Matthew G. Nowak

Land-cover changes predict steep declines for the Sumatran orangutan (Pongo abelii)

New surveys provide higher estimate of Sumatran orangutans than previously, but future scenarios indicate decline continues. Positive news about Sumatran orangutans is rare. The species is critically endangered because of forest loss and poaching, and therefore, determining the impact of future land-use change on this species is important. To date, the total Sumatran orangutan population has been estimated at 6600 individuals. On the basis of new transect surveys, we estimate a population of 14,613 in 2015. This higher estimate is due to three factors. First, orangutans were found at higher elevations, elevations previously considered outside of their range and, consequently, not surveyed previously. Second, orangutans were found more widely distributed in logged forests. Third, orangutans were found in areas west of the Toba Lake that were not previously surveyed. This increase in numbers is therefore due to a more wide-ranging survey effort and is not indicative of an increase in the orangutan population in Sumatra. There are evidently more Sumatran orangutans remaining in the wild than we thought, but the species remains under serious threat. Current scenarios for future forest loss predict that as many as 4500 individuals could vanish by 2030. Despite the positive finding that the population is double the size previously estimated, our results indicate that future deforestation will continue to be the cause of rapid declines in orangutan numbers. Hence, we urge that all developmental planning involving forest loss be accompanied by appropriate environmental impact assessments conforming with the current national and provincial legislations, and, through these, implement specific measures to reduce or, better, avoid negative impacts on forests where orangutans occur.

Feeding and Resting Postures of Wild Northern Muriquis (Brachyteles hypoxanthus)

Increased body size in Brachyteles has been regarded as an important evolutionary adaptation that allowed a greater reliance on leaves compared to other more frugivorous Atelidae, but its association with muriqui positional behavior and substrate use is still unknown. Here, we present original data on the feeding and resting postures of the northern muriqui (Brachyteles hypoxanthus) and evaluate predictions about the relationships between body size, postural behavior, and substrate use derived from previously published data for other atelids (e.g. Alouatta, Ateles, and Lagothrix). The study was undertaken from August 2002 to July 2003 on a large group of well‐habituated muriquis inhabiting the Reserva Particular do Patrimônio Natural – Felíciano Miguel Abdala in Minas Gerais, Brazil. Consistent with our predictions, we found that B. hypoxanthus was highly suspensory during postural feeding (60.9%) and commonly used tail–hind limb suspension/horizontal tripod (38.0%) or tail–forelimb/hind limb suspension (21.4%). However, although tail‐suspensory postures permitted the muriquis to use the terminal canopy and small‐sized substrates, these areas were also accessed via tail‐assisted above‐branch postural behaviors involving multiple substrates. Unexpectedly, tail‐suspensory postures were found to be frequently associated with large substrates, tree trunks, and the understory. We suggest that Brachyteles’ ability to access food resources from all areas of a feeding tree and from tree crowns at different canopy levels may account for their ability to efficiently exploit food resources in seasonal disturbed forest fragments of the Brazilian Atlantic Forest today. Am. J. Primatol. 75:74‐87, 2013.

The Evolution of Gibbons and Siamang

Gibbons and siamang (family Hylobatidae ) are the smallest of the extant apes and have geographically always been restricted to Asia. Their origin dates back to the beginning of the middle Miocene about 16.2 mya, a critical time when ape and human ancestors greatly diversified and when hominoids had left the African continent for the first time. The early Miocene was a warm period with equatorial forests expanding northward allowing early hominoids to colonize Europe and, through the Arabian-Indian corridor, also reach Asia. In contrast to the early Miocene, the late Miocene became a time of change caused by tectonic activity, changes in water currents, and cooling temperatures, which significantly impacted the World’s climate and in Asia led to the development of the characteristic monsoons, bringing pronounced seasonality to the region. At the end of the Miocene, many hominoid lineages had perished, but the small apes of Asia prospered and began to diversify. We suggest three key adaptations that were responsible for hylobatids success and survival: (1) perfection of forelimb suspensory locomotion (FSL) including regular, continuous ricochetal brachiation , (2) reduction of group size to small 2–4 adult groups, and (3) reduction in body size . These adaptations probably already characterized stem hylobatids since these traits are present in all extant hylobatids, before the family, after ~10.5 mya rapidly diversified into the four monophyletic genera Nomascus , Symphalangus , Hoolock , and Hylobates . The three key adaptations of hylobatids were probably all related to an overall net reduction of energy expenditure, which we hypothesize was the ultimate road to hylobatids’ success and survival under ecological conditions that were becoming increasingly challenging for large-bodied apes toward the late Miocene and into the Pliocene. In an arboreal environment, suspensory and ricochetal brachiation are more energy efficient than plantigrade quadrupedalism. In addition, brachiation increases access to difficult-to-reach, ripe fruit because it allows foraging along the thin twigs of a tree’s terminal branches. Perfection of the ancestral hominoid suspensory locomotion thus had a double-positive effect: it reduced energy expenditure and allowed hylobatids to become more competitive in fruit acquisition compared to less agile primates. Reducing average group size probably allowed hylobatids to decrease daily travel and within-group feeding competition, both mechanisms potentially adding to a decrease of net energy expenditure for foraging. Territoriality is a common corollary of pair-living in mammals and is also a characteristic of hylobatids, which further boosted a positive energy balance for the small Asian apes , because territoriality allows optimizing foraging efficiency through long-term knowledge of the spatiotemporal distribution of food sources and increased predictability of food availability. Finally, we hypothesize that the reduction in body size was perhaps the most important single adaptive response to the pressures of decreasing temperature and increasing aridity of Asian environments during the Miocene–Pleistocene transition. Body size reduction not only lowered absolute caloric need, particularly for females, but simultaneously facilitated optimizing forelimb suspensory locomotion to feed in terminal branches, which becomes uneconomical beyond the hylobatid body size. A small body may also have been the ultimate precondition for the evolution of small group size , because once females started to live in separate territories apart from each other for ecological reasons, it probably became inefficient for males to aim to defend more than one female. Pair-living may have relaxed sexual selection pressures on male morphology , which is so common in primates living in single-male/multi-female and multi-male/multi-female groups and also characterizes the large-bodied apes where large male body size often correlates with access to more females. The suite of morphological but also socio-ecological traits seen in hylobatids differ in significant ways from many traits seen in the great apes and comprehensively explain the extraordinary evolution of the small Asian apes .

