Hominidae I

(Great apes)

Class Mammalia

Order Primates

Family Hominidae

Thumbnail description
The largest members of the order; sharing the physical features that characterize all primates, great apes are distinguished by their impressive array of mental abilities

Size
Combined head and body length ranges from 28–67 in (70–170 cm); 68–388 lb (31–175.2 kg)

Number of genera, species
3 genera; 6 species

Habitat
Forest, savanna, swamp

Conservation status
Critically Endangered: 1 species; Endangered: 5 species

Distribution
Equatorial Africa, Borneo, and Sumatra

Evolution and systematics

The great apes have traditionally been grouped in the family Pongidae, clearly distinguishing them from the prosimians, monkeys, lesser apes, and humans. Three genera and four species were usually recognized within this family, which included the orangutans (Pongo pygmaeus), gorillas (Gorilla gorilla), chimpanzees (Pan troglodytes), and bonobos (Pan paniscus). In the past, bonobos have been referred to as pygmy chimpanzees, a misnomer attributed to their gracile appearance. They were not recognized as a distinct species until 1929.

Like any branch of science, taxonomic classification involves the search for greater comprehension. Incremental progress relies on debate among colleagues, constructive criticism, and innovative methods for measuring the relationship between species, such as DNA and chromosomal analyses. Based on these factors, Pongidae has been replaced by the family Hominidae, which more accurately describes the evolutionary relationship between all of the great apes and humans.

The family Hominidae consists of four genera and seven species. Orangutans, the only Asian great apes, are divided into two species based on their geographically distinct ranges. Those from the island of Borneo are Pongo pygmaeus, while those from the island of Sumatra are Pongo abelii. The remaining members of the family Hominidae are all African in origin. Chimpanzees, Pan troglodytes, are divided into four subspecies, P. t. troglodytes, P. t. verus, P. t. vellerosus, and P. t. schweinfurthii. Bonobos, Pan paniscus, are the remaining species in the genus. There are two species of gorilla, western gorillas (Gorilla gorilla), and eastern gorillas (Gorilla beringei). Two subspecies of eastern gorillas exist, mountain gorillas (G. b. beringei) and eastern lowland gorillas (G. b. graueri). The final species in the family Hominidae is Homo sapiens sapiens, otherwise known as modern humans. This species is discussed in a separate chapter.

Within the family Hominidae, two subfamilies further clarify the evolutionary and associated geographical origins of the species. The Asian orangutans are members of the subfamily Ponginae, while all of the African species are members of the subfamily Homininae. Based on comparisons of DNA from each species in both subfamilies, it is clear that orangutans diverged from the members of Homininae approximately 14 million years ago (mya). Within Homininae, gorillas split from Pan and Homo approximately 7 mya. Humans were the next to diverge approximately 6 mya. The final group to emerge was Pan, with bonobos and chimpanzees diverging approximately 3 mya.

Although commonly misunderstood, humans are not the "end-product" of great ape evolution. Humans did not evolve from chimpanzees (or any other living species of great ape), and the other species of great ape are not evolving into humans. Each species within Hominidae has evolved on its own distinct pathway, although all share a common ancestry.

Physical characteristics

Great apes are large when compared to other primates, although a range of sizes and body forms exist within the

family. Without question, gorillas are the largest of the great apes. Adult males may exceed 350 lb (158 kg), while adult females are generally about half that size. Bonobos are the most slender great apes, having very long limbs and a gracile appearance. Adult males average less than 100 lb (45 kg) and adult females are usually about 20% smaller.

All of the great apes share an overall similarity in body type, having arms that are longer than their legs. When standing with both hands and both feet on the ground, their limb proportions slope their back downwards, and their head is positioned as the highest point of their body. While this is their most common posture, all are also able to stand bipedally and walk upright for short distances. Whether on the ground or in the trees, great apes are able to move their arms with maximum rotation at the shoulder joint, unlike most other species of primates that have a more limited degree of flexibility. This superior range of motion allows great apes to suspend themselves with their arms, and move using a style of locomotion referred to as semi-brachiation. The gibbons, genus Hylobates, are the only true brachiators.

