RESEARCH ARTICLE Demography of Simakobu (Simias concolor) and the Impact of Human Disturbance

(simakobu or pig-tailed langur) is considered an exception because mostly adult male–female pairs have been reported. However, based on their phylogenetic position and marked sexualdimorphism, simakobu are also expected to form OMGs with multiple females. The preponderanceof small groups could be the result of human disturbance (hunting or habitat disturbance) reducinggroup size in the recent past. To investigate this possibility, we documented the demography of tenwild simakobu groups from January 2007 until December 2008 at an undisturbed site, the PeleonanForest, Siberut Island, Indonesia. We assessed the population-specific size and composition of groupsand documented demographic changes due to births, disappearances, and dispersals throughout our2-year study. We found OMGs with 3.0 adult females on average in addition to all-male groups, butno adult male–female pairs. The ratio of 0.5 infants per adult female (and 0.64 births per female-year in focal groups) suggested that birth rates were similar to those of other Asian colobines. In 5.1group-years, we observed six dispersal events and six temporary presences (i.e., less than 3 months’residency). Both males and females dispersed, and juveniles seemed to disperse more frequently thanadults. To assess the impact of human disturbance on simakobu demography, we compiled data forseven additional populations from the literature and compared them using multiple regressions. Adultsexratio and the number of immatures pergroup wereinfluenced negativelyby hunting and positivelybyhabitatdisturbancewhilereproductiverateswerenotsignificantlyaffectedbyeithervariable.Thesefindings suggest that adult male–female pairs may result from hunting pressure reducing group size,andthatconservationactiontoreducehuntingintheMentawaiIslandsisneededtoensurethesurvivalof this critically endangered species.