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

Abstract

Asian colobines typically live in small one-male groups (OMGs) averaging five adult females, but Simias concolor (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 sexual dimorphism, simakobu are also expected to form OMGs with multiple females. The preponderance of small groups could be the result of human disturbance (hunting or habitat disturbance) reducing group size in the recent past. To investigate this possibility, we documented the demography of ten wild simakobu groups from January 2007 until December 2008 at an undisturbed site, the Peleonan Forest, Siberut Island, Indonesia. We assessed the population-specific size and composition of groups and documented demographic changes due to births, disappearances, and dispersals throughout our 2-year study. We found OMGs with 3.0 adult females on average in addition to all-male groups, but no 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 5.1 group-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 than adults. To assess the impact of human disturbance on simakobu demography, we compiled data for seven additional populations from the literature and compared them using multiple regressions. Adult sex ratio and the number of immatures per group were influenced negatively by hunting and positively by habitat disturbance while reproductive rates were not significantly affected by either variable. These findings suggest that adult male-female pairs may result from hunting pressure reducing group size, and that conservation action to reduce hunting in the Mentawai Islands is needed to ensure the survival of this critically endangered species.

Ecology of an endemic primate species (Macaca siberu) on Siberut Island, Indonesia

“…comm. ), b median range not mean range from Whitten (1982c), c mean day range is monthly average for 10 months, range means minimum and maximum of all single “full day follows” during this period, d diet for P. potenziani from Hadi et al (2012), with values from Fuentes (1997) in brackets, e forest strata use for the three species was recorded at the same study site of the Siberut Conservation Programme, in the Peleonan Forest, North Siberut.: Erb (Data sources: Simias concolor 2012), Erb et al (2012b), Hadi (2012), Hadi et al (2012) and Tenaza & Fuentes (1995: Fuentes (); Presbytis potenziani 1997), Hadi (2012) and Hadi et al (2012: Tenaza (); Hylobates klossii 1975), Tilson (1981), Whitten (1980), Whitten (1982a), Whitten (1982c); Macaca siberu : this study.

Figure 5 Distribution of height classes (in m) of trees, trees and palm trees combined and the percentage of the daily daytime activity of the group of Siberut macaques per height class shown. Tree data are based on all 12 botanical plots.

Figure 6 Percent frequency of the relative forest height used (ground, lower-story, mid-story, canopy) for the whole group, and for adult males, adult females and juveniles separately.

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“…Tenaza 1975;Tilson 1977;Watanabe 1981;Tilson and Tenaza 1982;Whitten 1982a;Whitten 1982b;Whitten 1982c;Tenaza and Tilson 1985;Fuentes 1997;Hadi et al 2009a;Erb et al 2012b;Hadi et al 2012). In contrast, studies on the Siberut macaque are mainly limited to investigations on population size, acoustic traits, phylogenetic relationships and some preliminary observations on ecology (Whitten and Whitten 1982;Abegg and Thierry 2002b;Roos et al 2003;Ziegler et al 2007;Schneider et al 2008;Waltert et al 2008;Quinten et al 2009).…”

Individual, Contextual, and Age-Related Acoustic Variation in Simakobu (Simias concolor) Loud Calls

“…Simakobu at this site reside in one-male groups (OMGs) with 3.0 females (range = 2–5) and 7.9 individuals on average, as well as in all-male groups (AMGs) averaging 4.5 individuals [50]. Female dispersal is common, and group home ranges are small (<10 ha), exhibiting little overlap between adjacent groups [45], [50].…”

“…Female dispersal is common, and group home ranges are small (<10 ha), exhibiting little overlap between adjacent groups [45], [50]. Study subjects were 10 adult males residing in OMGs ( N  = 9) and AMGs ( N  = 1).…”

“…Study subjects were 10 adult males residing in OMGs ( N  = 9) and AMGs ( N  = 1). Four of the study groups were habituated to observers in 2006/2007, and all individuals were identified (details in [50]). Two of these groups (OMG-H = 130 follow days; and AMG-D = 104 follow days) were followed beginning in February 2007, and most group members were habituated by May 2007.…”