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Issues in primate foraging and diet. Review of foraging strategies and natural diet in monkeys apes and humans edited by Andrew Whiten and Elsie M. Widdowson

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American Journal of Primatology 33155-159 (1994)
Issues in Primate Foraging and Diet
Review of Foraging Strategies and Natural Diet in Monkeys, Apes and Humans, edited by
Andrew Whiten and Elsie M. Widdowson. New York, Oxford University Press, 1992, Vlll
138 pp, $65,cloth.
These are the proceedings of a Royal Society discussion meeting held in 1991.
The papers are generally of good quality, and I found much of interest in the book.
I recommend without hesitation that libraries of primatology or human evolution
obtain a copy. Prospective individual buyers should realize, however, that despite
its title, the book is not actually an analytical review of how primate species in
general go about finding and recognizing essential food items. First, it addresses
mainly human diet. The nonhuman primates examined in detail (chimpanzees,
gorillas, and baboons) are those thought likely to illustrate the background from
which human feeding patterns emerged. Second, the emphasis of the book is primarily on diet and nutrition rather than on foraging “strategies” at the behavioral
level. Thus, more space is devoted to analyses of food composition than to the
foraging tasks posed by the spatial distribution of nutrients in the field or the
detailed behavioral responses which primates use to solve these tasks. The volume
contains few quantitative descriptions of travel paths or of species-specific activity
patterns, and there is no in-depth review of proximate mechanisms underlying the
selection or avoidance of particular food constituents by primates. In sum, readers
interested in choice behavior rather than in nutrition as such would have expected
a more direct focus on the specific, perceived feeding options which primates face,
and on the patterns of choice which imply a particular type of evaluative or “decision-making” process.
The book contains 14 chapters in four sections. The individual papers are very
heterogeneous in content and could have been published in separate journals.
Section 1 includes a review of nonhuman primate diet and nutritional requirements, two papers on the diets of present day African apes, and a report on some
of the chemical and seasonal factors influencing dietary choices in baboons. Section 2 contains two papers on the archaeological record of human diet and one
paper on the energetics of hominid encephalization. Section 3 examines the diet
and food gathering patterns of present day hunter-gatherer groups. Section 4 contains one paper on dietary change in humans at the beginning of agriculture and
two general papers on nutrition in contemporary humans.
The editors view the accurate measurement of nutrient requirements, of nutrient intakes, and of the quantitative relations between these two variables as one
of the cornerstones of primate dietary research. In this respect, Whiten and Widdowson align themselves with previous editors and with many current-day researchers. It is interesting, however, to read Oftedal’s critical appraisal of this
research aim and his review of the pertinent literature. Although the methods of
the reviewed studies are not always described sufficiently for readers to interpret
the findings adequately, one message is that the accurate measurement of primate
0 1994 Wiley-Liss, Inc.
156 / Menzel
nutrient requirements and nutrient intakes in the field may be very difficult to
Regarding nutrient intakes, one must record the amount of each food item type
ingested by animals, not simply the time spent feeding on certain items. More
importantly, the energy available from every type of ingested food should be determined by digestion trials rather than simply by bomb calorimetry. For example,
commercially prepared fiber is only partially digestible rather than 100% digestible. At present, there are few data available for wild foods to take the issue much
further. It is not clear from this paper how one could determine the available
energy content of non-eaten items with any accuracy, given that digestion trials
are not feasible.
Regarding nutrient requirements, Oftedal reports that the U.S. National Research Council has determined the quantitative requirements for 24 of 45-47
required nutrients for primates under laboratory conditions. It would be useful in
a future paper to explain in much greater detail the nature and adequacy of these
methods. It remains unclear from this review whether the energetic requirements
of free-ranging primates could ever be measured accurately.
The central question of Oftedal’s chapter is whether physiological requirements in the field predict observed intakes in the field. Thus, do intakes closely
match, fall below, or greatly exceed requirements? Oftedal discusses the issue
using protein as an example. Because there are few data on the protein requirements of adult primates, Oftedal is forced to estimate protein requirements based
on assumptions about the digestibility of protein and about the energy concentration of wild foods. He suggests that if these assumptions are reasonable, then
free-ranging primates are usually not protein limited and typically obtain sufficient amounts for growth and reproduction from the leaves they consume. The
analysis, however, is based only on the leaves which primates select and ingest
rather than on all available leaves. In other words, the analysis takes the animal’s
selectivity in choosing foods entirely for granted. I suggest that a further question
of interest is whether Oftedal’s conclusion holds in relation to all leaf species, such
that the the protein content of all non-eaten leaves is also higher than required. If
this conclusion holds true, then in principle animals could meet their protein
requirements by feeding a t random. This result would be interesting because in
fact, howling monkeys and baboons deviate from a random feeding pattern and
appear to select high protein items.
