SOCIAL ORGANIZATION

In Chapters 7 and 8, Alcock discusses the comparative approach to the study of behavior. It can be very effective in determining which factors most strongly influence behavior. Interactions of numerous ecological factors and the evolutionary and ecological history of a species determine patterns of social organization. A comparative approach is generally used to construct scenarios of the evolution of social organization within taxa. We can use the comparative approach to examine the ecological factors that have shaped the evolution of primate social systems.

PRIMATE SOCIAL SYSTEMS

There has been extensive comparative analysis of primate social systems (e.g., Smuts, B. B., et al. 1987. Primate Societies. Chicago). Primates exhibit a broad spectrum of mating systems, social organization, and group size (see Fig. 1 on handout). Predation apparently plays a major role in patterns of sociality in primates. Small forest-dwelling primates rely on vegetative cover, camouflage, and inconspicuous behavior to avoid predators, and they are solitary. Social primates often rely on mutual vigilance to detect predators; many species have specialized alarm calls to signal danger. Male baboons and patas monkeys often act as sentinels while other troop members forage and either defend the group against predators (baboons) or distract predators and lead them away from the rest of the troop (patas monkeys).

The abundance and dispersion patterns of food are also important. Large groups can form only if the area within one individualís foraging range can support additional individuals, e.g., if food is patchily distributed and patches are ephemeral, several individuals may be able to coexist within an area. If food patches can support more than one individual, then the degree of grouping probably depends on the costs and benefits of being in groups in relation to other factors, such as predation.

Food and predation may influence social organization in similar ways or in opposing directions. For example, if the food resources can be defended effectively by a single individual that is small, cryptic and vulnerable to predators, both food and predation would favor solitary living on exclusive territories. If food is patchy and patches are ephemeral, individuals in groups may be less vulnerable to predation than solitary individuals and larger groups may exclude smaller groups from particular patches of resources. On the other hand, competition for food within groups may favor smaller group size in species that rely on mutual vigilance or defense to reduce predation -- in this case food and predation exert opposing pressures on group size.

Recent scenarios have considered females and males separately. A femaleís fitness is based primarily on her ability to secure resources and minimize risks of predation for herself and her dependent young. A maleís fitness generally is determined by the number of successful matings, particularly on species with little or no paternal care. We can categorize primate social organization into several types. Recent scenarios have considered females and males separately. A femaleís fitness is based primarily on her ability to secure resources and minimize risks of predation for herself and her dependent young. A maleís fitness generally is determined by the number of successful matings, particularly on species with little or no paternal care. We can categorize primate social organization into several types.

A. Nocturnal prosimians

Nocturnal prosimians are small forest-dwelling, insectivorous primates. Individual females are generally territorial, defending small territories that meet their energetic requirements. Many small forest-dwelling primates are nocturnal and avoid predation by cryptic behavior, which also would favor a solitary lifestyle. The small forest-dwelling, insectivorous primates avoid predators by quietly foraging alone at night. Most nocturnal predators hunt by sound (e.g., owls); a group of prosimians would make more noise and would, therefore, likely attract predators more frequently. Therefore, being in a group would be disadvantageous in relation to predation. Typically, foraging females forage alone and arenít accompanied even by their dependent young.

In this category male territories are as large or larger than those of females. Males defend boundaries of female territories against other males and thus have exclusive access to 1 or (rarely) more females within their territories.

Tree shrews (Tupaia glis) are prosimians. Their mating system can be described as facultative monogamy. Facultative monogamy, also exhibited by marmosets and tamarins, results when a species exists at very low densities with male and females being spaced such that only a single member is available for mating. Males defend territories, primarily by scent marking and occasionally chases, against other males. Females have non-overlapping home ranges, i.e., territories, and a femaleís territory generally is entirely within that of a male. Generally, one male and one female share a territory, each individual defending it against intruders of the same sex. Males with larger territories tend to share their territories with two females and thus are bigamous. Kawamichi and Kawamichi (1979. Anim. Behav. 27:381-393) describe tree shrews as ìfree-ranging pairsî and suggest that this type of social system may be widespread among prosimian primates.

