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Concept: Mentawai Islands


Primate loud calls have the potential to encode information about the identity, arousal, age, or physical condition of the caller, even at long distances. In this study, we conducted an analysis of the acoustic features of the loud calls produced by a species of Asian colobine monkey (simakobu, Simias concolor). Adult male simakobu produce loud calls spontaneously and in response to loud sounds and other loud calls, which are audible more than 500 m. Individual differences in calling rates and durations exist, but it is unknown what these differences signal and which other acoustic features vary among individuals. We aimed to describe the structure and usage of calls and to examine acoustic features that vary within and among individuals. We determined the context of 318 loud calls and analyzed 170 loud calls recorded from 10 adult males at an undisturbed site, Pungut, Siberut Island, Indonesia. Most calls (53%) followed the loud call of another male, 31% were spontaneous, and the remaining 16% followed a loud environmental disturbance. The fundamental frequency (F0) decreased while inter-unit intervals (IUI) increased over the course of loud call bouts, possibly indicating caller fatigue. Discriminant function analysis indicated that calls were not well discriminated by context, but spontaneous calls had higher peak frequencies, suggesting a higher level of arousal. Individual calls were distinct and individuals were mainly discriminated by IUI, call duration, and F0. Loud calls of older males had shorter IUI and lower F0, while middle-aged males had the highest peak frequencies. Overall, we found that calls were individually distinct and may provide information about the age, stamina, and arousal of the calling male, and could thus be a way for males and females to assess competitors and mates from long distances.

Concepts: Gender, Acoustics, Proboscis Monkey, Reproduction, Pig-tailed Langur, Old World monkey, Mentawai Islands, Colobinae


Male vocal displays play an important role in sexual selection in many species. If there are costs or constraints, calls may convey honest information about the caller. We studied the loud calls of male simakobu (Simias concolor), a sexually dimorphic primate that resides in one-male groups, on Siberut Island, Indonesia.

Concepts: Sexuality, Pig-tailed Langur, Proboscis Monkey, Sexual selection, Colobinae, Old World monkey, Sexual dimorphism, Mentawai Islands


Investigating which factors influence feeding competition is crucial for our understanding of the diversity of social relationships. Socio-ecological models differ in their predictions whether predation risk directly influences feeding competition and which factors exactly predict contest competition. We investigated feeding competition in Siberut macaques (Macaca siberu), a species endemic to Siberut Island (West Sumatra, Indonesia). Siberut macaques experience low predation risk, as major predators (felids, raptors) are absent. They are therefore appropriate subjects to test the prediction that low predation risk reduces feeding competition. To estimate contest potential, we quantified size, spatial distribution and density of food plants, and the availability of alternative resources. We recorded behavior in food patches using a modified focal tree method. Food patches, sorted by decreasing average feeding group size, included large trees (40% of focal plant observations), lianas/strangler (16%), medium trees (9%), small (palm) trees (20%), and rattan (15%). Most food patches were clumped but occurred at low densities relative to the area of average group spread. Thus, availability of alternative food patches was low. Although food patch characteristics indicate high contest potential, the observed aggression rate (0.13 bouts between adults/h) was low relative to other primates. Average feeding group size was small relative to total group size, and feeding group size matched crown volume. Perceived predation risk was low, based on spatial and feeding behavior of juveniles. Together, these results suggest that predation risk may influence feeding competition. Social and temporal factors (patch feeding time), but not ecological factors (fruit abundance in patch and forest, alternative resources) predicted aggression frequency in food patches. Overall, comparative data are still relatively scarce, and researchers should collect more data on group spread, sub-grouping, perceived predation risk, and aggression in food patches before we can draw final conclusions about the role of predation risk for feeding competition. Am. J. Primatol. © 2015 Wiley Periodicals, Inc.

Concepts: Scientific method, Mentawai Islands, Density, Ethology, Prediction, Ecology, Futurology, Predation


Kloss gibbons (Hylobates klossii) are endemic to the Mentawai Islands in Indonesia and have been subject to human predation for more than 2000 years in the absence of any other significant predators. We investigate the behavior of Kloss gibbons that may be attributed to avoiding human predation. We observed Kloss gibbons in the Peleonan forest in the north of Siberut Island, the northernmost of the Mentawai island chain, over 18 months in 2007 and 2008, and collected data on their singing behavior, the number of individuals present during different conditions and their responses to humans. We examine behaviors that may reduce the risk of predation by humans during singing (the most conspicuous gibbon behavior), daily non-singing activities and encounters with humans. The individual risk of being stalked by hunters is reduced by singing in same-sex choruses and the risk of successful capture by hunters during singing is reduced by singing less often during daylight hours and by leaving the location of male pre-dawn singing before full light (reducing the visual signal to hunters). Groups in the Peleonan also fission during non-singing daily activity and rarely engage in play or grooming, enhancing the crypticity of their monochromatic black pelage in the canopy. We also observed a coordinated response to the presence of humans, wherein one adult individual acted as a “decoy” by approaching and distracting human observers, while other group members fled silently in multiple directions. “Decoy” behavior occurred on 31% of 96 encounters with unhabituated Kloss gibbons that detected our presence. “Decoy” individuals may put themselves at risk to increase the survival of related immatures (and adult females with infants) who have a greater risk of predation. We argue that, in combination, these behaviors are an evolved response to a long history of predation by humans. Am. J. Primatol. © 2014 Wiley Periodicals, Inc.

Concepts: Siberut, Human behavior, Sumatra, Gibbons, Kloss's Gibbon, Hylobates, Gibbon, Mentawai Islands


Kloss gibbons (Hylobates klossii) are endemic to the Mentawai Islands in Indonesia and are one of only two gibbon species in which mated pairs do not sing duets. This is the first long-term study of the factors influencing the singing activity of Kloss gibbons within a northern Siberut Island population and follows two previous studies in central Siberut nearly 30 years ago. We collected data on the presence/absence of male and female singing within the study area on 198 days and within a focal group on 47 days. Rainfall during the time period in which they normally sing inhibits singing in both males and females. Our study supports the hypothesis that male and female songs function in intrasexual resource defence, as singing is associated with singing by same-sex neighbours, and same-sex choruses are more likely to occur after one or more days of silence (from that sex), suggesting there is pressure for individuals to communicate with same-sex neighbours regularly. Singing was not coordinated within a mated pair, suggesting that vocal coordination of the pair has been lost with the loss of the duet and that Kloss gibbon songs do not convey information to neighbours about the strength of the pair bond. On days when males sang predawn, females were more likely to sing after dawn and earlier in the morning. Additionally, the number of groups singing in female choruses was positively associated with the number of males that had sung in the predawn male chorus. We suggest that female songs have an intersexual territory defence as well as an intrasexual function.

Concepts: Hermaphrodite, Female, Reproduction, Male, Mentawai Islands, Gender, Sex, Gibbon