One implication of Oftedal’s review is that there may be gross errors in the
measurement of the energy content of uneaten “potential” foods. It would be useful
in a future report to examine the consequences of measurement errors for approaches based on optimal foraging theory. Although substantial errors might
seem to undermine the practicability of approaches which assume that animals
maximize their long-term rate of energy intake, the relative importance of the
energy content of single items depends critically on the state of other ecological
variables. The potential energy gain from including food type X in the diet is not
only a matter of the chemical composition of single items; it also depends on
morphological variables such as shell hardness and on spatial variables such as the
number of items per patch [Leighton, 19931. For example, the median number of
figs per plant can vary from 8 to 85,000 depending on the species [Leighton, 19931,
and the number of items per patch may outweigh modest differences in energy
value. In sum, the relative importance of nutritional measures versus patch size
and other variables for making theoretical predictions of behavior is in need of
Two papers on African apes are included for possible insights into the diet of
Primate Foraging and Diet / 157
the most recent ancestor shared by humans and nonhuman primates. These are
specialized data papers rather than broad reviews. Wrangham, Conklin, Chapman,
and Hunt conclude from fecal analyses that Kibale forest chimpanzees respond to
shortages of tree-fruit by increasing their intake of piths from terrestrial herbaceous stems. The authors hypothesize that piths provide a n important source of
energy from fermentable fiber. They speculate that differential reliance on piths
was correlated with the divergence of thin-enamelled African apes from other,
thick-enamelled hominoids in the Miocene.
Tutin, Fernandez, Rogers, Williamson, and McGrew present findings on diet
from a seven-year study on sympatric lowland gorillas and chimpanzees in Gabon.
Again, most of the analysis is based on fecal samples. At the Lope Reserve, both
chimpanzees and gorillas are generalized, opportunistic frugivores. There is extensive dietary overlap, with more than 90%of eaten fruit species shared in common. The authors discuss the notions that among sympatric frugivores, dietary
divergence is greatest when succulent fruit is scarce and that such divergence
between gorillas and chimpanzees a t Lope is due t o the ability of gorillas to survive
on a n almost exclusively folivorous diet. In contrast to Wrangham e t al., Tutin e t
al. do not draw special attention to the role of piths as compared to leaves or seeds
in explaining chimpanzee responses to fruit scarcity.
Baboon diet is covered by Whiten, Byrne, Barton, Waterman, and Henzi to
illustrate how a medium-sized catarrhine adapts its food selection patterns to
savannah habitats “similar to those exploited during important phases of hominid
evolution.” The paper provides a clear, interesting descriptive-analytic approach
to identifying combinations of specific food components (protein, lipids, fiber, phenolics, and alkaloids) which jointly influence diet selection. This report illustrates
the use of discriminant analysis. The method of comparing the nutrient content of
closely related plant species which are eaten versus not eaten deserves careful
The second section, “evolution of hominid foraging and diet,” is intended to
characterize the transition from the diet last shared with other primates (as described in the first section) to the diet of human hunter-gatherers (as described in
the third section). A review by Andrews and Martin infers hominrlid dietary evolution from tooth enamel thickness and, to a lesser extent, from microwear patterns. The authors provide the first published data on enamel thickness in Proconsul africanus and P. major and new data on Dryopithecus, Oreopithecus and
Graecopithecus. According t o Andrews and Martin, however, the confidence placed
in their own dietary interpretations is limited by a relative lack of fossil material
for the time of the human and African ape divergence and by a degree of uncertainty over the functional significance of thick enamel.
A report by Blumenschine examines nutritional aspects of scavenging in early
humans and the question of whether transport and sharing of carcasses occurred
on a regular basis. Data on carcass consumption sequences by living carnivores,
and estimates of carcass parts that could have been available to early human
scavengers, are used to interpret Plio-Pleistocene archaeological bone assemblages
at Olduvai Gorge. The paper assumes a fair degree of familiarity with the topic. if
not a working knowledge of the author’s prior studies. Additional explanation of
methods would have been useful to help readers interpret the data tables and the
many inferences which are made on the basis of indirect evidence. Blumenschine
concludes that marrow (fat) yield was more important than flesh (protein) yield in
decisions about carcass processing; he provides a discussion of hypotheses on the
socio-economic function of archaeological sites.