The African species (gorillas, chimpanzees, and bonobos) are primarily terrestrial, although they do spend considerable amounts of time in the trees. When moving on the ground, they "knuckle-walk," meaning that they support their body weight on the knuckles of their hands, and the soles of their feet. Orangutans, the largest arboreal species on Earth, have a number of physical adaptations that make them uniquely suited for life in the trees. Spending most of their lives in the forest canopy, these apes have exceptionally long arms, and the majority of their strength is located in the upper body. Their fingers and toes are also elongated, allowing them to secure themselves with a vise-like grip. When moving high above the ground, orangutans use their hands and feet interchangeably in a style that is referred to as quadrumanous, literally translated as "four-hands." Unlike the African apes, orangutans descend from the trees infrequently. When moving on the ground, they do not knuckle-walk, but either hold their hands and feet open and flat as they move, or close them completely and support themselves on their fists.

The general appearance of the great apes varies by species. All of the African apes have dark hair, and orangutans are aptly called the "red ape" due to their rusty orange appearance. Gorillas and orangutans show extreme sexual dimorphism, with males reaching an adult size that is usually twice that of most females. In addition to their size, adult male gorillas are most easily recognized by their prominent sagittal crest, which becomes dome-shaped as they mature. Male orangutans develop very long hair, a beard, cheek pads, and an obvious throat sac, as they reach adulthood. These physical indications of sexual maturity all serve to intimidate rival males, as well as to attract potential mates. Male and female chimpanzees and bonobos lack these extreme differences in size and appearance, with males being approximately 20% larger than females. However, bonobos and chimpanzees are distinct in their appearance. The skin coloration on the faces of adult chimpanzees may be lighter than the very dark complexion of bonobos, who also have pinkish lips. Bonobos have longer hair on their head, with an obvious part in the middle. In terms of general appearance, chimpanzees are robust and muscular, while bonobos appear much slighter by comparison.

Distribution

The current distribution for gorillas, chimpanzees, and bonobos is limited primarily to the tropical forests of Africa. Bonobos have a very restricted range, occurring only in the Democratic Republic of the Congo (formerly known as Zaire), south of the Congo River. Chimpanzees range across the middle band of the continent, from Senegal in the west, to the Democratic Republic of the Congo near the center of Africa, to southern Sudan, Uganda, Burundi, and Tanzania in East Africa. Gorillas may be found as far west as Nigeria, Cameroon, and Gabon. Their range extends to the Central African Republic, and east into the Democratic Republic of the Congo, Rwanda, and Uganda.

Orangutans are the only species of great ape that exist outside of Africa. Wild populations exist only on the islands of Borneo and Sumatra. Due to massive loss of habitat, most orangutans exist in areas of forest that are threatened by human development.

Habitat

Great apes are generally found in forested areas that are located on, or near, the equator. However, behavioral flexibility allows for some variation in habitat utilization that may be seen among populations of the same species. Availability of appropriate foods is the primary factor that limits the number of individuals that may occupy any given area. The range of great ape habitat includes primary forest, bamboo forest, lowland swamp, grasslands, and woodland savanna.

Gorillas, chimpanzees, and bonobos utilize all levels of their habitat, but are primarily terrestrial. Each species is well adapted for moving and foraging in the trees, although the amount of time that each spends off the ground varies. Chimpanzees and bonobos, smaller and lighter than gorillas, utilize the trees throughout their lives. As male gorillas mature, their increased bulk prevents them from using the trees as easily, and they may spend the majority of their time on the ground. This difference is well illustrated by the fact that all great apes construct their night nests in the trees, except for adult male gorillas, which commonly build their sleeping nests on the ground.

Chimpanzees occur in very diverse habitats throughout their range. They may be found in primary or secondary forest, tropical rainforest, grasslands, or woodland savanna. Bonobos are not found outside of primary or secondary forest. Lowland gorillas occur in primary and secondary forest as well, but also utilize marshy habitats, such as areas known as bais (pronounced "buys"). Bais are large, natural clearings within the forest that are brightly lit and populated by plant species that grow in very wet areas. Gorillas, as well as other forest species, consume these preferred foods, sometimes wading waist high in water to collect handfuls of the succulent plants. Mountain gorillas live at higher altitudes than chimpanzees, bonobos, or lowland gorillas, existing in montane and bamboo forest. Unlike these other species, they have much longer and thicker hair to protect them from the colder temperatures.

