The social organization for each type of great ape is distinct, and has evolved in response to a variety of factors such as territory defense, competition for mates, and food availability. The social system that emerges to balance the costs and benefits of these factors maximizes the potential for the reproductive success of each individual and the survival of the group. Clear distinctions between social systems are evident when comparing gorillas, orangutans, chimpanzees, and bonobos. While generalizations about each of these systems can be used accurately, it is also important to remember that a normal range of behavior exists, and variation may occur in response to different environmental conditions or pressures.

The members of the genus Gorilla demonstrate flexibility in their social organization. Most commonly, gorillas are found in groups that consist of one dominant adult male, several adult females, and their offspring. Gorillas travel within a specific home range, but do not defend an exclusive territory. Group size varies, and can range from a total of three or four individuals to more than 30, although the number of individuals in most groups usually falls between these extremes. In these polygamous social situations, the dominant male is referred to as the "silverback," a title that refers to the normal change in hair color that occurs as males mature. These males occupy the highest ranked position in their group, which generally allows them exclusive breeding access to the females. Silverbacks lead the group as they travel and forage, but their most important function is to protect the group against attacks by rival males, which commonly involve at tempts at infanticide. This classic model of gorilla social life may lead to the inaccurate assumption that the largest, strongest male simply controls the females in the group through brute strength. While male size and associated strength may contribute to social rank in general, the relationship between an alpha male and a group of females is mutually beneficial. At a minimum, in a polygamous social system, females provide the male with an opportunity to reproduce, and the male provides safety and security for the females and their offspring. Males demonstrate their capability to function as the leader of a group through a combination of social finesse, paternal attention to offspring, physical vigor, and effectiveness in repelling rival males. Based largely on the behavior of the silverback, females choose to remain in his group, or to emigrate into the group of another adult male. Instances of females living alone, or social groups composed only of females, are unknown for gorillas.

Male gorillas, by contrast, demonstrate a number of different social strategies. As they reach adolescence, males may abandon their birth group, and begin to travel alone. During this phase of their life, these individuals avoid direct encounters with silverbacks, but attempt to attract females in order to create their own group. In some instances, lone males congregate with each other, and form bachelor groups that travel and forage together. These associations are probably the least stable of all gorilla social groupings, and have a higher rate of change over time. Much more commonly, males pursue a distinct strategy, and simply remain in their birth group into adulthood. Silverbacks may be remarkably tolerant of these younger males, termed "blackbacks," who are usually their sons or brothers. These young adult males are subordinate in rank to the silverback, and may be dominated by adult females as well. Their primary social role is to provide additional protection and vigilance for the females and infants, who are also likely to be related to them. Gorilla groups with more than one adult male can be remarkably stable, and persist for many years. As time passes, the group may even have two silver-backs holding the highest ranked positions and sharing leadership roles. As these groups become very large over time, primarily as a result of births and female immigrants, it becomes increasingly likely that a fission will occur. When this rare event happens, females follow their preferred male, establishing the core membership for a traditional, polygamous grouping. The male-female bond provides the foundation for all of gorilla society.

Unlike gorillas, the social behavior of orangutans is centered almost entirely on individuals acting independently, and there are no long-term bonds between adults. Historically, orangutans have been described as "the solitary ape." While it is true that they are the least social of all the great apes, it is an exaggeration to suggest that they are truly solitary. Adults interact infrequently, compared to the rates of association observed for the other great apes, and it is more accurate to describe orangutans as living in an extended social system with individuals dispersed over large home ranges. Interactions between adult males are usually volatile, and contact between them involves threats or aggression. Adult females can be more tolerant of each other, and may even forage for fruits in the same tree. But, this level of association is the exception rather than the norm, and can vary depending on individual personality. Overall, there is no evidence that females form close bonds or provide each other with any form of social support. However, these behavioral traits are clearly obvious in the relationships that mothers form with their offspring, to whom they show great devotion and affection. Mothers and offspring may travel together for 7-8 years, at which time these young adolescents begin to move away and establish their own home range.

The unique social structure exhibited by orangutans is directly linked to the quantity and quality of food that they need to sustain their big bodies. As the largest arboreal species on Earth, these great apes require a sizable amount of food each day. Fruit is their most preferred choice, although its availability can be inconsistent. As a result, orangutans distribute themselves in such a way as to maximize their ability to find sufficient amounts. Given the impressive amount of fruit that one orangutan can consume in a day, two or more adults regularly traveling together could easily exhaust the quantity that is found within their home range. The availability of fruit is the major factor that limits the number of orangutans that can simultaneously occupy any portion of the forest. Orangutan social behavior does have some flexibility when this ecological factor fluctuates. If fruit becomes super abundant in a specific area of the forest, social tensions may ease. Adult females become more tolerant of each other, and may form a small aggregation as they forage independently. This same scenario does not apply to adult males.

