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(Comparative Anatomy and Physiology Brought Up to Date--continued, Part 5B)

The same form can serve multiple/different functions.

Canine teeth. The quote by McArdle in the preceding section serves as a good introduction to this topic. Canine teeth are a good example of one form serving different functions. In true carnivores, canine teeth help in cutting the flesh of prey. However, some frugivores find large canines to be helpful in processing fruit. Regarding 3 different species of platyrrhine monkeys, Anapol and Lee [1994, pp. 250-251], note:

The primary food resource of Chiropotes santanas consists of young seeds from unripe fruit that are obtained by removing the tough pericarp with the anterior dentition (van Roosmalen et al., 1988; Kinzey and Norconk, 1990; Kinzey, 1992)...

The most predominant feature of the dentition of Chiropotes satanas is the exceptionally robust upper and lower canines, also present in both Cebus species studied here...

For primates harvesting sclerocarp, robust canines provide an advantage for opening exceptionally hard unripe fruits to gain access to seeds (Kinzey and Norconk, 1990).

Primate teeth. Also see Richard [1985, p. 190] for comments on why frugivorous primates need sturdy teeth, specifically incisors. Richard [1985, p. 194] also notes that "features of tooth morphology indicate diet habits strongly but not perfectly." Mann [1981] reports that chimpanzee and gorilla teeth are very similar despite significantly different diets.

Teeth of hominids. Mann [1981] also contrasts the sharply different analyses of the diets suggested by the dental systems of early hominids. He cites Jolly [1970, 1972], who argues for a predominantly plant diet (but non-vegetarian; Jolly specifically included vertebrate meat in the diet), versus Szalay [1975], who interprets the same dental evidence as adaptations to hunting and a meat-predominant diet. (See Mann [1981] for Jolly, Szalay citations.)

Garn and Leonard [1989] discuss the form/function connection as it applies to the dentition (teeth) of both modern and prehistoric humans. Their remarks bring the issue into clear focus [Garn and Leonard 1989 (pp. 338-339):

[E]ven the [Homo] erectus fossils did not have long, projecting gorilla-like canines or the highly convoluted wrinkled molar surfaces that suggest dependence on roots, shoots, and large leaves. Our dentitions are now at most indicative of an omnivorous but soft diet and they are equally suitable for gnawing at bones, gorging on fruit, or consuming fried rice, boiled wheat, and unleavened cakes. However, and despite their more specialized dentitions, chimpanzees prove to be far more omnivorous in the wild than we had earlier reason to imagine. (3)...

We therefore grant the [Homo] erectus fossils a considerable year-round supply of animal flesh, but we have no knowledge of how much vegetation was also dug, pulled, picked, or stripped...

Climatic data and faunal associations indicate that these earlier people of our genus were not browsers. They correspond to the notion of "early man" as hunters, at least in part...

Thus we see that a particular form can serve multiple functions. As we will discuss in a later section, the human hand is an excellent example of one form that can serve a myriad of functions.

Subtle changes in form can produce or support significant changes in the function of organs or body systems.

Milton [1993, p. 89] reports:

This research further shows that even without major changes in the design of the digestive tract, subtle adjustments in the size of different segments of the gut can help compensate for nutritional problems posed by an animal's dietary choices.

The above quote is even more relevant in light of the fact that human (and other primate) guts are "elastic" and their dimensions can change in reaction to dietary changes. This will be explored in a later section.

Richard [1995] notes that juvenile apes have well-differentiated cusps in their molars, while adult apes have flat molars that display heavy wear. However, the larger teeth and stronger masticatory muscles in the adults may serve the same purpose as the weaker muscles and differentiated cusps in the juveniles. This is an example of soft-tissue changes compensating for hard-tissue changes, i.e., tooth wear.

The function served by a particular form can vary dramatically according to specific feeding behavior.

We have already discussed examples of this: the platyrrhine monkeys using canine teeth to eat hard fruits, and the discussion of the considerable versatility of human dentition. Additional examples are:

We will see later that failure to consider differences in feeding behavior makes major parts of the comparative proofs of diets invalid.

Analysis of dietary adaptations is non-trivial.

