Social Spacing

 Different groups of colobus monkeys exhibit different systems of social spacing, primarily in response to environmental factors and predation pressure. The red colobus monkey is organized in multimale-multifemale troops living on overlapping home ranges. These groups may range in size from 8-80 individuals, with up to 300 individuals occupying a square km. at any given time (Struhsaker, 1975). These home ranges vary in size depending on group size. One reason for this social organization is that food in the lush rainforests they live in is superabundant, making a territory useless to defend. The second major reason for this social organization is the high predation pressure the red colobus monkey suffers from chimpanzees. Chimpanzee predation may account for 18-40% of the red colobus's yearly mortality, necessitating large groups for defense (Wrangham, 1990). The maintenance of these tight male-male bonds is reinforced by the fact that the groups are patrilineal, resulting in a high degree of relatedness among males within a group. Though foreign males may be accepted into a troop, the majority of males are genetically related.

The black colobus monkey and olive colobus monkey do not suffer from such high predation pressure, but they also exhibit multimale-multifemale groups living on home ranges. These groups tend to be much smaller, however, with black colobus groups ranging in size from 5-30 members, while the olive colobus live in groups of 8-15. The superabundance of food for both of these types of colobus monkeys is generally the reason for this social spacing. In certain environments, the black colobus monkey lives on home ranges due to the lack of legitimate territories, because food is highly clumped, but sporadically distributed, which would make any food clump too small to provide enough nourishment and any group of clumps too large of an area to defend (McKey, 1982).

Black-and-white colobus monkeys are generally reported as being highly territorial and living in one-male units with harems of 2-4 related females. The reason for territories in these groups is that black-and-white colobus monkeys tend to live in dryer climates in which seeds constitute a larger part of their diet than other colobus monkeys. The sacculated stomach of the colobus monkey allows it to break down the thick cellulose walls of these seeds, but this digestion requires a great deal of time spent resting. Consequently, it is valuable to defend a territory to limit the amount of daily traveling needing to be engaged in. Questions have been raised about whether the description of black-and-white colobus monkeys as territorial is apt. In one study, 16 of 18 groups observed were found to have more than one adult male living in a group, and the groups lived on home ranges (Von Hippel, 1996). The site of that study was the Kakamega Forest, Kenya, a continuous forest. Such a habitat, with its superabundant food, is more suitable for living in home ranges. Territorial one-male units, on the other hand, manifest themselves more often in narrow, riparian forests, which feature viable and defensible territories. In a literature review of black-and-white colobus research, Mulhern(1991) also noted that every study of black-and-white colobus social behavior included at least two observations of territories with more than one adult male.