In order to meet their nutritional needs and reduce social tension, female orangutans normally occupy large home ranges. Individuals do not defend entire territories, but they may certainly compete for specific forage sites, and attempt to repel rivals. In general, female home ranges regularly overlap although interactions between these females are infrequent. The situation for males is similar, although magnified due to their higher levels of intolerance for each other. Adult males may occupy home ranges that are three or four times bigger than those used by females, and they appear to be less stable. Males may suddenly move from their usual range, and travel far into the forest with no apparent explanation. An area in the forest may have a male that can be considered a resident, although transient males may also occur. The resident male may or may not be dominant to the transient males. Adult males regularly produce a "long call" vocalization that is estimated to travel as far as 0.6 mi (1 km) through the forest. This booming call advertises the presence and location of a specific male, and it is presumed to have a variety of functions. The long call allows males to space themselves throughout the forest. Males can choose to avoid or initiate an aggressive interaction when they locate another male. The call also serves as an advertisement to adult females, who also choose to approach or move away from a male who could be a potential mate. Females do have a number of vocalizations, but nothing that resembles the imposing long call produced by the males.

Relationships between adult males also play a pivotal role within chimpanzee society, but one in which the primary emphasis is on coalitions and mutual support within a commu-

Chimpanzees (Pan troglodytes) wade through a stream. (Photo by K & K Ammann. Bruce Coleman, Inc. Reproduced by permission.)

nity rather than competition and avoidance. Chimpanzees live in a highly complex social system described as a "fusion-fission" society. In this dynamic social setting, individuals within a community freely intermingle with all others. Communities may have dozens of members depending on local ecological conditions, and the smaller groupings that regularly form within a community are called parties.

Chimpanzee parties form and re-form on a continual basis, reflecting the needs and preferences of the individuals within a community, and the size and composition of parties are almost never the same two days in a row. Depending on the situation, party size can range from five or less, to more than half of the total community. A number of factors are involved, although the most influential of these is the local food supply, specifically correlated with the amount of fruit that is available in an area. An increase in food promotes larger parties, while a decrease encourages fewer individuals to congregate. Hunting and meat-eating are associated with relatively large parties that stay together for longer than average periods of time. In addition to food, opportunities for sexual interaction also play a very important role in the formation of parties. Ovulating females attract significant amounts of attention from males, creating a very charged social situation. Other important factors that have an effect are the demographics within the community, such as the total number of males, ovulating females, and mothers with infants. Dangers from predation also may be involved, since chimpanzees are certainly at risk from leopards, and lions in some cases. However, this link has yet to be thoroughly investigated and firmly established. In general, the size, composition, longevity, and duration of each party are influenced by a number of factors that must be balanced against the desires and social goals of each individual chimpanzee.

Parties can be grouped into several specific types, with each promoting a different social function. Male chimpanzees generally prefer each other's company, and regularly form all male parties. Coalitions between males are very common, although they vary over time and change when it is politically advantageous. Male parties are one way that males can advertise and strengthen their coalitions. Males that share a strong bond may

An eastern lowland gorilla (Gorilla beringei graueri) family. (Photo by Eric & David Hosking/Photo Researchers, Inc. Reproduced by permission.)

also groom each other, share food, and provide support during threatening or aggressive encounters with other individuals. Male parties also patrol the peripheral areas of their home range, keeping track of neighboring communities. Although chimpanzees do not maintain well-defined territories, they will engage in attacks on parties from outside of their community. When males are on patrol, they appear very cautious, and move silently as they travel. They are highly vigilant, and clearly uneasy while outside of their normal home range. In a number of cases, male patrols appear to have targeted and planned attacks on individuals from other communities. Many of these attacks have been fatal.

Females form the core for other types of parties, which also usually involve their offspring. A mother and her dependent young may travel as a family, with no other individuals in attendance. "Nursery parties" also may form, in which multiple adult females with infants and juveniles join together. On occasion, adolescent or adult females with no offspring may join this congregation as well. Ovulating females draw the immediate attention of adult males, and easily disrupt all male groupings. Parties may form in these situations, con sisting of one or more sexually receptive females as well as multiple adult males. A single male and female may form a consortship while she is fertile, distancing themselves from all other members of the community. Mixed sex parties are also apparent in non-sexual situations, such as when many individuals forage on an abundant source of food. The size of these groupings varies, but may include a large percentage of the total community. Lastly, individual chimpanzees may simply prefer to spend time traveling alone, forming a party of one.

These complex and highly flexible social behaviors demonstrate that chimpanzees frequently make decisions that are calculated to increase personal gain. In addition to adjusting party size and composition based on food availability, chimpanzees pursue their personal social agendas as well. These may be associated with sexual behavior, gaining status within the hierarchy of the community, or simply finding ways to reduce stressful interactions with other individuals. Chimpanzees live in an intense social environment, where dominant and subordinate rankings are regularly reinforced. Adult males are generally dominant within the community, and maintain their status using a variety of techniques, including strategy, alliances, and aggression. Changes in party composition that reflect preferences in social partners demonstrate one important way that males, in particular, may relieve tension and stress. These behaviors also provide evidence of the sophisticated cognitive skills that chimpanzees must possess to manage the intricacies that form the basis of their society.

