ANIMALS: PRIMATE DIETS

ANIMALS: PRIMATE DIETS. The daily food quest is generally the single most important activity for any animal. For this reason, natural selection should strongly favor any feature that enhances an animal's success at food acquisition. In essence, it has been suggested that different animal species can be viewed as "natural experiments" aimed at securing some portion of the always limited dietary resources available on the planet at any one time.

Humans are members of the Primate order and are placed in the suborder Anthropoidea, along with monkeys and apes. (When speaking of Primates as an order, the word is capitalized; when speaking of primates generally, the word is lowercased.) Because of our close relationship to other anthropoids, particularly the great apes (Pongidae), the dietary behavior of wild primates is of strong interest because it can provide clues about the types of dietary problems humans may have faced and resolved in their own evolution as well as insights into the probable dietary behavior of human ancestors and the nutritional composition of their wild foods.

The Role of Diet in Primate Evolution

Most primates are highly arboreal animals and carry out most activities in the canopy of tropical forest trees. Fossil evidence suggests that the earliest Primates evolved from small insectivores, initially attracted into the canopy by pollinating insects. Many morphological traits specifically associated with Primates, as, for example, keen stereoscopic vision, manipulative, grasping fingers, and a reduced dependence on the sense of smell seem to relate to selection for features associated with the efficient capture of items such as insects in an arboreal environment. However, because insects tend to be discretely distributed in the forest canopy and are also small in size, only small and relatively solitary primate foragers can be supported on such a diet.

Perhaps to circumvent these limitations, it would appear that natural selection favored a decided change in the dietary behavior of the ancestral primate, giving rise to the anthropoids. Though insects may be scarce, any animal in the tropical forest is continuously surrounded by potential plant foods. The evolution of anthropoids appears to have resulted from some early primate's ability to penetrate the arboreal plant food niche and then radiate (diversify) such that its descendents (monkeys and apes) came to dominate a strong subset of the most nutritious and energy-rich plant foods available in the forest canopy.

The idea that monkeys and apes are basically plant eaters may come as a surprise because, when we think of plant-eating animals, primates do not generally come to mind. Rather we think of cows and horses or rabbits and kangaroos. However, a wealth of field studies on many different species of wild primate show that all extant (living) monkeys and apes take by far the greatest percentage of the daily diet from plant foods, eating small to negligible amounts of animal matter. It is fair to say that primates are omnivores (taking the diet from both animal and plant foods) because most primates do eat some animal matter each day along with plant foods, but it is important to realize that most material entering the digestive tract of monkeys and apes is of plant rather than animal origin.

Primates and Plant Foods

An orientation toward plant foods is characteristic of all anthropoids irrespective of body size. Many smaller monkeys take as much as 65 percent of the diet from concentrated plant foods such as gums and ripe fruits. For example, the smallest living monkey, the pygmy marmoset (Cebuella pygmaea, body weight less than 2 oz.) feeds primarily on calorie-rich plant gums and saps, supplemented with ripe fruits and insects. Larger monkeys as well as all apes take most of the diet from young leaves and fruits, supplementing these with seeds, flowers, and other plant parts, and often also eggs, insects, and small vertebrates. Orangutans and gorillas are estimated to take some 99 percent of their annual diet exclusively from plant foods—wild fruits, tender young leaves, piths, and the like—while for chimpanzees, this figure is some 94 percent. Though wild chimpanzees are known to hunt, kill, and eat small mammals, including monkeys, and also to eat termites and ants, such animal foods typically contribute only a small percentage to their annual diet, most of which is made up of ripe fruits. It is likely that many monkeys and apes would eat more animal foods if they could capture them more easily. Thus the low amount of meat in the average wild primate diet may relate more to the difficulties of getting animal food than to any aversion to eating it.

When we examine the digestive tracts of most plant-eating animals, animals such as deer, cows, or rabbits, we find that their digestive tracts show notable specializations to aid them in extracting sufficient nutrition from all of the plant foods they eat. With few exceptions, however, Primates do not show such dramatic digestive specializations—yet, Primates too are plant eaters. How are primates able to exploit plant foods successfully without the extensive digestive specializations we find in most other plant-eating animals?

Primates Solve Dietary Problems with Their Heads

The answer is simple—primates tend to specialize on only the highest quality plant foods—foods that offer the most nutrition in exchange for the time and energy invested in finding and eating them. Yet, very high-quality plant foods in the tropical canopy are bound to be less abundant than lower-quality plant foods. How then are primates able to afford the costs of their high selectivity?

To put it simply, primates use their heads. An unusually large complex brain relative to body size characterizes all primates. It is hypothesized that an unusually large brain was favored in primate evolution in part to aid in remembering the types and locations of higher quality plant foods as well as when such foods were ready to be harvested. Using their large brains, monkeys and apes can keep track of when particular important food trees are producing their nutritious new leaves or sugar-rich flowers or ripe fruits and can plan the most direct travel route to reach them at the opportune moment for harvest.

Thus primates use long-term memory and planning to lower the costs associated with seeking out the best foods in the forest canopy, and because the foods they eat are of relatively high quality, elaborate digestive specialization generally is not required to digest them efficiently. A few lineages of primates do show fairly elaborate digestive specializations. These lineages—as for example, the Colobinae or "leaf-eating" monkeys—have diets composed in large part of leaves, including many mature leaves that tend to be low in nutrients and hard to digest.

