Note: The above papers are lengthy and provide considerable detailed, technical information. Only a summary of the major points in each analysis is provided here. Those with a specific interest in gut morphology can learn much from the above papers. And frankly, if you do read the papers cited above, you will realize just how simplistic the analyses presented in the typical comparative "proofs" of diet, really are.

• GI tracts from numerous species were analyzed. Gastrointestinal (GI) tracts were analyzed from 180 individuals of 78 different species; 112 of the individuals in the analysis were primates.

• Morphology points to 3 basic dietary groupings. Differences in morphology reflect adaptation to different diets. Foods can be classed into 3 groups, by structure and composition:
  • Animal matter, including invertebrates and vertebrates. Such foods are easily digested, and a short, simple gut is adequate for such foods [faunivory].

  • Fruits--the reproductive parts of plants, including seeds and tubers. Fruits are high-sugar foods, and tubers are high-starch foods that are converted into sugar in digestion; such foods are readily digested and absorbed in the intestines [frugivory].

  • Leaves--including the structural parts of plants, i.e., grasses, stems, bark, and gums. These foods require fermentation in a large stomach or the large intestine/colon [folivory].

• Dietary categories reflect a continuum, not sharp divisions. The categories of dietary adaptation described above--faunivory, frugivory, folivory--are not discrete (distinct or black-and-white) categories, but reflect a continuum that stretches from animal foods (hard to catch but easy to digest) to folivory (easy to find but hard to digest), with frugivory an intermediate form (fruit is available, but often only in limited quantities).

• Typical gut of faunivores. Chivers and Hladik [1980] describe the typical gut pattern for a faunivore (p. 338):

  The basic pattern of gut structure among faunivores consists of a simple globular stomach, tortuous [containing many twists/bends] small intestine, short conical caecum, and simple smooth-walled colon.

• Folivore gut characteristics. Regarding folivores, the long-chain carbohydrates found in leaves and structural plant parts require bacterial decomposition (fermentation) for digestion and assimilation. There are two primary adaptations for fermentation: chambers in the fore-gut (stomach) or mid-gut (caecum and colon).

• Frugivore guts are variable. Regarding frugivores, Chivers and Hladik [1980, p. 340] note that:

  This group contains most primates, but none of them subsist entirely on fruit. All frugivores supplement their diets with varying amounts of insects and/or leaves, but have no distinctive structural specialization in the gut, although its morphology may show considerable variation between species.

  Contrast the above real-world information with the position of certain fruitarian extremists who claim that a nearly 100% fruit diet is optimal/most "natural" for humans. In actuality, it appears there is no such thing in nature as a 100%-fruit frugivorous primate.

• Faunivory and folivory are the endpoints with frugivory intermediate. Pure faunivory and pure folivory reflect the two extremes of adaptation. However, as diets are a continuum, Chivers and Hladik argue that putting all intermediate diets in the (too broad) category of omnivores is inappropriate. They note [Chivers and Hladik 1984, p. 216]:

  No mammal mixes large quantities of both animal matter and leaves in its diet without including fruit. Since faunivory and folivory seem to represent contrasting and incompatible [morphological] adaptations, the quantity of fruit in such a mixed diet is always considerable.

  The objection of Chivers and Hladik to the term omnivore appears to reflect a different view of the definition of the term than is used by many other authors. This will be discussed later in this section.

• Determining/measuring relevant gut characteristics is non-trivial. Chivers and Hladik [1980] includes a discussion of the major challenges involved in measuring the relevant gut parameters--volume, surface area, etc. Additional challenges include: the gut itself is elastic (i.e., it may stretch when you try to measure it), consists of irregular shapes which may be complicated by folds or specialized structures, and so on. In the real world, choosing what parameters to measure and how to measure them, in the area of gut morphology, is decidedly non-trivial. The simplistic comparative proofs of diet don't bother to tell you such details.

• Basic raw data collected: For the subject animals, the sizes of stomach, large intestine, and small intestine were all measured in terms of surface area, weight, and volume. Data on overall size of the animal--length and weight--were recorded as well.

  Chivers and Hladik [1980] includes a discussion on the importance and reliability of the various types of measures--surface area, weight, volume--and conclude that volume is the least reliable measure because of confounding factors--i.e., it is hard to distinguish and compare the volume of a fermenting chamber in a folivore versus the volume of absorbing regions in a faunivore.

