Diet and Primate Evolution
Variation in the choices of food on a daily, seasonal, and yearly basis is one of the greatest differences between primate species. Primate diets have generally been divided into three main food categories-fruit, leaves and fauna (including insects, spiders, and bird's eggs for the most part). The different diets also are referred to as Frugivores, Folivores, and Insectivores (fruits, leaves and insects respectively). These gross dietary categories are correlated with aspects of primate activity patterns such as home range and group size. There are different problems that have to be overcome in order to obtain a balanced diet on a day-to-day basis. New leaves and mature leaves, for example, may have different nutritional bases or availabilities. Some fruits appear in large clumps while others are spread thinly over a larger area. Aside from diet, primates have tended to either adapt through specialized dentition and digestive systems or rely on a variety of different foods. Those that chose this second route have tended to have larger brain sizes relative to their overall body size. This may be due to the need to know where different types of food can be found at different times throughout the year.
Have you ever looked at a picture of a gorilla or money that has what appears to be a "beer belly" look? Those primates that specialize on eating large amounts of vegetation have difficulty digesting their meals. Different solutions have been found including double stomachs or multichewes, but the problem usually requires an individual to sit back and let the digestive system work-and on the die expel gas.
It hasn't been until recently that it was realized that primates, who evolved in the canopy, actually had a quite difficult time finding adequate nutrition. Due to natural selection, which strongly favors those traits enhancing foraging efficiency, and this difficulty finding sustenance, the characteristics regarded as belonging to primates were evolved.
Most primates species either maximize the efficiency of their digestive track or maximize the quality and the volume of food processed in a day. Also. Those species, which increase the volume of food, consumed also tended to have larger brains because of a need to remember, not only what, but also where good food sources are and when they are in season.
Knowing the trends of evolution, the next thing to do is to look at how it relates to humans. It can be seen that out closest living relative, the chimpanzee makes up 94% of his diet with plants. Most of this is fruit with high sugar and fiber content, meaning that chimps take in hundreds of grams of fiber per day; contrary to the 10 grams or less the advantage American consumes. This suggests that our diet should be higher in vitamin C, pectin, and fiber like our relatives in the wild. Ironically, the same natural selection, which favors the most energy-dense and low fiber diets, may now be causing us to suffer from too little fiber.
Plants have been a major food eaten by primates. The primates were almost exclusively herbivorous. A strong focus on plant foods is characteristic of all primates-- and there is strong consensus that the primates were very strongly herbivorous (Milton 1987, 1993). Many plants are lacking vital nutrients, such as vitamins and the protein building blocks known as amino acids, that the monkeys and other animals require. Some plants lack enough carbohydrates to make them worthwhile as energy producers. Yet in all of these cases the primates would still search for specific plants that exhibited one or more of these traits, rather than just rely on the plant life that was within easy reach.
The fact that the primates would make an active search from a variety of plants. One particular type of plant may have lacked was often complemented by the positive aspects of another plant. For example, fruits are low in fiber and protein and yet they are rich in valuable carbohydrates. If the primates had relied solely on eating fruit then they would have lacked sufficient protein and vital amino acids. To make up for that particular type of shortage, the monkeys eat certain leaves that are high in protein and fiber and that are also more abundant than the fruit. Together, the fruit and the leaves make for a much balanced diet for the primates.
In order for the primates to rely on just one particular type of plant as a food source, the primates would have to travel quite a distance to obtain enough food from multiple trees of the same plant type. In addition, trees of a certain species tend to produce fruit or leaves during specific seasons of the year and then they are either without fruit or possibly without leaves. If primates were to depend on a single source of food supply, either fruit or leaves, then they would have to starve while waiting for a new crop of fruit or leaves to grow. With a varied diet, the primates are able to eat different plant types year round, and get a complete set of nutrients too.
