Relating Chimpanzee Diets
     To Early Human Diets  
   By: Nancy Lou Conklin-Brittain, Richard W. Wrangham, Catherine C. Smith:
                      Department of Anthropology, Harvard University.

It is generally accepted that modern humans evolved from some
chimpanzee-like ancestor.  

Consequently we use our data on the nutritional ecology of modern
chimpanzees to draw conclusions about how nutrition, and in particular,
how macronutrient chemistry, may have been involved in human evolution.  

We will be discussing only the plant component of the diet; although modern
chimpanzees are omnivorous, plant foods provided the great majority of the
food seen eaten by these primates during our year-long study (ca. 99%).  

The data presented in the first half of this report comes from a study we have
recently completed comparing the diet of chimpanzees with that of three
sympatric frugivorous monkey species in Kibale Forest, Uganda.  

The study species were: chimpanzee, blue monkey, redtail monkey, and
gray-cheeked mangabey.  

There was relatively little diet overlap in species lists between chimpanzees
and the monkeys as a group.  

More species of seeds and leaves were consumed by monkeys, while more
species of pith, (soft spongy core of a flowering plant), were consumed by
chimpanzees.  Even within the fruit categories, there was little fruiting-tree
species overlap (Wrangham).  

The chimpanzees were ripe fruit specialists while the monkeys split their
time equally among ripe fruit, unripe fruit and seeds, and leaves (Wrangham).

The chimpanzees tracked the availability of ripe fruit quite closely, while the
monkeys did not.  (Wrangham).

For most of the year an average of 25% or less of the monkeys' feeding time
was spent eating chimpanzee foods.  However, when fruit was abundant, the
blues and redtails tracked the chimpanzee ripe fruits where as the
mangabeys did not.  

When ripe fruit was not available, the chimpanzees significantly turned to
piths as their primary fallback food.  

Meanwhile, the redtails increased their unripe fruit and seed intake as ripe
fruit availability decreased.  The blue monkeys did not track ripe fruit, instead
they tracked leaves, with seeds and unripe fruit being their fall-back food
when leaves were scarce.  Mangabeys did not track any single food category
significantly (Conklin-Brittain).  

Dietary diversity for all species increased with more feeding records.  
However, in relation to observation time, all monkeys had higher dietary
diversity than chimpanzees.  

In summary, the monkeys had much more similar diets to each other than
they did to the chimpanzee, whether one considers the percentage of food
items shared or the percentage feeding time per month on shared foods.  
There exists a striking diversity in the plant parts and plant species
consumed by these frugivorous monkeys as compared to the chimpanzees
in Kibale forest.  Chimpanzees were much more frugivorous.  

Given that chimpanzees and monkeys differed in their diets, we ask here
whether these differences affected their intake levels of macronutrients.  

We found some striking similarities, for example, the crude lipid or fat content
of the diet.  

There were three important points regarding the crude lipid content of the

1. There were no significant differences among the primate species in the fat
content of their diets.  

2. The seasonality in fat intake coincided with an increased ripe fruit

3. The amounts of fat in the diets were very low, even at peak consumption
levels; peak was only about 8.5% lipid, and the average annual intake was
only about 2.5%.  

As a point of reference, humans do not need more than 3-5% fat on a dry
matter basis in their diet, enough to provide the one essential fatty acid and
the fat-soluble vitamins.  Modern, westernized humans consume 15-25% fat
on a dry matter basis, usually referred to as 30-45% of calories consumed,
far in excess of need or recommendation (Butrum).  

Dietary crude protein (CP) content showed there were no significant
differences among the monkeys but the difference between the
chimpanzees and the monkeys was significant.  

There was no significant seasonal variation.  As a point of reference, adult
humans need only about 9.5% protein on a dry matter basis, so the
chimpanzees' plant diet was probably not deficient in protein, simply lower in
content than the monkeys.  

The high protein content of the monkey diet was the result of eating leaves
as a fallback food (Conklin-Brittain), given that leaves were generally quite
high in protein.  