The third paper in section 2 by Foley and Lee treats brain expansion within a n
158 I Menzel
ecological framework. The authors examine the energetic costs of encephalization
in hominid evolution and how these costs may have been afforded. Foley and Lee
discuss meat eating, tool use, and especially a slowing of development rate as
possible ways for caretakers t o offset some of the added costs of rearing a larger
brained offspring. The paper compares the cumulative energetic costs of brain
maintenance in humans, in other hominids and in chimpanzees. Foley and Lee
base their calculations on the size of the brain a t birth and at the beginning of each
subsequent year, and they assume a stepwise increase in brain size. Recalculating
the costs as integrals of a continuous growth curve would be more accurate and
would even strengthen their case. An implicit assumption in this paper is that
differences in relative brain size (EQ) among species will correlate nicely with
differences in their learning and memory skills, even within the same taxonomic
family. This should be regarded as hypothesis rather than as established fact. No
relevant experimental data on cognitive performance are presented, and speciesspecific capabilities may not be easy to arrange in rank order along a single quantitative scale of measurement. The authors would probably agree that a careful
characterization of the diversity of learning and memory skills in primates is a
necessary half of any anatomical explanation.
The third section, “food of hunter-gatherers,” begins with a review of traditional diet in Australian aboriginal hunter-gatherers. ODea reviews how traditional dietary patterns have contributed to chronic diseases in Aborigines after
westernization. Next, Hawkes, O’Connell and Blurton Jones describe an experimental study on the benefits of large game versus small game hunting in the
Hadza of northern Tanzania. They ask why men specialize in the hunting of big
game even though 1) men obtain meat for their family more consistently by taking
small game and 2) men are allowed to take meat from big game kills even if they
do not regularly contribute t o the kills. Hawkes et al. provide a game-theoretic
analysis of this “collective action” problem and suggest that the adaptive value of
big game hunting in such cases does not lie, as is commonly assumed, in nutritional advantages to hunters and their families. The authors speculate that mating
advantages accruing to the hunters might provide the better explanation.
Milton presents data on diet in four groups of Amazonian forest-dwellers. The
systems Milton describes are complex, and the report appears to be an initial effort
to identify patterns and to raise hypotheses for future consideration. Milton suggests that there are no single-factor environmental explanations for differences in
the primary crops used and in the animal types hunted and that “more attention
should be paid t o historical factors. . . local environmental conditions and general
patterns of inter- and intra-group cultural dynamics.”
Speth briefly reviews data suggesting “the existence of an upper safe limit to
total protein intake in pregnancy” in humans and calls for comparative data on
protein selection and avoidance strategies in chimpanzees.
The fourth section, entitled “human diets: prehistory to present day,” gives
accounts of dietary change beginning with the development of agriculture. Ulijaszek reviews the nutritional consequences of the shift from hunting and gathering
to animal domestication and crop cultivation. Southgate presents a variety of data
on human food composition. Widdowson characterizes nutritional and energetic
deficiencies in modern humans. In the last two papers there is little discussion of
how evolved or acquired dietary patterns determine food choice and affect health
in modern environments.
In conclusion, this volume provides a valuable overview of some current research topics on natural diet and nutrition. I recommend it to anyone working on
primate foraging or human diets. The fact that wild primates show as few defi-
Primate Foraging and Diet / 159
ciency diseases as they do implies that they are capable of picking up a wealth of
information about the value of potential foods. My criticism of the present collection is that there is relatively little concrete discussion of the specific behavioral
tendencies and forms of “knowing” which contribute to nutritional well-being in
nonhuman primates.
Some further characterization of the environment in behaviorally relevant
terms is desirable. Even if one could describe the spatial array of available calories
and of digestible nutrients in the habitat, this would fail to describe the information which directly guides many aspects of foraging. It seems critical, for example,
to specify the types of visual information available in the structure of the habitat
for discerning the location and properties of potential foods and to show how different species pick up this information. Primates must also assess the value of
environmental opportunities in relation to present or future requirements. Some
nutritionists might be less intrigued by behavior which “anticipates” and circumvents deficiencies by means of greatly exceeding the minimum nutrient requirements than by the ability of primates to remedy imposed internal deficiencies
through changes in diet. The latter abilities, which appear more “reactive” than
anticipatory, remain largely unexplored in primates and are of great interest.
Nevertheless, if the design of a nutrient deprivation experiment sharply restricts
the animal’s sources of environmental information about changes in food quality
and simultaneously limits its feeding options, this can be regarded as examining a
special case of dietary choice, and the degree to which it models foraging tasks and
the operation of perceptual systems in the wild should be examined carefully.
Clearly, feeding is more than nutrition. Feeding patterns do not always closely
match nutrient requirements or immediate nutritional consequences, as Hawkes
et al. point out. The extent to which nutrient requirements predict preferences and
activity patterns a t the level of the whole animal remains an open question, and
the behavior of choice must be brought more into the picture.
The writing of this review was supported by Swiss National Science Foundation grant 31.27721.89.
Charles R. Menzel
Ethology and Wildlife Research
Institute of Zoology
University of Zurich-Irchel
Zurich, Switzerland
Leighton, M. Modeling dietary selectivity by
Bornean orangutans: evidence for integration of multiple criteria in fruit selection.
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