Competition and group size in Thomas's langurs ( Presbytis thomasi ): the folivore paradox revisited

Abstract

Among primates, group size is highly variable. The standard ecological model assumes that better predation avoidance as group size increases favours living in larger groups, whereas increased travel costs and reduced net food intake due to within-group competition for resources set the upper limit. Folivorous primates, however, tend to defy this generalisation in that some live in small groups despite low costs of feeding competition. To resolve this 'folivore paradox', it has been suggested that folivore group size is limited by social factors such as male harassment or infanticide, or that females can disperse more easily and thus maintain group size near optimum levels. In this paper, we examine the effects of group size on home range size, day-journey length, activity budget and diet in wild Thomas's langurs (Presbytis thomasi), which live in one-male multi-female groups with a limited life cycle. We examined only data from the stable middle tenure phase when factors such as the strength of the breeding male or the way in which groups were formed did not influence ranging and activities. During this phase, group size affected day-journey length and home range size, and had a minor effect on diet, but did not influence time spent feeding or resting, allogrooming or birth rates. Hence the upper limit to group size during the middle tenure phase in Thomas's langurs is not set by feeding competition. The folivore paradox is not due to frequent female dispersal in Thomas's langurs. The timing of female dispersal is not as expected if it serves to keep group sizes near the ecological optimum, and groups seem to be below this optimum. Instead, female reproductive success is presumably maximised in small to mid-sized groups because larger groups show a clear trend to experience higher risk of take-over, often accompanied by infanticide. Because females can redistribute themselves among nearby groups when groups reorganise each time a new male starts up a new group, females can keep the group small. Thus, a social factor, risk of infanticide, seems to provide the selective advantage to small group size in Thomas's langurs.

Socioecological influences on the dispersal of female mountain gorillas—evidence of a second folivore paradox

“…Similarly, the socioecological models do not predict different transfer preferences for nulliparous versus parous females (Wrangham 1980;van Schaik 1989;Sterck et al 1997;Steenbeek and van Schaik 2001). Therefore, to maximize sample sizes for analyses of immigration rates and transfer destinations, we included all categories of transferring females, regardless of whether they were nulliparous or parous and natal or nonnatal (e.g., as in Watts 2000; Stokes et al 2003).…”

“…Nonetheless, some folivores live in groups that are considered smaller than necessary to avoid within-group scramble competition, despite a substantial risk of predation (Janson and Goldsmith 1995). This apparent inconsistency has been known as the "folivore paradox" (Steenbeek and van Schaik 2001), and we will call it the "original" folivore paradox.…”

Habitat Use by White-Thighed Colobus in the Kikélé Sacred Forest: Activity Budget, Feeding Ecology and Selection of Sleeping Trees

“…For this reason colobine monkeys would be expected to form large groups, which is not always the case. This is the "folivore paradox" (Steenbeek and van Schaik 2001;Snaith 2008).…”

Mechanisms determining relationships between feeding group size and foraging success in food patch use by Japanese macaques (Macaca fuscata)

“…Generally, sharing food resources with other group members causes within-group feeding competition, which reduces the foraging success of individuals within the group (Janson, 1988;Janson & van Schaik, 1988). Then, the within-group feeding competition increases with increasing group size (Wrangham, 1980;van Schaik, 1983;Janson & Goldsmith, 1995;Steenbeek & van Schaik, 2001). However, individuals in a group have some benefits in terms of the efficient discovery of food patches (Clark & Mangel, 1986) and protection from predators (Terborgh, 1983;van Schaik & van Hooff, 1983).…”

“…Foraging success is also reduced through direct competition by aggressive interactions and dominance rank (contest competition), and through indirect competition by the sharing of food resources with other group members (scramble competition; Janson & van Schaik, 1988). Both types of feeding competition increase with group size (Wrangham, 1980;van Schaik, 1983;Janson, 1985;Janson & Goldsmith, 1995;Steenbeek & van Schaik, 2001;Vogel & Janson, 2007). …”