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 bono-bos. 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.

The orangutan (Pongo pygmaeus) has a short thumb and long fingers, enabling it to hook easily over branches. (Photo by © George D. Lepp/ Corbis. Reproduced by permission.)

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 bers 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.

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 mem-

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