Orangutans possess a number of physical and behavioral traits that make them particularly well adapted for life in the trees. They live in the canopies of rainforests, montane forests, and lowland swamps, and are the only great apes that are primarily arboreal. Their distinctive anatomy allows them to perform all of their most essential behaviors, such as traveling, foraging, and mating, while suspended in the treetops. Orangutans are fully capable of movement on the ground,

although it is awkward by comparison. This clear disadvantage restricts the range of habitats that they can successfully occupy when compared to the African apes.

Behavior

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 attempts 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 silverbacks 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 community

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

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, consisting 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

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 behavior

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.

Feeding ecology and diet

The foods that are eaten by the great apes generally include a wide variety of items such as fruits, assorted types of vegetation, bark, seeds, insects, and meat. However, great apes in certain habitats may have access to a more limited array of foods due to local ecological conditions, which affect the composition of their diet. For example, mountain gorillas (G. beringei) live at higher elevations than other great apes, where vegetation, rather than fruits, are the predominant foods. Their environment can be described as a giant salad bowl, filled with abundant amounts of leaves, shoots, pith, and vines, which are easily collected. Western gorillas (Gorilla gorilla), eastern gorillas (Gorilla beringei), and eastern lowland gorillas (G. b. graueri) all live at lower elevations that have a greater array of food types, including fruits, leading to more diverse diets. While many different types of food may be consumed, gorillas have not been seen to eat meat. In general, the food items in gorilla habitats are plentiful, and relatively easy to acquire, process, and consume. As a result, feeding competition between individuals is fairly low, and gorillas have no need for complex foraging techniques in order to meet their nutritional needs.

Bonobos also depend on vegetation as a staple in their diet, although they spend large amounts of their time foraging for fruit, which is more highly preferred. In general, competition for food is fairly low and large numbers of individuals may be seen feeding together. This is also illustrated by the fact that bonobos have high rates of food sharing with each other, a feature associated with their unique social system. Bonobos also eat a wide variety of non-plant foods, such as caterpillars, earthworms, and perhaps even shrimp found in shallow streams. Meat from prey such as squirrels and small antelopes is highly prized. These animals appear to be taken opportunistically rather than through active hunting. This limited food resource is not freely shared, and is likely to be dominated by adult females, who may totally exclude males when meat is being eaten. Although coveted, meat is estimated to make up a very small percentage of the normal diet for bonobos. Like gorillas, bonobos are able to successfully meet all of their dietary needs through foraging techniques that do not require any forms of tool-using behavior. The absence of tool-use by gorillas and bonobos in the wild can confidently be attributed to a lack of necessity, rather than lack of mental ability, since both easily demonstrate mastery of tools in captivity.

Compared with gorillas and bonobos, diet and food availability for orangutans exerts a much more restrictive influence. These large, arboreal great apes rely predominantly on fruit, such as figs (Ficus spp.) and durian (Durio spp.), which are patchily distributed both in space and time. Orangutans expend most of their foraging effort in finding, processing, and consuming fruit when it is available. As a result, access to this limited resource can be highly competitive, and limits the number of orangutans that can successfully forage in any area of the forest. When fruit is absent, a wide variety of other foods, such as leaves, seeds, bark, and insects, are consumed. All of these are less preferred and incite less competition. On rare occasions orangutans have been seen to eat animal prey such as a loris (Loris spp.), although these events are described as opportunistic rather than active hunting.

One of the ways in which orangutans meet their nutritional needs is through the use of tools to extract otherwise unavailable foods. Two specific forms of tool-manufacture and tool-use have been well studied in the wild. In the first, orangutans have been seen to construct and use probing tools that are inserted into tree trunks in order to remove insects, larvae, and honey. These tools were used to break open and probe the nest inside a tree hole, as well as for extracting the honey and insect prey. These nest holes were located far above the ground, and in most cases the orangutans climbed into position and utilized the tool while it was held with their teeth. In the second case, these apes made and used a short, blunt tool to remove the calorie-rich seeds encased inside of the spiny (Neesia sp.) fruit. These fruits are very hard, with imposing spikes on their skin. As they ripen, slits open in the sides that allow the seeds to fall onto the ground. Since the orangutans are unable to simply open the fruit and remove the seeds, they use a tool to scrape the seeds from inside the fruit, avoiding the spines on the outside as well as irritating fibers that are present on the interior. Using this strategy, they are able to exploit a calorie rich food that would be otherwise unavailable. Although these fruits are available to a number of populations, this behavior is only present at specific sites. This strongly suggests that the presence of this particular form of tool use is the result of innovation and inter-generational learning, the basic elements of culture.

