(Comparative Anatomy and Physiology Brought Up to Date--continued, Appendix 4)
|APPENDIX 4: Statistical methods for determining gastrointestinal (GI) quotients in Martin et al. 
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- Martin et al.  eliminate the differences in body size via the use of quotients (defined below); however, allometric (non-linear) scaling models are used to calculate estimated organ sizes for the GI quotients.
- Using major axis methods, linear equations were fitted between the (dependent variable) log of the surface areas of the stomach, intestines, cecum, and colon, and (independent variable) log of body weight. Such equations relate the size (surface area) of each organ to overall body weight, in an equation analogous to Kleiber's Law. They found a slope of 0.75 (i.e., Kleiber's Law) to be within the 95% confidence intervals for the data set for each GI component, except for the colon. They chose to use 0.75 for producing estimates for the colon anyway, for consistency with the other three gut components, which scaled according to Kleiber's Law.
- They next calculate 4 GI quotients for each animal in the study. These are defined as:
surface area of GI component--actual
surface area of GI component--predicted
where the predicted surface area is based on actual weight and scaling equations (above) similar to Kleiber's Law. Separate GI quotients are computed for the stomach, intestines, cecum, and colon. Note that quotient values greater than 1 indicate actual surface area is greater than predicted, while quotient values less than 1 indicate that actual surface area is less than predicted.
- Multivariate (simultaneous multiple-variable) techniques were used to attempt to group the data set (4 GI coefficients for each animal in the sample) in a way that would provide insight into the actual diets of each animal, as follows:
- The technique of multi-dimensional scaling (MDS) was used to analyze the multivariate GI-quotient data set. The results from MDS roughly agreed with the dendrogram results (below), but were difficult to interpret because a large group of primates clustered together.
- A dendrogram--a "tree" diagram--based on distances between points in the study data set was calculated. (See Figure 11, p. 81 of Martin et al. .) Developing a dendrogram is the first step in a procedure known as clustering. The object of clustering is to determine groupings for multivariate data. The next step is to examine the possible groupings in the dendrogram to see if the clusters can be interpreted in a meaningful way.
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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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