As part of a larger study of the behavioral ecology of the spectral tarsier,
Tarsius spectrum,a population census was conducted. A modified form of the fixed point count and quadrat census methods was conducted within a 1-km2 area in Tangkoko Dua Saudara Nature Reserve, in northern Sulawesi, Indonesia. In 25 one-hectare plots, 14 groups were located, containing a total of 39 individuals. Therefore, the mean number of groups per ha was 0.56 and the mean number of individuals per ha was 1.56. The number of groups for the entire sampled area (100 ha) was calculated to be 56, while the total population within the sampled area was calculated to be 156 (i.e. 156/km2). Group size, defined as the number of individuals sharing a sleeping site, varied from 2 to 6 individuals. Group composition varied from 1 adult of each sex to 1 adult male and 2 adult females and their offspring. Fourteen percent of groups contained more than 1 adult female. In one of the groups with two adult females, both females gave birth to infants. The home ranges of 13 individuals, obtained using radio telemetry, were between 1.6 and 4.1 ha, with an average size of 2.3 ha for females and 3.1 ha for males. Results from this study indicate that spectral tarsiers are still relatively abundant at Tangkoko Dua Saudara Nature Reserve. Nonetheless, the limited geographic distribution of this species, the lack of information on its relative abundance in other areas of its range, as well as the high rate of habitat disturbance throughout its range, suggests that the conservation status of the spectral tarsier should be changed from indeterminate to vulnerable.