Among the great apes, chimpanzees utilize the widest variety of potential foods, made possible by the most diverse collection of behaviors related to food gathering, extraction, processing, and consumption for any species except humans.

Numerous types of food are eaten, including leaves, bark, sap, flowers, nuts, insects, meat, and fruit, which is highly preferred. In the wild, chimpanzees demonstrate superior overall ability in the manufacture and use of tools, especially tools used in food acquisition. The modification and use of slender grasses and stems by chimpanzees to "fish" termites out of their mounds is the classic example that shattered the long held assumption that humans were the only tool-maker and tool-user. While the catalog of tool-using behaviors demonstrated by chimpanzees is simply too large to list here, nut cracking exemplifies the importance of food-related tool-use.

Nut cracking involves the skilled use of a hammer and anvil to carefully open hard shelled nuts without damaging the food inside. This ability is learned over time through a combination of observing other chimpanzees, practice, and in some instances, direct assistance from another individual. This behavior does not occur in all populations of chimpanzees, even in areas where nuts and potential tools exist. Therefore, it is assumed that innovation and social learning are necessary for this behavior to become fixed within a population. The chimpanzee communities that do know how to crack nuts gain an important source of fat, sugar, protein, and amino acids. When nuts are in season, an individual chimpanzee may crack nearly 300 each day, which supplies most of the calories and protein that the individual needs.

Hammers and anvils may be stones, pieces of wood, or a combination of a stone hammer and wood anvil. Hardened, exposed tree roots are also used as anvils, creating a fixed tool-using site. Chimpanzees may transport their tools to where nuts are found, or they may carry nuts to where tools can be found. In a small number of instances, chimpanzees in Bossou, Guinea have attempted to use a stone anvil that was not flat, and the nuts rolled off before they could be cracked. These individuals inserted a third stone as a wedge that both leveled and stabilized the anvil, which was then used successfully. This complex behavior is termed "meta-tool" use. Nut cracking illustrates the interaction between chimpanzee mental skill, tool-use, and the ability to exploit otherwise unavailable foods. This phenomenon is not limited only to tool-use, but includes social behaviors as well.

Chimpanzees actively engage in hunting, and eat a variety of animals such as bushpigs, small antelopes, and monkeys. Unlike the opportunistic meat-eating seen with other great apes, chimpanzees are known to coordinate their efforts and then share meat with other party members. The most commonly sought after prey are the arboreal colobus monkeys, (Colobus spp.). As with most chimpanzee behaviors, there can be considerable variation both within and between populations. Individuals may successfully hunt alone, a collection of individuals may hunt in an uncoordinated fashion, or several chimpanzees may form a hunting party and cooperate with each other to drive colobus monkeys towards a group member who may be waiting in a nearby tree to make the capture and kill. After the hunt, the meat is divided among the members of the party, but is not shared equally, and some individuals may be completely excluded. Hunting and meat-eating may provide an important source of calories for some individuals, but unlike nut cracking, is not an essential part of the overall chimpanzee diet. Rather, meat appears to be a highly preferred food that is shared strategically. This behavior may strengthen alliances, maintain social status, or increase an individual's opportunities for reproduction.

Across their ranges, all of the great apes exhibit some level of variation in relation to their diet. In some instances this may be related to local food availability, but in others, cultural norms between populations explain behavioral differences related to feeding and foraging. Preferences, skills, or specific techniques for food acquisition are transmitted between generations, establishing patterns of behavior that may be extremely resilient over time. Research devoted to the ways in which these behaviors emerge and are learned by other individuals provide one of the best opportunities for understanding the mental skills and abilities of great apes in the wild.