A tremendous diversity exists within the dietary choices of primates. As a general rule, small animals require a high nutrient flow but a lower caloric input, while larger animals can survive on a poor quality, high-density diet. This is linked to metabolic rate, whereby the smaller the primate, the faster the metabolism (Gaulin and Konner 1977). Diet can also be associated with energy levels. It is notable that gorillas and oranges exhibit low energy levels and consume low quality foods, while active chimpanzees consume a high-quality diet. There are exceptions to the generalization that small primates will select a high quality diet, while larger primates will rely primarily on low quality foods. Aye-ayes consume a large proportion of insects in their diet and are considerably larger than most other primate. Yet, they do eat wood boring insect larvae, which are less mobile and provide a higher yield. As well, the potto consumes a high proportion of insects and sap. This may somehow be linked to a faster metabolic rate (Gaulin and Konner 1977). Dietary constraints, such as competition, should also be considered. Natural selection would favor variation in size and the ability to exploit alternative dietary niches. One such paleontological example focuses on the early horse or equid. Ancestors of the contemporary horse were much smaller than today and consumed shoots (growing plants) and fruits. Certain lineages established a trend towards grazing. Within these lineages that exploited dense grasses (low nutritive value), an increase in size, similar to modern populations, is evident (Gaulin and Konner 1977).
The primate gut is very sensitive to the differences between C3 and C4 plants. Specifically, the microflora in human guts is sensitive to these differences. Human primates can effectively digest fiber from vegetables such as cabbage and carrots, but less
efficiently break down that from cereal fibers such as bran. This suggests that the consumption of cereal grains is a recent departure from more traditional plant foods consumed by a majority of primates.
Potentially, this is linked to an increase in energy requirements, with no increase in dietary quality. The surface area of the small intestine must increase in order to maximize
absorption of vital nutrients. Yet, the colon is actually a derived trait and not an ancestral trait (Milton 1987).
Is there a primate analogue to the human gut? Quite similar proportions, with respect to small intestine and small gut mass to body ratio, can be found in the capuchin monkey. These monkeys have a high quality diet of rich foods such as fruits, oil rich seeds, and insects. Baboons also have a very selective diet. Interestingly, both Savannah baboons and capuchin monkeys are known for their manual dexterity, efforts in food preparation, and extensive selective searching. The similarity in gut morphology is not associated with a common ancestor, but more likely has arisen from commonality in high quality diets (Milton 1987).
Dietary changes have been sited as the impetus behind bipedal locomotion. If early humans exploited high quality, low-density foods, this would require a home base and extensive travelling. Bipedalism could serve as a more energetically efficient method
for gathering food items. Based on her discussion of the capuchin monkeys, Savannah baboons, and the use of the hand, Milton (1987) appears to support this hypothesis. Extinction of robust australopithecines has also been linked to dietary shifts. Milton (1987) suggests that robust australopithecines may have opted for a lower quality diet. This is indicated by the massiveness in the morphology of dental and facial bones due to consumption of tough plant foods. Such a dietary selection may have led to the direct competition preceding extinction. There is a decrease in cheek tooth size, thinning of
dental enamel, expansion of cranial capacity, and increase in body size. Factors such as these confirm a dietary change,
potentially linked with a novel technology, social innovation such as sharing or development of language skills, or both.
The study of primate diets is an important aspect of paleonutrition. Information gleaned from research on primates has been linked to such diverse topics as the anatomical proportions of the human digestive tract, to the advent of language and bipedalism. Although some connections are somewhat tenuous, primate studies can provide a living perspective on the direction of human evolution.
There's a link between the diet and teeth. The typical of most primates lack of dietary specialization. They tend to eat a wide assortment of food items. The teeth are not specialized for processing only one type of food, a pattern corrected with the lack of dietary specialization.
Most primates possess fairly generalized teeth. For example, the cheek teeth have low, rounded cusps. Equipped with this type of premolar and molar morphology, most primates are capable of processing a wide variety of foods ranging from rough or hard items, such as leaves and seeds, to more easily processed fruits, insects, and even meat.
Although the majority of primate species tend to emphasize some food items or others, most eat a combination of fruit, leaves, and insects. Many obtain animal protein from birds and amphibians as well. Some (baboon and, especially, chimpanzees) occasionally kill and eat small mammal, including other primates. Others, such as African colobus monkeys and the leaf-eating monkey (langus) of Southeast Asia, have become more specialized and subsist primarily on leaves.
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