B. Frugivorous Great Apes and Spider Monkeys (see handout, Figs. 16.1 and 16.2)

Among the great apes, orangutans are solitary, and single males form brief consortships with estrous females. Orangs feed on fruits, which are rare and discrete. In contrast, troops of gorillas are composed of 1-2 males and several females; a dominant male monopolizes the group. Gorillas feed primarily on understory herbs which are evenly distributed and show little local variability in quality. Males protect members of the group from predators.

Females of species in this category are typically solitary foragers, feeding primarily on fruits. In general, they have highly specialized diets, typically with low quantities of insects and leaves in the diet -- they generally cannot tolerate secondary compounds in their diet. Females typically allow juveniles to follow them, even after weaning. Females have large home ranges with overlapping core areas. There is little predation, and home ranges are too large to be defensible, i.e., for regular visits to boundaries. Group size increases temporally at rich food sources, but females have no enduring social ties with one another.

Chimpanzees seem to have a much more elaborate social organization. Chimps live in ìcommunitiesî of about 20-100 individuals who share a common home range. They feed primarily on tree-ripened fruit. Within chimpanzee groups, however, there is little association among females. Females are basically solitary, forming ìpassive aggregationsî when feeding in the same fruiting trees. Except for mother-daughter cooperation, there is little evidence of female-female cooperation. Females typically emigrate to other communities before maturity. An interesting sidelight: infanticide has been noted on 3 occasions; all instances were by the same mother-daughter pair when the daughter was beginning to establish her core area. Males tend to stay in the natal community, and males cooperate in defending home range boundaries.

Bonobos (ìpygmy chimpsî -- a misnomer) differ from chimps in that females tend to form much larger, more permanent feeding associations. Females establish and maintain social bonds through mutual grooming and sexual stimulation (see Parish, A. R., 1993. Ethol. Sociobiol. 15:157-179). Bonobos inhabit continuous evergreen broadleaf forests where there is a well-developed herbaceous understory. The understory herbs are a staple and are important food sources when fruit is unavailable. Females groom each other and commonly share food. Within communities males and females form more stable subgroups than chimps, but male-male affiliations are weaker than those of chimps. These differences seem to be related to the bonoboís use of understory herbs, which are evenly distributed and canít be defended economically. Thus, there is reduced feeding competition both within and between groups, facilitating within-group affiliations among females and minimizing the need for male-male alliances to defend the community's home range.

Males of different species in this group have varying strategies. Male orangutans defend territories against other males, including territories of 1 or more females within their territory. In contrast, male chimps form large coalitions and defend a large territory with numerous females. Large groups dominate small ones at territory boundaries, and territory size is positively correlated with group size. Females tend to avoid boundary areas, indicating that their home range is partially influenced by patterns of intergroup aggression by males.

C. Monogamous or polyandrous groups

About 22% of primate species have this type of social organization. The gibbon is a familiar example. Females are highly aggressive toward each other and are conspicuous participants in encounters at territorial boundaries. Exclusive access to defensible resources seems to be the key to female-female interactions. Males travel with females, generally directing their aggression at males during territory encounters. In some species male provisioning of young may also enhance the tendency towards monogamy. Gibbons live in nuclear family, territorial groups and exhibit obligate monogamy - it appears that the female cannot raise a litter without the aid of another adult, and the degree of paternal care is too great for the male to be bigamous. In some species 2 or more males share a territory and the female, resulting in a monogamous system.

D. Multifemale Groups (see handout, Figs. 13.1 and 10.1)

About half of primate species have this type of social organization and all are diurnal. Most of the species in this group show ìfemale-bonding,î i.e., long-term associations among females in their natal home ranges. Some species are characterized instead by ìfemale transferî in which females leave their natal group, usually individually, either due to attraction by a neighboring adult male (e.g., gorilla) or male herding (hamadryas baboon). Females may live in groups to effectively compete with neighboring females and/or decrease vulnerability to predators. Various authors favor one hypothesis or another, and there are few data to evaluate either idea. Perhaps predation is decreased but competition for resources within the group increases as group size increases. When groups meet, larger groups do dominate smaller groups but the frequency and significance of such encounters is not well understood.

