A tremendous diversity exists within the dietary choices of non-human
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).
Non-human primates consume large quantities of plant remains which are
variable in nutrient quality. Growing plants and fruits have a higher
nutritional percentage of protein to fiber. Certain non-human primates obtain
a considerable proportion of their diet from highly dispersed insects that
are
nutritively superior to fruits and leaves. Tree sap, a rare dietary
constituent, is higher still in concentrated sugars than fruit. Gaulin and
Konner (1977) predict that smaller animals will exploit resources such as
fruits, insects, and sap, while larger primates will consume mature
plants, of
greater abundance, but less nutritive value. Diet can also be associated with
energy levels. It is notable that gorillas and orangs 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 non-human 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).
Milton (1987) investigated hominid evolution by examining gut morphology. She
performed various experiments illustrating that 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.
As there are always exceptions, caution should be maintained when employing
gut dimensions in diet determination. Several differences in gut morphology
exist between humans and other hominoids. For instance, humans have the
greatest gut volume represented by the small intestine, while in gibbons and
orangs the greatest gut volume is in the colon. Size of the gut to body mass
is relatively small in humans. There is an
increased size of the small intestine and decreased size of the colon.
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. The size of a human
neonate colon is very similar to that of pongids, but as the primate body
grows, there is a regression in the size of the human colon and an elongation
of the pongid colon (Milton 1987).
Is there a non-human 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).
Although these similarities are noteworthy, an evolutionary picture can not
necessarily be constructed. Many animals have demonstrated the ability to
rapidly alter gut proportions in response to changes in the quality of diet.
This indicates that the proportions of the contemporary human small intestine
could be either an ancient or a current trait. As of publication of Milton's
(1987) article, no one has investigated whether there are intestinal
differences in extant human populations consuming varying diets. Western
populations which consume less than 10 grams of fiber per day may vary
anatomically from certain African rural populations which consume over 170
grams of fiber per day.
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. Gracile australopithecines, alternatively, may have continued to
consume even higher quality foods while increasing food search efficiency.
Skeletal morphology has changed from australopithecines to early humans.
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.
Micro-wear analysis is a common analytical tool employed in determining diet.
Hamilton (1987) critiques this method and outlines the severe limitations.
This tool can be useful to exclude certain methods of food processing, but is
limited in the ability to determine types of foods actually consumed. For
instance, if an animal is cracking a particular type of nut using dental
force, diagnostic striations may be exhibited on the teeth. Yet, if the
animal
is cracking the nut with a rock and then consuming it, there may be no
evidence that this food was a component of the diet. It is difficult to make
generalizations and Hamilton cautions that differences may exist within
various populations, ages groups, or sexes.
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.