The Torso-Orthograde Positional Behavior of Wild White-Handed Gibbons (Hylobates lar)

Torso-orthograde (TO)-positional behavior is a unifying characteristic of extant hominoids. Previous studies have highlighted the unique use of forelimb-suspensory dominated locomotion and posture among hylobatids, a tremendously successful radiation of small hominoid primates, often neglecting the importance of other TO-positional behaviors, causing hylobatid locomotion and posture to appear more stereotypic and less versatile, relative to the closely related large-bodied nonhuman hominids (great apes). However, early and recent studies of hylobatid positional behavior have lacked the categorical detail necessary to effectively analyze and compare their TO-diversity and contextual use of TO-positional behaviors to those of other primates and in particular the large-bodied nonhuman hominids. To address the deficit of knowledge of the complete positional repertoire of hylobatids, we observed in detail the positional behavior of a large sample of adult white-handed gibbons (Hylobates lar) at Khao Yai National Park, Thailand. We evaluated the TO-positional repertoire of 24 adult gibbons (11 females and 13 males), and the contextual use of their TO-positional repertoire. Our results indicate that lar gibbons possess and use a diverse TO-positional repertoire that is comparable to that of large-bodied hominids. A collective and flexible use of TO-locomotion and -posture allows lar gibbons to maximize their exploitation of the arboreal canopy. We argue that hylobatids unique suspensory locomotion and posture coupled with a previously undocumented TO-versatility more accurately reflect the pattern of positional behaviors responsible for the small apes’ successful radiation and subsequent diversification across all of South and Southeast Asia’s forest habitats.

Locomotion and Posture in Ancestral Hominoids Prior to the Split of Hylobatids

Torso-orthograde (TO)-positional behavior and an associated suite of postcranial morphological specializations, both of which are often linked to the evolution of large body size, are considered hallmarks of hominoid evolution. These features are thought to have played an important role in the Old World monkey-ape divergence and the development of upright bipedal locomotion in our own lineage. Compared to early theorists, who considered hylobatids an appropriate model for investigating the initial stages of hominoid evolution, more recent reconstructions of the last common ancestor of hominoids have advocated a larger-bodied, more generalized ape very different from hylobatids as the most likely morphotype. While field observations from the large-bodied non-human hominids (i.e., Asian and African apes) confirm that they do in fact utilize an expansive TO-positional repertoire, no detailed data from hylobatids has been available until now to fully evaluate this hypothesis in light of all hominoid taxa. Using recently published positional behavior data from a population of adult white-handed gibbons ( Hylobates lar ) from Khao Yai National Park, Thailand, and integrating these data with a diverse anthropoid primate comparative sample, we reevaluate differences in positional behavior of extant cercopithecoids and hominoids. The comparative dataset agrees with recent suggestions that all living apes share a diverse TO-positional repertoire, relative to that of cercopithecoid monkeys. The question, whether or not TO-anatomical specializations and behavior among the Hominoidea represent homologous or homoplastic traits or both, could not yet be answered conclusively, despite the fact that homologous evolution is the more parsimonious explanation, due to a lack of support from the fossil record. Nevertheless, an upright (TO) and enhanced exploitation of the arboreal tree canopy by our ape ancestors were likely key evolutionary novelties that helped to shape the origin of hominins. Only an expansive understanding of the diversity of both extant and extinct apes will help to comprehensively unravel our enigmatic origins.

Preliminary Data on the Highland Sumatran Orangutans (Pongo abelii) of Batang Toru

Orangutans are the only great apes occurring in Asia (Caldecott and Miles 2005) and knowledge of their behavioral ecology comes from a number of long-term studies on Sumatra and Borneo (Wich et al. 2009).

LiDAR patch metrics for object-based clustering of forest types in a tropical rainforest

Abstract Tropical rainforests support a large proportion of the Earth’s plant and animal species within a restricted global distribution, and play an important role in regulating the Earth’s climate. However, the existing knowledge of forest types or habitats is relatively poor and there are large uncertainties in the quantification of carbon stock in these forests. Airborne Laser Scanning, using LiDAR, has advantages over other remote sensing techniques for describing the three-dimensional structure of forests. With respect to the habitat requirements of different species, forest structure can be defined by canopy height, canopy cover and vertical arrangement of biomass. In this study, forest patches were identified based on classification and hierarchical merging of a LiDAR-derived Canopy Height Model in a tropical rainforest in Sumatra, Indonesia. Attributes of the identified patches were used as inputs for k-medoids clustering. The clusters were then analysed by comparing them with identified forest types in the field. There was a significant association between the clusters and the forest types identified in the field, to which arang forests and mixed agro-forests contributed the most. The topographic attributes of the clusters were analysed to determine whether the structural classes, and potentially forest types, were related to topography. The tallest clusters occurred at significantly higher elevations (>850 m) and steeper slopes (>26°) than the other clusters. These are likely to be remnants of undisturbed primary forests and are important for conservation and habitat studies and for carbon stock estimation. This study showed that LiDAR data can be used to map tropical forest types based on structure, but that structural similarities between patches of different floristic composition or human use histories can limit habitat separability as determined in the field.