The chimpanzees, on the other hand, consumed pith as a fallback food,
which was on average low to moderate in protein.  We believe the
chimpanzees ate piths because they were high in one of the more easily
digested fiber fractions (hemicellulose), therefore contributing to their
carbohydrate and energy intake (Wrangham, 1991).

Considering the carbohydrate intake, for the water-soluble carbohydrate or
simple sugars, there was pronounced and significant seasonality in the
simple sugar content of the chimpanzee diet.  

This peak coincided with peak ripe fruit availability.  The monkeys did not
take advantage of the fruiting peak to increase their sugar intake, and there
were no significant differences among the monkeys.  

Total nonstructural carbohydrates include the simple sugars, starch and
most soluble fibers.  Soluble fibers include pectins, gums, beta-glucans, and
other non-starch polysaccharides.  

Humans can digest (via hind-gut fermentation) 0-100% of soluble fibers,
depending on the fiber.  We assume that because chimpanzees have a much
larger capacity for hindgut fermentation (Milton), they can digest (via
fermentation) most soluble fibers, so we included them in this category of
potentially digestible carbohydrates.  

Again the chimpanzees showed significant seasonality, increasing their
digestible carbohydrate intake when given the opportunity.  

This is important because the most healthful human diet is believed to be
one based on complex carbohydrates with small amounts of fat and protein
(Butrum, 1988).

Neutral-detergent fiber is also referred to as total insoluble fiber or plant cell
wall.  The most surprising similarity in diets among all four primates; they all
consumed diets at the same insoluble fiber level, around 32% throughout the

There were no significant seasonal differences.  This was surprising
because the monkeys were small, about 10-20% the body weight of the

In general smaller animal species consume diets lower in fiber than those of
larger animals.  We do not know whether to consider the monkeys' diets as
high in fiber or the chimpanzee diet as low, though we suspect the latter
(Conklin- Brittain).

To summarize: all of our study species consumed a low fat, high fiber diet
compared to humans.  

The chimpanzees' diet was higher in digestible carbohydrates when there
was an increase in ripe fruit availability.  In addition, the chimpanzees
maintained a fairly low and constant protein intake, due to their focus on
fruit, with pith as a fallback food.  

No plant part was high in lipid.  Seeds were the highest, but 8.4% is not very
high.  As expected, the leaves had the highest crude protein.  The leaf
fraction was mostly young leaves, while piths on average contained less
than half that amount of protein.  

Flowers were surprisingly high in protein, but eaten only sporadically.  The
average ripe fruit had exactly the same protein content as the annual average
intake for the chimpanzees.  

Ripe fruit was the sweetest, and piths were the second sweetest.  Ripe fruit
was also the lowest in fiber, but 33.6% is quite high for a fruit pulp.  Not
coincidentally, 33.6% is also the annual average fiber content of the
chimpanzees' diet.  

As a prerequisite to considering the Australopithecus diet, we will briefly
discuss hunter-gatherer diets, modern but traditional human diets, and the
minimum nutrient requirements of humans.  

Modern humans do not have high protein or fat requirements, as already
mentioned.  The value of 9.5% CP in the chimpanzee diet in our study is
consistent with the prediction by Oftedal (1990) that all primates should have
relatively low protein requirements because they have slow growth rates
compared to other mammals (Case, 1978).  

Although a need for protein or fat is often assumed to explain increasing
amounts of hunting throughout hominid evolution, primates do not have
metabolic demands for high levels of protein or fat.  

Eaton (1988) proposed an ancient hunter-gatherer diet in the book The
Paleolithic Prescription.  We can now evaluate their hypothetical diet in the
light of what we have just learned about the chimpanzee diet and with what
is known of modern human nutrient needs.  

The hunter-gatherer diet Eaton et al. proposed contained 35% meat and 65%
(wild) plant foods.

The protein intake is almost 4.5 times higher than required by humans, so we
cannot make meaningful comparisons there.  

The fat intake is also high compared to the chimpanzee diet and to that
required by humans, even though the authors used nutrient values from wild
game meat instead of domestic meat, so the fat intake is moderate compared
to a modern human diet.  