Bonobos also live in a complex and dynamic social environment, with some behaviors that are similar to those exhibited by chimpanzees, and others that are completely distinct among the great apes. Like chimpanzees, bonobos are gregarious and live in fusion-fission communities. However, the bonds between males are weaker by comparison, and relationships between females are much more influential in their society. Adult male bonobos have a social role that is largely defined through their mother, and the closest male-male relationships are usually between maternal brothers. Males generally stay in their birth group throughout their adulthood, and their social rank is largely determined by the status of their mother. Unlike chimpanzee males, who may form coalitions with any other male in their community, kinship is a very influential factor in the formation of male-male relationships for bonobos.

As female bonobos mature, the bond with their mother weakens, and they normally emigrate from their birth group. During this time, young females who have yet to reproduce may move between communities and begin the process of forming alliances with other, unrelated females. As these strong bonds develop over time, they form the core of the bonobo community. While a female hierarchy exists, it is less obvious than the dominance structure found among chimpanzees, and their age and residency status appear to be the most important factors that determine their rank. Females are clearly capable of monopolizing food resources, and may singly or cooperatively dominate males. While both males and females exhibit aggressive behavior, male dominance over females is uncommon. The unified force that females represent in bonobo society is explained, at least in part, by the development of strong and persistent social bonds between unrelated individuals, a behavioral strategy not seen among males.

Bonobo behavior at the party and community level has some similarities and differences when compared to chimpanzees. The community members of both species regularly divide themselves into parties, and the total number of individuals in each party can be correlated with the amount of food that is available. As is the case for chimpanzees, abundance promotes larger congregations of bonobos. In places where there is a risk of predation, especially from humans, bonobo party size is reported to increase. Overall, party size ranges from a minimum of 2-6 individuals to a maximum of 11 or more. Communication between party members also appears to vary by species. Chimpanzees effectively use loud vocalizations, as well as drumming on tree buttresses, to exchange information between parties. Using these methods, field researchers report that the community may stay in contact even while divided into multiple parties. It is also speculated that drumming may exchange specific information about the direction in which individuals travel, as well as when they

The western gorilla (Gorilla gorilla) is an herbivore. (Photo by Mark Newman/Photo Researchers, Inc. Reproduced by permission.)

stop to rest. While bonobos may communicate between parties, their vocalizations are less effective for long distance exchange. It appears that most of their efforts are focused on communicating with other members of the same party. Bonobo parties also are more stable than those of chimpanzees, with membership changing less frequently. Most are usually a mixture of males and females rather than a nursery, family, or other configuration. Female bonobos are not reported to travel alone, although males occasionally exhibit this behavior. Unlike chimpanzees, bonobo parties are much more likely to fuse back together as a community each night. An additional distinction is that bonobo communities are more flexible in their behavior towards each other. While aggressive interactions have been documented, peaceful interactions between communities have also been seen.

The most notable behavioral difference between bonobos and all of the other great apes is their reliance on sexual behavior as a means for promoting social affiliation. In addition to sexual behavior between adults that occurs for reproduction, all members of a community regularly engage in pleasurable, non-reproductive sexual interactions that function as a way of offering appeasement, reinforcing bonds between individuals, and easing social tensions. Erotic behaviors may occur between a male and a female, as well as male-male and female-female pairings. These species-typical interactions begin well before bonobos are capable of reproduction, and continue throughout adulthood. However, specific associations may be avoided, such as sexual behavior between mothers and their sons.

More than any of the other great apes, bonobos appear to live in a society with a greater emphasis on reciprocity. Sex is used as a form of social currency that can be offered to defuse tension between individuals, promote reconciliation, encourage alliances, and ease competition. This unique social strategy clearly assists migrating females as they move between communities and form bonds with resident females. It also may account for the high frequency with which males and females associate and form mixed sex parties. Aggression is clearly not absent from bonobo society, erotic behavior simply allows it to be reduced in frequency and severity. In some instances, expressions of reciprocity may be very literal. While bonobos are known to have high rates of food-sharing be-

An adult male orangutan (Pongo pygmaeus) at Bukit Lawong, north Sumatra. (Photo by B. G. Thomson/Photo Researchers, Inc. Reproduced by permission.)

havior in general, they may specifically offer preferred foods to another individual in a direct exchange for sex.

While it is clearly true that bonobos engage in social behaviors that are not seen among the other great apes, all of the great apes are faced with the same survival challenges. Reproduction, foraging, protection from predators, and territory defense are the basic concerns that shape all social systems. For example, the comparatively low rates of association between orangutans are associated with a limited availability of preferred foods. In contrast, gorillas may forage in large groups, with little competition, due to the abundance of the vegetation that they consume. The behaviors that characterize each great ape society have evolved as a balance between the costs and benefits of living socially. This dynamic process may have been a primary factor that influenced the emergence of complex cognitive skills. Individuals with greater mental flexibility are able to out-compete rivals, giving them an overall advantage in surviving and reproducing. This benefit favored the development of mental abilities that allowed individuals to think strategically, expressed by such behaviors as coalitions, alliances, cooperation, and even deception. The sophisticated minds that are commonly and accurately associated with great apes are assumed to have their basis in the complicated social behaviors that are used to promote survival and reproduction.

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