For the diurnal primates, John Terborgh, following others, has suggested that group size is a compromise between enhanced vigilance for predators and increased competition for food with increasing group size. Virtually all authors agree that the mating system of monkeys is a result of the grouping tendencies of females in response to ecological conditions. If female groups are small, a single adult male may be able to maintain exclusive access, resulting in a uni-male troop (a mixed-sex group with only one adult male). As a group size of females increases, exclusive access by a single male becomes more difficult for that male and multi-male troops (e.g. olive baboons, savannah baboons) typically result (see handout). Within multi-male troops there are typically dominance hierarchies; reproductive success of males seems to be positively correlated with dominance, but coalitions of subordinate males may be effective in neutralizing the effects of dominance on access to mates.

Terrestrial diurnal primates tend to benefit from sociality because vulnerability to predation is high. Hamadryas baboons inhabit arid regions of Africa. At night several bands congregate as large troops and share the sleeping cliffs. A band travels to and from foraging areas as a unit and then fragments into small one-male units comprised of one adult male, one or more adult females and their immature offspring. Males lead movements throughout the day. Bands aggregate at night because sleeping cliffs are sparsely distributed within range of the species but are large enough to accommodate many individuals. Bands separate during the day because food is sparsely distributed - hamadryas baboons eat various portions of acacia trees, and optimal group size to maximize individual feeding efficiency is 2-3 individuals/acacia. Traveling in a band, however, reduces the likelihood of predation. The one-male units are the reproductive units, i.e., the mating system is harem (female defense) polygyny.

Savannah dwelling baboons (e.g. olive baboons) live in multi-male social units (troops) of similar size to a hamadryas baboon band. Each Savannah baboon troop has its own home range. Home ranges do overlap, but troops are usually antagonistic when they meet. Savannah baboons eat a wide variety of foods and depend on vigilance and group defense to avoid predation. The mating system is both polygynous and polyandrous; a mating pair forms only a brief consort.

Gelada baboons (Theropithecus gelada) inhabit high altitude grasslands in northern Ethiopia. They are grazers and inhabit treeless areas, using gorges for sleeping sites and refuges from predators. Social organization in hierarchical, consisting of three tiers: (1) one-male units (single breeding male, 1-10 breeding females, and their dependent young), and all-male units, (2) bands, those units that share a common home range (the average size of bands is 115 individuals, and average number of units/band is about 10 (mostly one-male units)), and (3) groups of bands that share the sleeping cliffs. ìHerdsî are feeding aggregations which usually consist of units from the same bands. The females form the core of gelada society; females of the same unit are closely related and form foraging coalitions together. The herds are the foraging units, and herd size is positively related to the exposure to predators and not closely related to characteristics of food availability.

Overview

As we progress through the series of movies on ungulate social organization, you'll see many parallels between the primates and ungulates. In both groups, small forest-dwelling forms tend to be solitary and monogamous, relying on crypticity to avoid predation. In more open habitats females form larger groups both in ungulates and primates. As we'll see, however, male ungulates do not form permanent associations with female groups, like male baboons do. We'll leave "why" as an open question for awhile.

Does understanding of the diversity and patterns in primate social organization provide a helpful framework for understanding social organization of humans? I think comparisons are of limited use for two reasons. First, our closest relatives, the chimps and bonobos inhabit similar habitats but differ strongly in sociality, particularly of females. Secondly, as you'll see in reading Chapters 7 and 8, ecology as well as evolutionary ancestry is important in shaping social organization. Because humans evolved as hunter-gatherers and are much more carnivorous than any of the great apes, we should probably look to the social carnivores as well as the great apes for insights about the evolution of human social organization. George Schaller (1969. The relevance of carnivore behavior to the study of early hominids. Southwestern J. Anthropology 25:307-341) was the first to emphasize this point.

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