Reproductive biology

Each of the great apes has a distinct mating system that is directly associated with their social organization and behavior. Sexual activity is not seasonal, all female great apes have a predictable reproductive cycle that is usually about a month in duration. Offspring are most often singletons, although twins have been reported on rare occasions. A female makes an enormous investment in each of her young, who will remain dependent on her for years. Males do not take a direct role in the care of their offspring, although they do provide indirect care in the form of group protection and usually demonstrate amazing levels of tolerance and good will towards infants and juveniles. Neither of these generalizations specifically apply to wild orangutans, since males rarely encounter, or interact with, youngsters.

In any mating system, both males and females need to protect and promote the genetic investment that they make in their offspring. Males accomplish this primarily by competing for sexual access to females, hoping to assure their paternity. Females are choosy about their mates, and base their preferences on physical as well as behavioral traits. Most importantly, infants and juveniles need to be protected from rival males, who may attempt to enter the group and commit infanticide. Although this behavior is repugnant, it does serve an evolutionary purpose since males that commit infanticide decrease their rival's reproductive success. Females that lose their infants begin cycling again, which may provide mating opportunities as well, a desirable possibility for the rival male. The combination of these factors has a profound influence on the structure of great ape mating and social systems.

Gorillas are the only polygamous great apes. Breeding groups are usually composed of one adult male, who is dominant, several adult females, and their offspring. The alpha male attempts to repel all rival males, guarding his opportunities for reproduction. In this system, mature males may normally be twice the size of adult females. This extreme sexual dimorphism is the result of female preference for larger males, as well as male-male competition. Larger males are able to provide better protection for the females and offspring in their group, minimizing the risk of infanticide. Female gorillas advertise their sexual receptivity through their behavior, rather than with any obvious physical changes. Females invite copulations by closely following a male, staring at him intently, and sometimes stimulating his genitals with their hand or mouth. Gorillas usually limit their sexual activity to the few days in each cycle when a female can become pregnant. Offspring are primarily dependent on the care provided by their mothers, although they may regularly interact with any of the other members of their group, including the dominant male.

Like gorillas, infant chimpanzees are at risk of infanticide from rival males in other communities. However, chimpanzees live in a promiscuous, rather than polygamous, mating system. Communities may have multiple adult males, females, and offspring, with males providing cooperative defense against rivals. Sexual dimorphism is greatly reduced, and males are only slightly larger. Female chimpanzees do not have concealed ovulation, and conspicuously advertise their fertile time (and associated sexual readiness) with a large, pinkish swelling of their genital area. This serves as a visual invitation to adult males, who compete for sexual access. During an ovulation, a female may copulate with a number of males, sometimes in succession. Therefore, paternity is never clear, and males protect all offspring equally. Usually, chimpanzees do not copulate outside of a female's fertile period. Offspring receive the majority of their care from their mothers, who spend much of their time traveling in family or nursery parties.

The situation for bonobos is very different, since adult females are constantly in a state of sexual receptiveness. Copulations are not in limited supply for bonobos, although mate choice is clear and males may compete for specific females. In these situations, higher ranked males copulate more often than lower ranked ones. Sexual dimorphism is greatly reduced, and infanticide has never been recorded in bonobo society. As with chimpanzees, paternity is never clear, and adult males show equal tolerance towards all youngsters. An additional distinction is that bonobo communities may also intermingle, with no apparent risk to infants. While young bonobos are predominantly dependent on their mothers, they also may interact with individuals both within and outside of their natal community.

Orangutans have a unique set of circumstances related to mating and reproduction. Males and females travel separately, moving within overlapping home ranges. Males usually occupy larger areas, thereby increasing their access to both females and food patches. Orangutans have a very dispersed social organization with a promiscuous mating system. They have extreme sexual dimorphism with males reaching twice the size of females. Competition between these males is intense. However, two forms of sexually mature male orangutans exist. In addition to those that show the full expression of sexual dimorphism, others fail to develop these characteristics even though they are capable of reproduction. These males may be suppressed by the presence of more dominant males, but can develop fully dimorphic traits if the dominant individuals leave the area. Females show a strong mate choice preference for the fully dimorphic males, and may willingly engage in copulations with them. Non-resident males, suppressed males, and juvenile males are generally less preferred, and may pursue females to copulate with them forcefully. Infanticide has never been recorded among orangutans, even though females regularly encounter different adult males. Females show no physical signs related to ovulation, and sexual behavior may occur at any point during the female's cycle. In the wild, female orangutans give birth only once every 6–8 years, the longest inter-birth interval of any land mammal. Offspring are completely dependent on their mothers for survival, but may occasionally interact with other juveniles within their home range.