However, some of the comparative "proofs" themselves do border on the trivial. More seriously, though, as Richard [1995] points out, evolutionary adaptations are compromise solutions that reflect the effect of multiple selection pressures, applied simultaneously. For example, a large gut might assist digestion. However, if the gut size increases sharply and in a disproportionate manner, the animal might not be able to outrun predators. Such an animal would quickly go extinct. This illustrates that one cannot look at single organs or subsystems in isolation. Rather, one must look at the whole organism, or at least attempt to do so.

An alternate way of stating the above is that adaptations (morphology) are the solutions to multivariate (multiple-variable) optimization problems, where:

Diets can temporarily change with habitat in the same species. To illustrate the latter point: a species may evolve under a specific diet, then be forced to change the diet (within the implicit range of adaptation) because of habitat changes driven by climate change. As Richard [1995, p. 190] notes:

But environments do change, sometimes quite rapidly, and an animal is not necessarily specialized to eat the foods we see it feeding on now; a species can change its diet (within limits, at least) faster than it can change its teeth.

Staple foods vs. critical but more rare foods. Richard [1995] also notes the difficult problem of determining which selection pressure is more relevant in analyzing adaptation--the consumption of a dietary staple, or a food that is consumed rarely but that is the sole source of specific nutrients, e.g., vitamin B-12.

The above reflect why examination of all the evidence of morphology, the fossil record, and paleoclimate are important in assessing adaptation.

Phylogenetic (structural) challenges in comparative studies.

The form of digestive features can be affected by non-diet factors. The comparison of animal features using data from individual species that belong to different taxonomic groups presents subtle but important technical (statistical) difficulties in an analysis. As Pagel and Harvey [1988, p. 417] note:

[A]ny comparative relationship obtained from an analysis that includes more than one taxon [i.e., phylogenetic category] may be explained, in principle, by any other factors that vary across those taxa...

For example, McNab (1986) pointed out that size-corrected metabolic rates of species of mammals are associated with diet. Elgar and Harvey (1987) however, show that McNab's results cannot be easily be separated by phylogeny...

If the character of interest varies only at a very high taxonomic level it may be extremely difficult to separate the favored explanation for the character from many other explanations associated with phylogenetic differences.

The basic problem here is that the level of aggregation [grouping] of the raw data used in the analysis interferes with the statistical independence required for analysis. (On top of which, the possible existence of common evolutionary ancestors for the species being compared may make the analysis even more complicated.) Readers interested in the statistical details should consult Pagel and Harvey [1988], and also Elgar and Harvey [1987] (as cited in Pagel and Harvey [1988]).

Difficulty of untangling cause and effect. The importance of the above as it relates to simplistic comparative "proofs" of diet is that it raises the question of cause and effect. In the comparative "proofs," are all of the observed differences in features really due to diet, or are some of them simply artifacts of cross-taxa (between-group) comparisons? That is, if the features in question may be impinged on, or shaped, at least in part, by things that affect a range of features in that taxonomic class beyond diet itself, then one must be able to sort out the cause and effect relationships, or valid conclusions cannot be drawn.

Counterexamples to the Paradigm
of Comparative Proofs of Diet

As briefly explained above, the comparative proofs of diet are subject to a number of logical and structural limitations. To further illustrate this, let's examine now two animals that illustrate the importance of feeding behavior, and how it can invalidate simplistic comparative proofs that assume the form/function linkage is strict.

Polar Bears: an example of semi-aquatic feeding behavior

The polar bear, Ursus maritimus, physically resembles its relatives, the brown bear and grizzly. Genetically, polar bears evolved from brown bears; see Talbot and Shields [1996], also Zhang and Ryder [1994] for information on the genetic similarities. A limited comparative study of polar bears vs. brown bears might find some evidence that the polar bear is more carnivorous than the brown bear. However, more importantly, without knowledge of the actual--and unusual, for bears--feeding behavior of the polar bear, one might conclude from a comparative study that the polar bear "should" be a predominantly terrestial animal rather than the semi-aquatic animal it actually is. (The primary evidence for aquatic adaptation by the polar bear is webbed feet; however one could presume that is an adaptation for walking on slippery ice, rather than swimming.)