Anthropoids are also characterized by extreme sociality. Most monkeys and apes live in social groups composed of various adult females and males and their immature offspring. Each group occupies a particular area in the forest and, as a general rule, its descendents will continue to occupy more or less the same area generation after generation. For this reason, the social unit so characteristic of monkeys and apes can be regarded as a type of dietary adaptation for it enables the older individuals in the group to pass on important information to young animals about the types, locations, and seasonal patterning of the higher quality plant foods in their habitat—information essential for their survival.

Each Primate Species Has a Particular Dietary Focus

Stressing that primates are largely plant-eaters is not to imply that all monkeys and apes have exactly the same diet, for they do not. Each primate species tends to have its own characteristic mix of plant (and animal) foods. In addition, each particular species typically specializes in only a certain subset of the higher-quality dietary resources in its environment. For example, the South American capuchin monkey, Cebus apella, has a high degree of manual dexterity and very powerful jaws. These features aid capuchins in cracking open the hard palm nuts that form an important part of their diet, which also includes ripe fruits, insects, and small vertebrates. Another monkey species often found living in the same forests as Cebus apella is the spider monkey, Ateles paniscus. Spider monkeys are characterized by a low degree of manual dexterity and small teeth, which are useless for cracking hard-shelled palm nuts. But spider monkeys specialize in soft fruits and tender young leaves, not hard palm nuts. They are also much bigger than capuchin monkeys, and if the supply of softer fruits is limited, they can chase capuchin monkeys out of fruiting trees and monopolize the fruit crop for themselves.

The Role of Diet in Human Evolution

As discussed, the closest living relatives of humans, the great apes, all eat plant-based diets. The common ancestor giving rise both to apes and humans is likewise believed to have been primarily plant-eating. All humans and apes have the same basic digestive tract—a simple acid stomach, a small intestine, a tiny cecum, and a sacculated (folded) large intestine (or colon), though the human small intestine is longer and the human colon smaller than is the case for apes. Humans and apes also show the same pattern of food digestion, indicating that, biologically, in terms of gut form and function, humans have departed little from the ancestral condition. But, unlike apes, humans eat a wide range of different foods and have many different types of diets. What factors may have contributed to this difference between humans and apes?

Some 3 million years ago, before the first evidence of human beings, we know that climatic changes caused the vegetation in many areas of Africa to shift from tropical forest to more open savanna vegetation. This produced notable changes in the types of plant foods available in these environments. In particular, finding sufficient high-quality plant foods throughout the year probably would have been difficult for a large apelike primate such as those we envision as prehuman ancestors. One solution to this problem would be to try and find some other source of high-quality food. Animal foods are such a source: they are very high quality and, because of the huge herds of grazing and browsing animals in the African savanna, animal foods are far more abundant than is the case in the tropical forest canopy.

The association of stone tools with the earliest fossil evidence for human beings (genus Homo, species Homo habilis ) suggests that early humans began to include animal as well as plant foods in the diet. Rather than just being an occasional food, as is the case for chimpanzees or other apes, meat and other animal products (tongue, bone marrow, brains, fat) appear to have become an integral part of the daily human diet. Initially, the earliest humans may have lived as scavengers and relied on meat from kills abandoned by carnivores. But increasing numbers of stone tools and processed mammal bones in the archaeological record suggest that humans gradually turned to the hunting of larger animals.

Animal protein is of high quality and easy for humans to digest. It also is a rich source of many essential vitamins and minerals. In addition, and most importantly, animal foods are volumetrically concentrated relative to plant foods and thus take up much less space in the feeder's gut. By eating some meat each day and combining this animal food with energy-rich plant foods, humans evolved as large-brained, active, and social hunter-gatherers. The fossil record shows that the human brain increased substantially over the course of human evolution. It would seem that, initially, the earliest human ancestors were somehow able to include a modest amount of animal food in the diet on a routine basis along with their usual plant foods. But these animal foods provided much high-quality nutrition and also freed up space in the gut for energy-rich plant foods, thus improving the feeder's diet in terms of both nutrients and energy. This energy was required to fuel the increasingly large human brain, which requires glucose as fuel. More astute individuals doubtless were more proficient at securing animal foods as well as energy-rich plant foods and they passed these mental abilities on to their descendents. Over time, this feedback process resulted in the gradual expansion of the human brain.

Humans as Cultural Omnivores

The evolution of humans actually follows the common primate trajectory—that is, using brain power to resolve many important dietary problems. Morphologically, humans lack the huge jaws, massive teeth, and sharp claws of true carnivores. But their increasingly large and complex brains permitted early humans to substitute technology for carnivore teeth and claws and use tools and creative intellectual solutions to capture and prepare their animal and plant foods for consumption.

In addition, early humans developed a unique form of food acquisition characterized by a division of labor. Typically, male members of the social unit sought animal prey while female members sought out higher-quality plant foods. Then all of the foods collected each day were shared with all members of the social unit, giving each person a good mix of all of the different nutrients required to remain in good health. By entering the adaptive zone of culture—cultural behavior made possible by virtue of the unusually large and complex human brain—humans gradually developed the capacity to exploit dietary resources of all types in ways not possible for any other animal. The success of this uniquely human form of dietary behavior is manifested by the fact that over time, by using their wits to devise successful diets, humans have been able to colonize almost every environment on earth. As long as all of the nutrients humans require are present in their foods, humans can thrive on an amazing variety of different diets.

See also Agriculture, Origins of ; Anthropology and Food ; Cuisine, Evolution of ; Evolution ; Food Archaeology ; Hunting and Gathering ; Nutritional Anthropology ; Paleonutrition, Methods of ; Prehistoric Societies .

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Katharine Milton