Conservation status

The IUCN considers the Sumatran orangutan (Pongo abelii) to be Critically Endangered. The remaining five species of great apes are considered Endangered. The conservation status of great apes can be easily summarized. Throughout their entire range, all species of great ape are likely to become extinct within a generation. The major threat to their survival is unrelenting competition with humans. Great ape habitat is being transformed for human agricultural and commercial uses, rapidly bringing all wild great ape populations into direct conflict with people. Hunting of great apes to fill cooking pots is completely unsustainable, and particularly acute throughout Africa. This trade in bushmeat has accelerated the already critical decline in all great ape populations. Scientists working in the field estimate that there are no more than approximately 200,000 chimpanzees left in Africa, perhaps 100,000 gorillas, 25,000 bonobos, and a combined total of 20,000 orangutans left in Sumatra and Borneo. Conservation of these species is primarily dependent on the governments of the countries in which they exist. The best hope for their survival is habitat protection and preservation which is supported by major conservation organizations throughout the world.

Significance to humans

The great apes are our closest living evolutionary relatives. The degree of genetic relatedness between humans, chimpanzees, and bonobos is greater than the relationship between gorillas, chimpanzees, and bonobos. Great apes make and use tools, form life-long social bonds, demonstrate grief, spontaneously adopt and care for infants, and show evidence of compassion for each other. They also wage war, rape, practice infanticide, and hunt baboons for food. In captivity, they have learned to use language and numbers, generously accepted humans as social equals, and taught us that the mental differences between humans and the other great apes are only in degree, not in kind. They provide us with the best measure of what is uniquely human, and what we must admit we share with them.

Species accounts

List of Species

Bornean orangutan

Pongo pygmaeus

subfamily

Ponginae

taxonomy

Pongo pygmaeus (Linnaeus, 1760), Borneo. Three subspecies.

other common names

French: Orang-outan; Spanish: Orang-után.

physical characteristics

Largest of the arboreal primates, demonstrating extreme sexual dimorphism. Females weigh 70–100 lb (31.8–45.4 kg), males may reach 200 lb (90.7 kg). Hair color ranges from reddish to brown. Adult females lack the prominent cheek pads and throat pouch that are obvious on mature males.

distribution

Borneo. Pongo pygmaeus pygmaeus, northwest Kalimantan; Pongo pygmaeus wurmbii, southwest Kalimantan; Pongo pygmaeus morio, Sabah, south to Sungai Mahakam.

habitat

Lowland primary forest canopy and swampy areas. Mature fruit trees must be present.

behavior

Dispersed social system. Males spend most of their time traveling alone, and are highly intolerant of each other. Long calls are used as a spacing mechanism. Females are found with their juvenile offspring.

feeding ecology and diet

Rely primarily on fruits, but also consume many types of vegetation. Meat-eating has been documented, but is exceedingly rare. Tools are used to extract imbedded foods.

reproductive biology

Females have concealed ovulation, and demonstrate mate choice. Mating is promiscuous and may occur throughout the female's reproductive cycle. Copulations may be forced, most often by juvenile or non-resident males.

conservation status

Endangered.

significance to humans

Currently hunted for meat, and for infants that are sold in the illegal pet trade.