How aquatic is the polar bear? Stirling [1988] describes an aquatic stalking procedure commonly used by bears in hunting seals (a major food for them). One type of aquatic stalk is done by swimming underwater, in a stealthy manner, then exploding onto the ice by the seal (often the seal escapes). In another type of stalk, the polar bear lays down flat on the ice and slides along shallow water-filled channels on top of the ice. Stirling also describes how polar bears capture sea birds by diving underneath and biting from below.

Garner et al. [1990] fitted 10 female polar bears with satellite telemetry collars in 1986 and tracked their movements on the Arctic pack ice floes, near Alaska and Siberia, for over a year. They calculated a minimum distance traveled by the bears on the shifting pack ice of the Arctic as ranging from 4,650-6,339 km (2,888-3,937 miles) over time periods ranging from 360-589 days. They note [Garner et al. 1990, p. 224], "[B]ears must move extensively to maintain contact with the seasonally fluctuating pack ice."

Thus we observe that polar bears are indeed semi-aquatic in behavior, able to travel long distances over ice and open water. Yet polar bears have almost none of the adaptations for aquatic life that (carnivorous) fully aquatic mammals have. (See Estes [1989] for a discussion of the adaptations of carnivorous aquatic mammals.)

The polar bear illustrates how an animal can lead a semi-aquatic life, with very few aquatic adaptations, simply via adaptive behavior. That is, adaptive behavior allows the polar bear to overcome what appears to be a limit (of sorts) on morphology. We will see later that the comparative proofs of diet make the serious mistake of ignoring adaptive human behavior.

The Giant Panda: a "carnivore" on a bamboo diet

The giant panda, Ailuropoda melanoleuca, is a member of the carnivore order, but the diet of the panda is predominantly plant food. Schaller et al. [1989, pp. 218-219] note that, "Pandas may consume plants other than bamboo, and they also eat meat when available (Hu 1981; Schaller et al. 1985)... F. scabrida bamboo was the principal food of most pandas."

Gittleman [1994] provides a comparative study of the panda. Citing Radinsky [1981], Gittleman notes that an analysis of panda skulls and bone lengths finds they are similar to other carnivore species. The life history traits, however, are significantly different for pandas (vs. other carnivores). Recall that life history traits are ignored by the comparative proofs of diet.

Let's now briefly consider the characteristics of the panda that suggest a coarse, largely herbivorous diet. Raven [1936], as cited in Sheldon [1975], lists the adaptations as: lining of the esophagus, thick-walled stomach, small liver, large gall bladder, and pancreas. Gittleman [1994] and Sheldon [1975] both mention the large molars and masseter muscles of the panda (for mastication). Gittleman [1994, p. 458] also mentions the "enlarged radial sesamoids (the so-called panda's thumb) used for pulling apart leaves."

So, a comparative study of pandas, if done in sufficient depth, might suggest that the panda diet probably includes substantial amounts of coarse vegetation. However, without knowledge of the habitat and actual feeding behavior, such a study would not constitute proof that the actual diet of the panda "should" or "must be" close to 100% bamboo. (The radial sesamoids are slim evidence on which to conclude that the diet is almost purely bamboo; one would be figuratively "hanging by the thumbs," or "hanging by the sesamoids" on such limited evidence.)

The example of the panda illustrates the problem of "resolution levels." A comparative study might suggest that the panda eats considerable vegetation, but such a study does not have the resolution to "prove" that the panda "must" eat a diet that is 100% bamboo vs. a diet that is 75% bamboo + 25% other foods. This is relevant because there are fruitarian extremists who suggest that the bogus comparative "proofs" they promote suggest the human gut is specialized for a nearly 100% fruit diet (vs. for example, a diet that is 60% fruit + 40% other foods). Such claims are unrealistic, not only because of the oversimplistic nature of the comparisons themselves as discussed earlier, but also also given the low resolution of the comparative "proofs."

Polar Bears and Pandas: Epilogue

The examples of the polar bear and panda illustrate that feeding behavior can:

Section Summary

Some of the major logical and structural limitations on comparative proofs of diet are:


(What Comparative Anatomy Does and Doesn't Tell Us about Human Diet)

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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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