• Gut differentiation coefficients calculated. Given that the primary gut morphology specializations related to diet range from a gastrointestinal (GI) tract dominated by the small intestine in faunivores, to a GI tract dominated by fermentation chambers in the stomach and/or cecum or large intestine (colon) in folivores, Chivers and Hladik define and calculate coefficients of gut differentiation as:
  measure of stomach + caecum + colon
  measure of small intestine
  where measure = one of weight, volume, or surface area. In other words, the numerator in the equation above represents a measure of the structures that tend to dominate in more folivorous species. The coefficient of gut differentiation compares the numerator against (divides it by) the denominator, which is a measure of the feature (small intestine) that dominates in species with more faunivorous (animal-based) diets. The resulting coefficient provides a simple (though crude) measure of gut morphology specialization.

  Coefficients of gut differentiation were calculated for each species in the study, and the results plotted by dietary specialization. In regard to the classification of animals by dietary specialization, Chivers and Hladik [1980, p. 377] note:

  While recognizing the special significance of the gross dietary categories to which each species can usually be assigned, particularly the most specialized forms, we have tried to avoid any implication that a classification into faunivores, frugivores, and folivores reflects exclusive diets.

• Size scaling to compare species adjusted according to body length. To be able to compare gastrointestinal tracts across species, the raw data was adjusted for body size--in this case, body length. (Note: depending on the source of data, body length might not be as accurate an indicator as adjusting for body weight--which scales better with the body's metabolic demands. The later research papers of Martin et al. [1985] and MacLarnon et al. [1986], which we'll examine shortly and which are extensions of the research described in Chivers and Hladik [1980, 1984], does use body weight to adjust for size differences between species.)

• Index of gut specialization. Incorporating this scaling adjustment, the GI tracts studied were analyzed for the three different dietary classes (faunivore, frugivore, folivore) to produce a numeric index--an index of gut specialization. (For details on the methodology used to derive the index of gut specialization, see Appendix 3.) The index of gut specialization is an attempt to produce an index that reflects the apparent morphological specialization for the animal in question. That is, it is intended to indicate what the animal's morphology suggests the animal's diet might be.

• Dietary index: what the animal actually eats. Having developed the index of gut specialization, which suggests what the animal might eat, Chivers and Hladik then turn their attention to the issue of what the animals actually do eat. As the three main dietary specializations are leaves, fruit, and fauna, they use a triangular plot whose 3 vertices represent 100% faunivory, 100% frugivory, and 100% folivory. Use of a triangular plot provides a nice 2-dimensional plot for the 3 dimensions required. Each animal is plotted in the triangle according to its actual diet. An update of the original graph [Chivers and Hladik 1980], from Chivers [1992], is as follows.

• Chivers and Hladik assert that overlaying an x-axis on the triangular plot provides the best univariate (i.e., one number per animal) summary description of actual diets. Note that because the plot is triangular and a projection of a 3-dimensional plot, the scales are more complicated than in a standard rectangular plot. In this case, the x-axis is defined as:

  x = (% leaves) - (% animals)

  The value of the x-axis for each point in the triangular plot of dietary characteristics (above) is used as the dietary index, an approximate indicator of the actual diet of the animal. Note also that the three percentages (fruit, leaves, animal matter) are not independent, as they must add up to 100%.

• Chivers and Hladik then compare the dietary index values (which indicate actual diet) against gut specialization index values (which indicate what diet would be suggested by gut characteristics) for 8 selected species. The results of the comparison are "good" (i.e., some similarities between the indices of diet and gut specialization). This provides some supporting evidence for the analysis.

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SEE REFERENCE LIST

SEE TABLE OF CONTENTS FOR:
PART 1 PART 2 PART 3 PART 4 PART 5 PART 6 PART 7 PART 8 PART 9

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GO TO PART 1 - Brief Overview: What is the Relevance of Comparative Anatomical and Physiological "Proofs"?

GO TO PART 2 - Looking at Ape Diets: Myths, Realities, and Rationalizations

GO TO PART 3 - The Fossil-Record Evidence about Human Diet

GO TO PART 4 - Intelligence, Evolution of the Human Brain, and Diet

GO TO PART 5 - Limitations on Comparative Dietary Proofs

GO TO PART 6 - What Comparative Anatomy Does and Doesn't Tell Us about Human Diet

GO TO PART 7 - Insights about Human Nutrition & Digestion from Comparative Physiology

GO TO PART 8 - Further Issues in the Debate over Omnivorous vs. Vegetarian Diets

GO TO PART 9 - Conclusions: The End, or The Beginning of a New Approach to Your Diet?

Back to Research-Based Appraisals of Alternative Diet Lore

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