Sumatran orangutan

Pongo abelii

subfamily

Ponginae

taxonomy

Pongo abelii (Lesson, 1827). Monotypic.

other common names

None known.

physical characteristics

Largest of the arboreal primates, demonstrating extreme sexual dimorphism. Reported to be slightly smaller than Bornean orangutans. Adult females lack the prominent cheek pads and throat pouch that are obvious on mature males.

distribution

Sumatra.

habitat

Lowland primary forest canopy and swampy areas. Mature fruit trees must be present.

behavior

Dispersed social system. Males spend most of their time traveling alone, and are highly intolerant of each other. Long calls are used as a spacing mechanism. Females are found with their juvenile offspring.

feeding ecology and diet

Rely primarily on fruits, but also consume many types of vegetation. Meat-eating has been documented, but is exceedingly rare. Tools are used to extract imbedded foods.

reproductive biology

Females have concealed ovulation, and demonstrate mate choice. Mating is promiscuous and may occur throughout the female's reproductive cycle. Copulations may be forced, most often by juvenile or non-resident males.

conservation status

Critically Endangered.

significance to humans

Currently hunted for meat, and for infants that are sold in the illegal pet trade.

Western gorilla

Gorilla gorilla

subfamily

Homininae

taxonomy

Gorilla gorilla (Savage and Wyman, 1847), Gabon Estuary, Gabon. Two subspecies.

other common names

French: Gorille; Spanish: Gorila.

physical characteristics

Largest of the terrestrial primates, demonstrating extreme sexual dimorphism. Females generally weigh about 150 lb (68 kg), males may approach 400 lb (181.4 kg). Hair is generally black over most of the body, red to brown on the crest of the head. Mature males sport silver hair on their back.

distribution

West Africa, including Nigeria, Cameroon, Gabon, extending into Congo, and Central Africa Republic. Gorilla gorilla gorilla, Cameroon; Gorilla gorilla diehli, border area between Nigeria and Cameroon, extending into the forest of the upper Cross River.

habitat

Found in primary and secondary forest, may venture into swampy clearings as well.

behavior

Polygamous social system. Mixed sex groups generally include one dominant male, multiple adult females, and offspring. Variations may be seen in which more than one adult male is present. Males may also travel alone, or congregate in bachelor groups.

feeding ecology and diet

Consume a variety of types of vegetation and fruits. Meat-eating has not been documented, and tool use appears to be absent.

reproductive biology

Females show no physical signs associated with ovulation, but give behavioral signals that invite copulation. Mating system is polygamous, and adult males repel rivals who may attempt to lure females away. Infanticide by rival males is well known.

conservation status

Endangered.

significance to humans

Populations are being severely affected by unsustainable hunting for meat.

Eastern gorilla

Gorilla beringei

subfamily

Homininae

taxonomy

Gorilla beringei (Matschie, 1903). Two subspecies.

other common names

English: Mountain gorilla, eastern lowland gorilla.

physical characteristics

Largest of the terrestrial primates, demonstrating extreme sexual dimorphism. The hair is primarily black, and may be quite long with a shaggy appearance. Mature males sport silver hair on their back.

distribution

East Africa, Democratic Republic of Congo into Rwanda and Uganda. Gorilla beringei beringei (mountain gorilla), Uganda, Rwanda, and Democratic Republic of Congo; Gorilla beringei graueri (eastern lowland gorilla), eastern Democratic Republic of Congo.

habitat

Found in primary, secondary, and bamboo forest, but at higher altitudes than the western gorilla.

behavior

Polygamous social system. Mixed sex groups generally include one dominant male, multiple adult females, and offspring. Variations may be seen in which more than one adult male is present. Males may also travel alone, or congregate in bachelor groups.

feeding ecology and diet

Primarily dependent on vegetation and some fruits. G. b. beringei relies entirely on vegetation. Meat-eating has not been documented, and tool use appears to be absent.

reproductive biology

Females show no physical signs associated with ovulation, but give behavioral signals that invite copulation. Mating system is polygamous, and adult males repel rivals who may attempt to lure females away. Infanticide by rival males is well known.

conservation status

Endangered.

significance to humans

Some portions of the population are being severely affected by unsustainable hunting for meat.