The fiber content is the interesting point for comparison.  The content is
about half of that in the chimpanzee diet.  This results from 35% of the plant
component being replaced by meat, in effect diluting the fiber content of the

We have seen that a wild herbivore diet, such as the chimpanzee in Kibale
Forest, is high in fiber because wild foods are high in fiber.  In order to dilute
that fiber level further, a new source of food must be found that is low in
fiber.  Meat is guaranteed to reduce the fiber content of a diet considerably
and to be fairly easily digested.  

Nevertheless, because wild vegetation is high in fiber, the Paleolithic
hunter-gatherer diet was still assumed to contain 150 g of fiber from its 65%
plant component, a huge intake by modern standards.  

Westernized diets normally include only 10-20 g of fiber per day.  
Consequently, it is useful to consider a traditional, non-westernized modern
diet from Zaire where the only domesticated component of the diet is
cassava, a tuber very low in fiber (Pagezy, 1990).  The rest of the diet is wild,
either game or wild plant food.  

Fiber levels have dropped from 33.6% NDF for chimpanzees to 18.5% for an
ancient hunter-gatherer diet to about 9% for these modern but traditional diet
containing meat but dominated by a low fiber root crop, cassava.  

We suggest that this pattern of fiber reduction represents what happened in
the transition from a chimpanzee-like ancestor to modern humans.  

With this information regarding the plant part of a chimpanzee diet and the
progressively lower fiber diet of later hominids, what can we project
regarding the Australopithecus diet?  

Hatley and Kappelman (1980) describe how Australopithecus may have
added underground storage organs (roots, tubers, rhizomes, etc) to their
diet, a food source underexploited by apes, past and present.  

A similar scenario is also described by Wrangham and Peterson (1996).  
Underground storage organs are also more common in woodland areas, the
habitat Australopithecus were living in, as opposed to rain forest, where
chimpanzees live (Hatley and Kappelman, 1980).  

To evaluate the nutritional feasibility of Australopithecus consuming
underground storage organs, we examined the macronutrient content of
wild roots and tubers.  

Assuming that Australopithecus evolved from a frugivorous ape like a
chimpanzee, what would have been the nutritional consequences of
consuming roots as a fallback food instead of pith?  

We have already seen that pith contains 44% fiber.  And we also know that
neither chimpanzees nor humans need a high protein intake, but that
complex carbohydrates figure importantly in chimpanzee selectivity
(Wrangham, 1991).  

Consequently, what would wild roots and tubers add to the diet that piths
cannot provide?  

On average the wild roots and tubers would be marginal in protein, but there
is quite a range of values, offering the possibility for selection.  

Fat is probably adequate.  The fiber values were determined using the very
out-dated method of crude fiber analysis that always underestimates fiber

We multiplied all the crude fiber values by 2, which is a conservative
correction value.  Three would be the highest reasonable correction factor
and would have resulted in fiber values of about 24% instead of 16%.  

Nevertheless, 24% is considerably lower than what the chimpanzee diet in
our study contained, and much lower than the 44% in piths, the chimpanzee
fallback food.  

Consequently the inclusion of roots or tubers in the diet would decrease the
fiber intake of a potential consumer and increase the nutrient density of the
diet, especially the carbohydrate intake.  

In summary it appears that underground roots and tubers would make an
important nutritional addition to the diet of Australopithecus, who might have
been able to live exclusively on roots and tubers during short periods of
aboveground food scarcity.  

Furthermore, the dental and micro-wear patterns exhibited by
Australopithecus are compatible with the additions of roots to a
chimpanzee-like diet.  

They would not have needed additional protein supplement to top-up their
protein intake to safe levels.  

In addition, the lower fiber values would improve the quality of their diet.  
This does not imply that a need to decrease fiber in the diet was a driving
force in the evolution of the hominid diet.  

However, with the serendipitous addition of underground storage organs to
the Australopithecus diet and the resulting increase in the nutrient density of
the diet, the stage was set for Homo to further reduce fiber levels and further
improving the nutrient quality of their diet.  

By: Nancy Lou Conklin-Brittain, Richard W. Wrangham, and
Catherine C. Smith:
Department of Anthropology, Harvard University.

Relating Chimpanzee Diets to Potential Australopithecus Diets