For folivores, socio-ecological models predict scramble competition and egalitarian dominance relationships with female dispersal, but female Asian colobines do not all nicely fit these predictions. Alternative explanations concern an absence of competition, and the Folivore Paradox, where group size is limited by social factors such as infanticide. Relevant data are scarce, but our review shows that colobine foraging costs increase with group size, yet female fitness may increase or decrease. Dominance relationships vary from despotic to egalitarian, and are individualistic. The lack of female nepotism in despotic species (or populations) still requires explanation. Female dispersal is found in egalitarian and some despotic species, yet costs may be low if females migrate into a group with familiar females. Asian colobine social organization seems to follow one of two patterns. First, in seasonal species, food limits group size, while infanticide does not; these groups may experience contest competition. Second, in a-seasonal species groups are uni-male, the infanticide risk is high and infanticide, not food, may limit group size. This proposal requires further testing and may also apply to uni-male frugivores. This overview of the socio-ecological patterns in Asian colobines highlights that ecological and social selection pressures in interaction determine primate social organization.

Introduction

The behavior of primates results from a compromise of requirements and risks. The requirements include the acquisition of food and mates and the risks include predators and dangerous conspecifics. Both the acquisition of food and the risk of predation will affect behavior on a daily basis. Food, however, may be found on potentially dangerous locations and foraging behavior may be influenced by strategies to reduce predation risk. This paper examines whether this is found for Thomas langurs (Presbytis thomasi).

Many diurnal primates live in groups to reduce the risk of predation (Alexander 1974; Janson and Goldsmith 1995; van Schaik 1983). Individuals in these groups may benefit from group members in several ways: the early detection of predators, alarm calls, confusion of the predator, the selfish herd effect and by joint mobbing and dislodging of the predator (Alcock 1998). A larger group size or more neighbors (Treves 1998) may provide better protection. Not all group members, however, may benefit equally and high-ranking individuals may occupy safer places within the group than low-ranking ones (e.g., Janson 1990a,b; van Schaik and van Noordwijk 1988).

Langur species are largely arboreal primates that are confined to Asia. Their predators include snakes, felids and canids. Snakes and the smaller felids can be found at all heights in the forest, but are probably less agile in trees than primates (e.g., leopards (Panthera leo): Muckenhirn 1972; personal observation of encounters of primates with reticulated pythons (Python reticularis) and a clouded leopard (Neofelis nebulosa)), whereas the tiger (Panthera tigris) and canids (jackals (Canis aureus): Stanford 1991; domestic dogs (Canis domesticus): Sommer 1985) are confined to the ground.

Introduction

Sex-biased dispersal

Dispersal is a common feature of bird and mammal societies. Philopatry, however, is generally assumed to be the most advantageous situation, because it assures knowledge of the home range and offers better opportunities for cooperation with relatives. Several benefits of foregoing these advantages have been proposed (better mating opportunities, including inbreeding avoidance; reduction of within-group competition; coercion) (Greenwood 1980; Pusey & Packer 1987a; Moore 1993). The balance between the costs and benefits of dispersal obviously differs between the sexes, because dispersal is often highly sex biased. Most bird species show a female bias in dispersal, whereas most mammal species show a male bias (Greenwood 1980; Dobson & Jones 1985; Liberg & Schantz 1985). The best explanation proposed so far for such a strong sex bias is inbreeding avoidance.

In mammals it is generally assumed that the costs of dispersal are higher for females than for males, because loss of a range or allies affects female reproductive success more than male reproductive success, explaining the prevalence of male-biased dispersal (Waser et al., 1986; Pusey & Packer 1987a; Clutton-Brock 1989a). Female dispersal, however, is found in some mammal species and a considerable number of primate species (Moore 1984; Clutton-Brock 1989a; Strier 1994).

Costs and benefits of dispersal

Different costs and benefits of female philopatry and dispersal can be distinguished. Females in gregarious animals such as diurnal primates can remain in their natal range or in their natal group.