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(Comparative Anatomy and Physiology Brought Up to Date--continued, Appendix 3)

APPENDIX 3: Methodology of Chivers' and Hladik's index of gut specialization

Allometric scaling considerations

To compare the dimensions of gastrointestinal tracts across species, the raw data obtained must be adjusted for body size (referred to as an allometric scaling; see Martin [1992] for an introduction to the topic of scaling). To determine the nature of such an adjustment, volume and surface area (of the components of the specimen GI tracts) were regressed (ordinary least squares) in logarithmic equations, fitting separate equations per dietary grouping against a measure of body size--the cube (3rd power) of body length. Note that the classification of animals into dietary groups was done manually, based on the information available on each animal's diet.

A logarithmic equation was used because of Kleiber's Law. Recall that Kleiber's Law relates metabolic energy (dependent variable) to body weight (a measure of size; independent variable), via an exponential/logarithmic equation. Note that the GI tract is the input source for the body's metabolic energy. Hence, by regressing, in log form, a measure of the size of a GI tract component (volume or surface area) versus body size (body length cubed), the resulting equation is analogous to, and in the general form of, Kleiber's Law.

Computing the index of gut specialization

To analyze the differences in gut morphology of faunivores vs. folivores, Chivers and Hladik next regressed the log of volume of (stomach + caecum + colon) versus the log of body size (body weight, estimated using a formula based on body length, cubed), separately, by dietary categories. The structure of the analysis--regressing faunivores separately from folivores, reflects the structural physical limits of the 2 possible extreme diet strategies/morphological adaptations (taking body size as fixed)--i.e., the faunivore adaptation for an animal with the given body size vs. the folivore adaptation appropriate for an animal with the given body size. Note that Chivers and Hladik actually did separate regressions here for folivores, faunivores, and frugivores. From the regressions, they develop an index of gut specialization, which simply scales the distance between the three regression lines.

The index of gut specialization is computed by taking the value, for a given body weight, of the frugivore regression line as 0, the folivore line as 1, and the faunivore line as -1. These values are then rescaled, via a non-linear scaling, to produce an index whose values are in the range -100 (faunivore) to 100 (folivore). Similar methods are applied to produce indices based on surface area, rather than volume, to yield two different values for the index of gut specialization for each animal.

Thus the index of gut specialization is an effort to produce an index, that is, one single number, that measures the degree of "gut specialization" for an animal. That is, it is an index whose value varies according to the morphological adaptations displayed by each animal, i.e., whether the animal has the adaptations typically associated with folivores, faunivores, or frugivores.

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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?

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