Grouping behavior of Sumatran orangutans (Pongo abelii) and Tapanuli orangutans (Pongo tapanuliensis) living in forest with low fruit abundance

In contrast to the African great apes, orangutans (Pongo spp.) are semisolitary: Individuals are often on their own, but form aggregations more often than expected by chance. These temporary aggregations provide social benefits such as mating opportunities. When fruit availability is high, costs of aggregating should be lower, because competition is less pronounced. Therefore, average party size is expected to be higher when fruit availability is high. This hypothesis would also explain why orangutans in highly fruit‐productive habitats on Sumatra are more gregarious than in the usually less productive habitats of Borneo. Here, we describe the aggregation behavior of orangutans in less productive Sumatran habitats (Sikundur and Batang Toru), and compare results with those of previously surveyed field sites. Orangutans in Sikundur were more likely to form parties when fruit availability was higher, but the size of daily parties was not significantly affected by fruit availability. With regard to between‐site comparisons, average party sizes of females and alone time of parous females in Sikundur and Batang Toru were substantially lower than those for two previously surveyed Sumatran sites, and both fall in the range of values for Bornean sites. Our results indicate that the assessment of orangutans on Sumatra as being more social than those on Borneo needs revision. Instead, between‐site differences in sociality seem to reflect differences in average fruit availability

Modelling ranging behaviour of female orang-utans: a case study in Tuanan, Central Kalimantan, Indonesia

Quantification of the spatial needs of individuals and populations is vitally important for management and conservation. Geographic information systems (GIS) have recently become important analytical tools in wildlife biology, improving our ability to understand animal movement patterns, especially when very large data sets are collected. This study aims at combining the field of GIS with primatology to model and analyse space-use patterns of wild orang-utans. Home ranges of female orang-utans in the Tuanan Mawas forest reserve in Central Kalimantan, Indonesia were modelled with kernel density estimation methods. Kernel results were compared with minimum convex polygon estimates, and were found to perform better, because they were less sensitive to sample size and produced more reliable estimates. Furthermore, daily travel paths were calculated from 970 complete follow days. Annual ranges for the resident females were approximately 200 ha and remained stable over several years; total home range size was estimated to be 275 ha. On average, each female shared a third of her home range with each neighbouring female. Orang-utan females in Tuanan built their night nest on average 414 m away from the morning nest, whereas average daily travel path length was 777 m. A significant effect of fruit availability on day path length was found. Sexually active females covered longer distances per day and may also temporarily expand their ranges

A matrix approach to tropical marine ecosystem service assessments in South east Asia

Ecosystem service assessments are increasingly used to support natural resource management, but there is a bias in their application towards terrestrial systems and higher income countries. Tropical marine applications are particularly scarce, especially in SE Asia. Given the growing coastal population and expansion in blue economy sectors in SE Asia, evidence to support effective marine planning, such as ecosystem service assessments, is urgently needed. Data deficiencies for marine systems, especially (but not only) in lower income countries is a significant obstacle for ecosystem service assessments. To overcome this, we develop an ecosystem service potential matrix which combines evidence taken from an extensive literature review together with expert opinion. The matrix includes both natural and modified habitats as the service providing units. The ecosystem service potential for habitats are scored at the macro level (e.g. mangrove) due to insufficient evidence to score micro-habitats (e.g. fringe, basin or riverine mangroves). The majority of evidence is available for biogenic habitats (mangroves, coral reefs and seagrass meadows) with comparatively little for sedimentary habitats. While provisioning, regulating and cultural services are scored, published evidence is more readily available for provisioning and regulating services. Confidence scores, indicating the uncertainty in the ecosystem service potential scores are included in the matrix. To our knowledge this is the first attempt to systematically capture the provision of ecosystem services from tropical marine habitats. Although initially developed for four marine biosphere reserves and protected areas in SE Asia, the generic nature of the evidence included suggests that the matrix constitutes a valuable baseline for marine ecosystem service assessments within SE Asia and provides a robust foundation for development in future work

