food was abundant than when food was scarce.     

Sleeping sites and lodge trees of the night monkey (Aotus azarae) in Bolivia

Between 1984 and 1987, we recorded the sleeping-site and lodge tree preferences of night monkeys at the Beni Biological Station, Bolivia. We characterized the structure of sleeping-site compared lodge trees to nonlodge trees, and determined the frequency of their use. Aotus azarae used branch and liana platforms on trees of the middle strate of the forest as sleeping sites, but the lodge trees provided sparse cover. Monkeys may manipulate either natural accumulations of material or bird nests to serve as sleeping sites. The characteristics of the sleeping site and of the lodge trees may be related to protection against predators and to thermal advantages. The distribution of lodge trees appeared to be related to access to food. Activities around the sleeping site could be related to marking behavior.     

Sleeping Sites and Latrines of Spider Monkeys in Continuous and Fragmented Rainforests: Implications for Seed Dispersal and Forest Regeneration

Spider monkeys (Ateles geoffroyi) use sites composed of one or more trees for sleeping (sleeping sites and sleeping trees, respectively). Beneath these sites/trees they deposit copious amounts of dung in latrines. This behavior results in a clumped deposition pattern of seeds and nutrients that directly impacts the regeneration of tropical forests. Therefore, information on the density and spatial distribution of sleeping sites and latrines, and the characteristics (i.e., composition and structure) of sleeping trees are needed to improve our understanding of the ecological significance of spider monkeys in influencing forest composition. Moreover, since primate populations are increasingly forced to inhabit fragmented landscapes, it is important to assess if these characteristics differ between continuous and fragmented forests. We assessed this novel information from eight independent spider monkey communities in the Lacandona rainforest, Mexico: four continuous forest sites and four forest fragments. Both the density of sleeping sites and latrines did not differ between forest conditions. Latrines were uniformly distributed across sleeping sites, but the spatial distribution of sleeping sites within the areas was highly variable, being particularly clumped in forest fragments. In fact, the average inter-latrine distances were almost double in continuous forest than in fragments. Latrines were located beneath only a few tree species, and these trees were larger in diameter in continuous than fragmented forests. Because latrines may represent hotspots of seedling recruitment, our results have important ecological and conservation implications. The variation in the spatial distribution of sleeping sites across the forest indicates that spider monkeys likely create a complex seed deposition pattern in space and time. However, the use of a very few tree species for sleeping could contribute to the establishment of specific vegetation associations typical of the southeastern Mexican rainforest, such as Terminalia-Dialium, and Brosimum-Dialium.     

Sleeping Sites of <Emphasis Type="Italic">Rhinopithecus brelichi</Emphasis> at Yangaoping,Guizhou

Arboreal primates spend about half of their lives at sleeping sites; hence, selection of sleeping sites is crucial for individual survival, and data concerning them is important for conservation efforts. We collected data on sleeping sites for a group of the endangered snub-nosed monkey (Rhinopithecus brelichi) at Yangaoping (27°58′N, 108°45′E) from January 2006 to December 2007. All sleeping sites were located in the mid-slope and in the shadow of ridges facing the northeast and southeast. The monkeys remained quiet while entering and occupying sleeping sites, and slept in evergreen species during the cold season (December–March). Trees in sleeping sites were similar in height and girth at breast height to those elsewhere, but some trees in lower areas were larger. The monkeys usually slept in close proximity to the last feeding spot, and their daily activities usually occurred around the sleeping site. Areas adjacent to sleeping sites were used more intensively than those not adjacent. Monkeys left the sleeping sites later in the morning in the cold season. These behavioral responses suggested that predation risk, thermoregulation, and climate stresses are the main determining factors in the selection of sleeping sites for this temperate monkey.     

Sleeping sites of <Emphasis Type="Italic">Rhinopithecus bieti</Emphasis> at Mt. Fuhe,Yunnan

Data on sleeping site selection were collected for a group of black-and-white snub-nosed monkeys (Rhinopithecus bieti; around 80) at Mt. Fuhe, Yunnan, China (99°20E, 26°25N, about 3,000 m asl) from November 2000 to January 2002. At the site mainly three vegetation types were present in an elevation-ascending order: deciduous broad leaf forest, mixed coniferous and broad leaf forest, and dark coniferous forest. In addition, bamboo forest presented in areas burned in 1958. Sleeping sites (n =10) were located in the coniferous forest, where trees were the tallest, bottommost branches were the highest, the diameter of crowns was the second largest, and the gradient of the ground was the steepest. Monkeys usually kept quiet during entering and staying at a sleeping site. The site choice and the quietness may be tactics to avoid potential predators. In the coniferous forest, however, monkeys did not sleep in the valley bottom where trees were the largest, but frequently slept in the middle of the slope towards the east/southeast, in the shadow of ridges in three other directions, to avoid strong wind and to access sunshine; in winter-spring, they ranged in a more southern and lower area than in summer-autumn. These may be behavioral strategies to minimize energy stress in the cold habitat. Monkeys often slept in the same sleeping site on consecutive nights, which reflected a reduced pressure of predation probably due to either the effectiveness of anti-predation through sleeping site selection, or the population decline of predators with increasing human activities in the habitat. The groups behavioral responses to interactive and sometimes conflicting traits of the habitat are site-specific and conform to expectations for a temperate zone primate.     

Sleeping sites of black-and-white snub-nosed monkeys (Rhinopithecus bieti) at Baima Snow Mountain, China

Data on sleeping sites of a group of black-and-white snub-nosed monkeys Rhinopithecus bieti (Colobinae, Primates) were collected between April–July and September–December 2001 to try to determine the factors affecting site selection at Nanren (99°04'E, 28°34'N, Baima Snow Mountain, China). Sleeping trees were tall emergents (27.5±3.2 m) with large diameter at breast height (57.9±16.9 cm) and broad crown diameters (6.3±1.4 m), and significantly larger than other trees. The use of large sleeping trees could provide effective shelters from attacks of terrestrial and aerial predators, heavy snow and rain. Sleeping sites were changed nightly and located at significantly lower altitudes in winter months than in other seasons, which might be a thermoregulatory adaptation or related to snow covering. Such sites tended to be at the middle of sun-oriented slopes, possibly trading off predator avoidance and foraging efficiency.     

Effects of water level on sleeping-site selection and inter-group association in proboscis monkeys: why do they sleep alone inland on flooded days?

A one-male group (BE-Group) of proboscis monkeys was studied along the Menanggul River, a tributary of the Kinabatangan River, Sabah, Malaysia, from May 2005 to 2006. It has generally been assumed that proboscis monkeys only set up their sleeping sites along the riverbank; however, when more than 1 m of water covered the forest floor for more than 700 m inland from the riverbank during the seasonal flood, the BE-Group slept inside the forest. It seems that the sleeping-site selection of the BE-Group was not influenced by food availability during the flooded months because the food availability by the vegetational survey did not vary much between flooded and non-flooded months. In addition, feeding behaviors of the focal monkey in the BE-Group also did not vary much between flooded and non-flooded days. On the other hand, the water level statistically influenced the sleeping-site selection. The proboscis monkeys remained in inland forest during the flooded days because of the reduced predation threat, as terrestrial predators such as clouded leopards are prevented from foraging by deep water covering the forest floor. On non-flooded days when the BE-Group slept at the riverbank, they frequently slept close to other one-male groups on the riverside trees. Contrastingly, when the group slept inside the forest on flooded days when the water level was high, they slept away from other groups. This difference in the need for one-male groups to sleep close to each other might be attributed to the decreased predation threat during high water level in the flooded days.