Chimpanzee

Pan troglodytes

subfamily

Homininae

taxonomy

Pan troglodytes (Blumenbach, 1775), Mayoumba, Gabon. Four subspecies.

other common names

English: Common chimpanzee, robust chimpanzee; French: Chimpanzé; Spanish: Chimpancé.

physical characteristics

Compact and muscular build. Hair is generally black, but may turn gray with age. Slight sexual dimorphism. Females

weigh 70–100 lb (31.8–45.4 kg) and males may be 80–130 lb (36.3–59 kg).

distribution

Large portion of Africa, beginning in Senegal and reaching the Democratic Republic of Congo, Sudan, Uganda, Rwanda, Burundi, and Tanzania. Pan troglodytes troglodytes, Cameroon, Nigeria, Central African Republic; Pan troglodytes verus, Senegal; Pan troglodytes schweinfurthii, Democratic Republic of Congo, Sudan, Uganda, Rwanda, Burundi, and Tanzania. Actual borders between subspecies are difficult to assess and overlap may occur in some areas.

habitat

Found in a range of habitats including primary forest, secondary forest, grassland, and woodland savanna.

behavior

"Fusion-fission" social system. Communities regularly break into smaller parties of varied composition. Males are generally dominant and form coalitions. Females are generally subordinate to adult males. Social interactions can be highly complex.

feeding ecology and diet

Exploit the widest variety of foods of any species of primate. Fruits, vegetation, insects, nuts, and animal prey are all included. Hunting and meat-eating are common. A diverse collection of tools are used to acquire and process regularly eaten foods.

reproductive biology

Promiscuous mating system. Females advertise their ovulation with prominent genital swellings. Males compete for sexual access to females, although females may mate with many males when they ovulate. Paternity is difficult to assess. Offspring are at risk of infanticide from rival males.

conservation status

Endangered.

significance to humans

Populations are being severely affected by unsustainable hunting for meat.

Bonobo

Pan paniscus

subfamily

Homininae

taxonomy

Pan paniscus Schwarz, 1929, south of the upper Maringa River, Democratic Republic of the Congo. Monotypic.

other common names

English: Pygmy chimpanzee, gracile chimpanzee, dwarf chimpanzee; French: Chimpanzé nain, chimpanzé pygmée; Spanish: Chimpancé pigmeo.

physical characteristics

Much more slender than the chimpanzee. Hair is black, and is distinctly parted down the center of the head. Slight sexual dimorphism. Females may weigh around 70 lb (31.8 kg), and males less than 100 lb (45.4 kg). The skin on the face is always very dark.

distribution

Democratic Republic of Congo.

habitat

Primary and secondary forest.

behavior

"Fusion-fission" social system. Communities divide into smaller parties that change less frequently than those of the chimpanzee. These parties usually have a mixed sex composition. While tension and aggression exist between individuals, it is reduced through frequent erotic interactions. These may be male-female, male-male, or female-female. Females are dominant, and males are subordinate. Communities may peacefully intermingle.

feeding ecology and diet

Primarily reliant on fruits and vegetation. Some meat is eaten, but comprises a very small part of the overall diet. Tools are not used to acquire foods.

reproductive biology

Promiscuous mating system. Sexual activity is extremely common, but is not indiscriminate. Incest taboos are in place, and males compete for access to specific females. Higher ranking males have increased sexual access for reproduction. Infanticide is unknown in bonobos.

conservation status

Endangered.

significance to humans

Populations are being severely affected by unsustainable hunting for meat.

Resources

Books

Boesch, C., and H. Boesch-Ackerman. The Chimpanzees of Tai Forest. New York: Oxford University Press, 2000.

de Waal, F., and F. Lanting. Bonobo: The Forgotten Ape. Berkeley: The University of California Press, 1997.

Goodall, J. The Chimpanzees of Gombe: Patterns of Behavior. Cambridge, MA: The Belknap Press of Harvard University Press, 1986.

McGrew, W. C., L. F. Marchant, and T. Nishida, eds. Great Apes Societies. Cambridge, UK: Cambridge University Press, 1996.

Rijksen, H., and E. Meijaard. Our Vanishing Relative: The Status of Wild Orang-utans at the Close of the Twentieth Century. Dordrecht: Kluwer Academic Publishers, 1999.

Robbins, M. M., P. Sicotte, and K. J. Stewart, eds. Mountain Gorillas: Three Decades of Research at Karisoke. Cambridge, UK: Cambridge University Press, 2001.

Wrangham, R. W., W. C. McGrew, F. B. M. de Waal, and P. G. Heltne, eds. Chimpanzee Cultures. Cambridge, MA: Harvard University Press, 1994.

Robert W. Shumaker, PhD