Six extant species of non-human great apes are currently recognized: Sumatran and Bornean orangutans, eastern and western gorillas, and chimpanzees and bonobos [1]. However, large gaps remain in our knowledge of fine-scale variation in hominoid morphology, behavior, and genetics, and aspects of great ape taxonomy remain in flux. This is particularly true for orangutans (genus: Pongo), the only Asian great apes and phylogenetically our most distant relatives among extant hominids [1]. Designation of Bornean and Sumatran orangutans, P. pygmaeus (Linnaeus 1760) and P. abelii (Lesson 1827), as distinct species occurred in 2001 [1, 2]. Here, we show that an isolated population from Batang Toru, at the southernmost range limit of extant Sumatran orangutans south of Lake Toba, is distinct from other northern Sumatran and Bornean populations. By comparing cranio-mandibular and dental characters of an orangutan killed in a human-animal conflict to those of 33 adult male orangutans of a similar developmental stage, we found consistent differences between the Batang Toru individual and other extant Ponginae. Our analyses of 37 orangutan genomes provided a second line of evidence. Model-based approaches revealed that the deepest split in the evolutionary history of extant orangutans occurred ∼3.38 mya between the Batang Toru population and those to the north of Lake Toba, whereas both currently recognized species separated much later, about 674 kya. Our combined analyses support a new classification of orangutans into three extant species. The new species, Pongo tapanuliensis, encompasses the Batang Toru population, of which fewer than 800 individuals survive. VIDEO ABSTRACT.

Great apes are threatened with extinction, but precise information about the distribution and size of most populations is currently lacking. We conducted orangutan nest counts in the Malaysian state of Sabah (North Borneo), using a combination of ground and helicopter surveys, and provided a way to estimate the current distribution and size of the populations living throughout the entire state. We show that the number of nests detected during aerial surveys is directly related to the estimated true animal density and that a helicopter is an efficient tool to provide robust estimates of orangutan numbers. Our results reveal that with a total estimated population size of about 11,000 individuals, Sabah is one of the main strongholds for orangutans in North Borneo. More than 60% of orangutans living in the state occur outside protected areas, in production forests that have been through several rounds of logging extraction and are still exploited for timber. The role of exploited forests clearly merits further investigation for orangutan conservation in Sabah.

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.

In this study, we have reported two direct observations of individuals from a one-male group of proboscis monkeys (Nasalis larvatus) being killed by clouded leopards (Neofelis diardi) in the riverine forest along the Menanggul river, a tributary of the Kinabatangan river in Sabah, Malaysia. One of the two individuals was an infant female and the other was a juvenile female. Based on literature reviews and the observations reported here, we suggest that clouded leopard and crocodile might be significant potential predators of proboscis monkeys of any age or sex and that predation threats elicit the monkeys' anti-predator strategies. Moreover, the observations of the monkeys' behaviour when the group is attacked by a predator suggest that the adult males in one-male groups play an important role as protectors.

We observed a unimale group (BE-Group) of proboscis monkeys (Nasalis larvatus) comprising an α-male, 6 adult females, and several immatures from May 2005 to May 2006. We followed the group for 2014 h along the Menanggul River, Sabah, Malaysia (118°30′E, 5°30′N). Observations focused mainly on ranging behavior. We determined availability and seasonal changes in plant species consumed by the members of the group by vegetation surveys in a 2.15-ha area along 200-500 m trails in the riverine forest. During the observation period, the group ranged ≤800 m from the riverbank, within a total range of 138.3 ha. The daily path length of the group ranged from 220 to 1734 m (mean, 799 m), and daily path length correlates negatively with fruit availability. The monkeys were apt to remain within a small range in fruit-abundant seasons. Because the monkeys preferred to feed on fruits of dominant plant species in the study area, their daily path length may decrease on days when they feed on fruits. The core areas of the group's home range were along the river because the monkeys typically returned to riverside trees to sleep. The group most often used areas that were nearer the riverbank and where the availability of fruits was higher. The most frequently used grids were the ones where the group often had sleeping sites and crossed the river. Avoiding predation may be the main reason for river crossing and selecting particular sleeping sites; hence not only food availability but also the risk of predation appears to influence the ranging of the BE-Group.