Determinants of gregariousness in the spectral tarsier (Prosimian: Tarsius spectrum)

Abstract

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.

Predation on a Wild Spectral Tarsier (Tarsius spectrum) by a Snake

“…In fact, tarsiers are generally considered to be predators of snakes as evidenced by Niemitz's classic photo of a tarsier consuming a neurotoxic snake [10]. Recent observations by Gursky [8,12], however, have noted numerous instances of alarm calling and mobbing in the presence of civets, snakes, birds of prey and monitor lizards. Shekelle [cited in 13] also noted several predation attempts by snakes, as well as a successful predation event by a monitor lizard.…”

Predation Experiments on Infant Spectral Tarsiers (Tarsius spectrum)

“…In studying predation, primatologists have paid particular attention to whether adult males defend group members during predator attacks [DeVore and Washburn, 1963;Busse, 1976;Kappeler, 2000;van Schaik and Janson, 2000], the influence of predation on body size, on activity pattern and on degree of arboreality [Crook, 1972;Alexander et al, 1979;Popp, 1983;Gaulin and Sailer, 1984;Cheney and Wrangham, 1987;Isbell, 1994], whether predation pressure functions as the primary selective force favoring group living [van Schaik, 1983;Dunbar, 1988;Janson, 1992;Gursky, 2002a] as well as whether predation occurs frequently enough to influence primate demographic patterns [Caughley, 1977;Janson, 1984;Cheney and Seyfarth, 1985].…”

“…Few studies have addressed the importance of predation pressure on nocturnal prosimian primates [Goodman et al, 1993;Wiens and Zitzmann, 1999;Schulke, 2001;Bearder et al, 2002;Gursky, 2002a]. This discrepancy is, in part, a reflection of the fact that many primatologists perceive that studying predation in nocturnal prosimians is less relevant because they employ crypsis [Cheney and Wrangham, 1987;Janson, 1992].…”

“…This discrepancy is, in part, a reflection of the fact that many primatologists perceive that studying predation in nocturnal prosimians is less relevant because they employ crypsis [Cheney and Wrangham, 1987;Janson, 1992]. However, numerous studies have demonstrated numerous noncryptic antipredator behaviors among nocturnal prosimians [Nekaris, 2001;Bearder et al, 2002;Gursky, 2002a]. For example, according to Charles-Dominique [1977], when carnivores attack pottos, they defend themselves with their scapular shield.…”

“…Slow lorises take predation defense one step further by producing a toxic glandular substance that when mixed with their saliva can put a human, or other predator, into anaphylactic shock [Alterman, 1995]. Similarly, galagos and spectral tarsiers are known to mob predators [Bearder et al, 2002;Gursky, 2002a]. None of these strategies can be considered cryptic.…”