Why male orangutans do not kill infants

Infanticide is widespread among mammals, is particularly common in primates, and has been shown to be an adaptive male strategy under certain conditions. Although no infanticides in wild orangutans have been reported to date, several authors have suggested that infanticide has been an important selection pressure influencing orangutan behavior and the evolution of orangutan social systems. In this paper, we critically assess this suggestion. We begin by investigating whether wild orangutans have been studied for a sufficiently long period that we might reasonably expect to have detected infanticide if it occurs. We consider whether orangutan females exhibit counterstrategies typically employed by other mammalian females. We also assess the hypothesis that orangutan females form special bonds with particular “protector males” to guard against infanticide. Lastly, we discuss socioecological reasons why orangutan males may not benefit from infanticide. We conclude that there is limited evidence for female counterstrategies and little support for the protector male hypothesis. Aspects of orangutan paternity certainty, lactational amenorrhea, and ranging behavior may explain why infanticide is not a strategy regularly employed by orangutan males on Sumatra or Borneo

Orangutans: geographic variation in behavioral ecology and conservation

This book describes one of our closest relatives, the orangutan, and the only extant great ape in Asia. It is increasingly clear that orangutan populations show extensive variation in behavioral ecology, morphology, life history, and genes. Indeed, on the strength of the latest genetic and morphological evidence, it has been proposed that orangutans actually constitute two species which diverged more than a million years ago — one on the island of Sumatra the other on Borneo, with the latter comprising three subspecies. This book has two main aims. The first is to carefully compare data from every orangutan research site, examining the differences and similarities between orangutan species, subspecies and populations. The second is to develop a theoretical framework in which these differences and similarities can be explained. To achieve these goals the book synthesizes and compares the data, quantify the similarities or differences, and seeks to explain them

Social organization and male-female relationships

Despite their semi-solitary nature, associations among orangutans are more common than expected by chance for most combinations of age-sex classes. Variation in party size is due to variation in food availability or sexual activity, reflecting the two main types of parties encountered in orangutans. Parties may involve mating or are formed around mothers and immatures of various ages, in which social play is the main social activity. Beyond direct association, Sumatran females tend to remain within audible range of the dominant flanged males, using his long calls to adjust their ranging. Females tend to be more philopatric than males, although it is not clear whether males disperse away from their natal range or end up including their natal range within a much larger home range. The accumulating evidence suggests that orangutans live in more than mere neighbourhoods, but in loose communities in which related females form clusters, share a preference for the same dominant flanged male, within whose earshot they tend to remain and whose ranging is more limited. Further study should reveal whether this Sumatra-derived picture also holds for Borneo

Male-male relationships in orangutans

Sexually mature male orangutans live in home ranges that are 3–5 times that of adult females and show very high overlap. Encounters among flanged males, while rare, are invariably antagonistic and may lead to injury or death. Their dominance relations are not always transitive, however, producing non-linear hierarchies, probably because they usually meet one on one. Flanged males in consortship with a receptive female frequently chase unflanged males trailing them, but never catch them, so that unflanged males often remain associated with the consort pair. Unflanged males are more tolerant among each other, although attacks also occur. The response of flanged males to long calls they hear depends on their dominance position: the dominant male approaches them, whereas the other males tend to avoid them. There is no conclusive information on the reaction of unflanged males. Data from Sumatra suggest that dominant flanged males and the males challenging them for local dominance were most commonly present in a given study area, suggesting that non-dominant males roamed more widely in search of uncontested access to females

Orangutan mating behaviour and strategies

Orangutans are a species with a very pronounced sexual dimorphism, in that fully grown males are about twice the size of females, but adult, sexually mature males come in two distinct morphs. Unflanged males lack the secondary sexual characteristics (e.g., cheek flanges, throat sack, long call) of the flanged males, but are sexually active, fertile and known to sire offspring. In some males, full development may be delayed until they are over 30. This ‘bimaturism’ is hypothesized to have arisen as a result of sexual selection in which female choice, male–male competition and male coercion have all played important roles. The data reviewed here show that male–male competition is highly affected by the reproductive condition of the females and female preference for associating with particular males. Potentially reproductive female orangutans are widely dispersed in space and time. Consequently, dominant flanged adult males cannot easily exert permanent control over access to reproductive females