Intraspecific Variation in the Mating System of Spectral Tarsiers

Historically, a species’ social system was perceived to be a fixed attribute and deviations were usually treated as aberrant or maladaptive. Over the last few decades, socioecologists have started to recognize that variation in social systems is normal and adaptive. Here I explore how ecological variation affects the mating system of a nocturnal primate, the spectral tarsier, Tarsius spectrum. Several studies of the spectral tarsier have illustrated variation in this species’ mating system. Although most groups exhibit a monogamous mating system, a small proportion of the population consistently exhibits a polygynous mating system. I conducted this study at Tangkoko Nature Reserve in Sulawesi, Indonesia during 2007. I found that group size was highly variable, ranging from 2 individuals to as many as 8 individuals per sleeping site. Although most groups (21 of 26) were composed of a single adult male and a single adult female, ca. 19% of the groups contained 2 adult females. Three of the 5 groups with 2 adult females contained 2 young infants. As might be expected, polygynous groups were larger than were monogamous groups. The mean group size of monogamous groups was 2.9, whereas the mean group size of polygynous groups was 6.0. Polygynous groups were also more likely to use Ficus caulocarpa trees than were the monogamous groups. Polygynous groups consistently used more sleeping sites as well as larger diameter sleeping trees, than did monogamous groups. The large-diameter fig trees are ideal homes for the spectral tarsiers in that they offer multiple entrances and exits as well as protection from the elements. Polygynous and monogamous groups exhibited no differences in insect biomass available, home range size, or height of sleeping tree. These results support the hypothesis that ecological variation is an important determinant of mating system within spectral tarsiers.