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.

Few mammals-cetaceans, domestic cats and select bats and rodents-can send and receive vocal signals contained within the ultrasonic domain, or pure ultrasound (greater than 20 kHz). Here, we use the auditory brainstem response (ABR) method to demonstrate that a species of nocturnal primate, the Philippine tarsier (Tarsius syrichta), has a high-frequency limit of auditory sensitivity of ca 91 kHz. We also recorded a vocalization with a dominant frequency of 70 kHz. Such values are among the highest recorded for any terrestrial mammal, and a relatively extreme example of ultrasonic communication. For Philippine tarsiers, ultrasonic vocalizations might represent a private channel of communication that subverts detection by predators, prey and competitors, enhances energetic efficiency, or improves detection against low-frequency background noise.

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An increasing number of primatologists have begun using radio telemetry to study the behavioral ecology of nocturnal prosimian primates. Radio telemetry has enabled the collection of data on these nocturnal and cryptic prosimians that was previously difficult or impossible to otherwise obtain. A critical assumption of studies employing radio telemetry is that the radio transmitters have no appreciable negative effects on the study animals and the data being collected are not being biased by the presence of radio transmitters. This assumption is made because comparable data from a non‐radio‐collared control group are impossible to obtain. In an attempt to determine the tolerable weight limit for radio collars for a small nocturnal primate, the spectral tarsier, Tarsius spectrum, a comparison of the behavior and body weight of individuals wearing collars of two different weights was conducted. This study was conducted in Tangkoko Dua Saudara Nature Reserve in Sulawesi, Indonesia. A total of 16 individuals from seven groups were trapped in mist nets, radio‐collared, and observed using focal follow sampling between April 1994 and June 1995. Each individual was observed for 4–6 months depending on the life span of the radio‐collar battery. The two radio‐collar weights appeared not to affect spectral tarsiers differentially. Average body masses in neither set of subjects differed between the days collars were attached and 6 months later, when they were removed. No differences in activity patterns, home range size, or prey capture rate were detectable between subjects wearing the different transmitters. These results suggest that the heavier radio collars used in this study did not have any appreciable effects on the behavioral patterns of this primate. Am. J. Primatol. 46:145–155, 1998. © 1998 Wiley‐Liss, Inc.

In this study, we examine how high-altitude ecology and anthropogenic edges relate to the density and distribution of pygmy tarsiers. Pygmy tarsiers (Tarsius pumilus) are extremely small-bodied primates (55 g) that are endemic to high-altitude forest and exhibit several differences from lowland Sulawesian tarsier species. From June to September 2010 and January to March 2012, we conducted a population census of pygmy tarsiers across multiple altitudes. Sampling took place within a 1.2 km(2) area encompassing altitudes of 2,000-2,300 m a.s.l. on Mt. Rore Katimbu in Lore Lindu National Park, central Sulawesi, Indonesia. We observed 22 individuals, with an estimated population density of 92 individuals per 100 ha. These results indicate that pygmy tarsiers live at a lower density than lowland Sulawesian tarsier species. Lower density was associated with decreased resources at higher altitudes, including decreased tree size, tree density, and insect biomass. Within the sample area, we found pygmy tarsiers in only 8 of 24 (33%) quadrats, suggesting a nonrandom distribution that probably overinflated this population density estimate. Pygmy tarsiers exhibited a clumped distribution near anthropogenic edges that were associated with increased insect abundance and biomass. Airborne insects were more abundant along forest edges than within the forest interior, and pygmy tarsiers were observed to forage along edges where there was a higher abundance of Lepidoptera and Orthoptera. Tarsiers may mitigate the decreased availability of insects at high altitudes by adjusting their ranging patterns to remain near forest edges.

Numerous species are known for their tendency to approach and confront their predators as a group. This behavior is known as mobbing. Snakes seem to be one of the more consistent recipients of this type of predator-directed behavior. This paper explores individual differences (sex and age) in the mobbing behavior of the spectral tarsier toward live and model snakes. This study was conducted at Tangkoko Nature Reserve (Sulawesi, Indonesia) during 2003-2004. During this research, 11 natural mobbing events and 31 artificially induced mobbing events were observed. The mean number of individuals at a mobbing was 5.7. The duration of mobbing events was strongly correlated with the number of assembled mobbers. Adults were more likely than other age classes to participate in mobbings. Males were more likely than females to participate in mobbings. Mobbing groups often contained more than one adult male, despite the fact that no spectral tarsier group contains more than one adult male. No difference in body size between extragroup males and resident males was observed, refuting the "attract the mightier" hypothesis. The number of mobbers did not affect whether the tarsier or the snake retreated first, countering the "move-on" hypothesis. The "perception advertisement" hypothesis was tentatively supported, in that live snakes were rarely seen in the area following mobbing calls, in comparison to when tarsiers either ignored the snake or alarm call.

Interspeci®c comparisons of mammals and primates suggest that the major determinants of gregarious behaviour are the distribution of food resources in space and time, predation pressure and infanticide avoidance. This study of gregarious behaviour in the spectral tarsier Tarsius spectrum at Tangkoko Nature Reserve, Sulawesi Indonesia suggests that all three may be important in understanding the relatively gregarious nature of spectral tarsiers. To test whether predation pressure was the major determinant of sociality for this species, I mimicked predation pressure by using predator models and playback predator vocalizations. In response to increased predation pressure from rubber models of snakes, the family group was observed mobbing the`predator'. On the other hand, when plastic bird of prey models were placed throughout the tarsier's territory the adult group members often increased their distance from one another. No signi®cant response was made by the spectral tarsiers in response to playbacks of predator vocalizations. To test whether infanticide avoidance was the major determinant of sociality for this species, I compared the distance between adult males and females for three reproductive phases. When adult females were lactating, adult males were observed to decrease the distance between themselves and the adult female, while the female often attempted to increase the distance. However, groups containing either a pregnant female or a non-reproductive (cycling) female did not differ in their gregariousness. To explore whether the temporal distribution of resources was the major determinant of sociality in spectral tarsiers I collected insects hourly. The diet of spectral tarsiers is restricted to insects. Distance between group members closely followed hourly and monthly changes in insect abundance. During hours when abundance was high, distance between group members decreased and vice versa. Additional long-term and more ®ne-grained data need to be collected to ascertain the relative importance of resources, predators and infanticide in determining sociality in the spectral tarsier.