Anatomical Differences

Comparative anatomy is a valid scientific tool, but simplistic applications are often fallacious. The basic question for our hypothesis is not whether comparative anatomy and physiology are valid tools, as they clearly are, but whether or not we accept that the species "dog" is an omnivore as expressed by many, or facultative carnivore, as advocated by us.
Evolutionary Nutrition for Cats and Dogs

Anatomical Differences between carnivores, omnivores and herbivores.

To explore our hypothesis and validate our assumptions, we need to explore some areas of anatomical differences​1​ between herbivores, omnivores and carnivores. An article titled “The Comparative Anatomy of Eating” by [Milton R.] Mills, M.D.​2​ , hotly debated and feverishly contested, demonstrate how much controversy can be found when discussing anatomic differences or similarities. To be fair, it should also be pointed out that some of those presenting comparative studies openly admit that such studies do not provide hard proof of the … (natural human, omnivore, herbivore, or frugivorous diet), but instead merely contribute evidence in favor of the particular diet they advocate.

Comparative anatomy is a valid scientific tool, but simplistic applications are often fallacious. The basic question for our hypothesis is not whether comparative anatomy and physiology are valid tools, as they clearly are, but whether or not we accept that the species “dog” is an omnivore as expressed by many, or facultative carnivore, as advocated by us.

We neither subscribe to, nor advocate the scientific correctness of the article cited. By understanding which anatomical features are associated with each kind of diet, we are able to simplistically classify animals as:

  • carnivore (meat eaters);
  • herbivore (plant eaters);
  • omnivore (meat and plant eaters).

According to Auburn University College of Agriculture, one can also classify animals based on their digestive physiology.

For example, nonruminant animals include (based on digestive physiology):

  • pigs – nonruminant animals that are omnivorous, thus consume both plant and animal matter;
  • poultry – nonruminant that are omnivorous, and they have complex foregut and simple intestinal tract;
  • dogs and cats – nonruminant animals that are carnivorous;
  • horses and mules – nonruminant animals, but they are herbivorous and have rather large and complex large intestines;
  • rabbits – a nonruminant animal that is a herbivore with a complex large intestine.

Examples of ruminant animals include (based on digestive physiology):

  • Capable to consume and digest plant materials and classified as herbivores;
  • Include cattle, sheep, goats, deer, games, elk, and many other wild species.

For the purpose of our hypothesis, we focus on several areas including comparative anatomy.

CarnivoreOmnivoreHerbivore
Physiological food: MeatPhysiological food: Meat, fruits, vegetables and nutsPhysiological food: Plants, grass and herbs
4 paws with claws4 paws with claws / hooves4 paws with hooves
walk in 4 pawswalk in 4 pawswalk in 4 paws
mouth open largemouth open largemouth open small
great sharp fangsgreat sharp fangsrudimentary, blunt canines
short and pointed incisorsshort and pointed incisorsbig and flattened incisors
blade shaped molarsblade shaped / crushing molarsflattened and strong molars
lower jaw embedded inside of the top, no lateral or forward mobilitylower jaw embedded inside of the top, no lateral or forward mobility / minimalupper jaw sits on the bottom, great lateral and forward mobility
shear. swallow without chewingshear. swallow / crushingno shear, chew much
small salivary glandssmall salivary glandsbig salivary glands
acid saliva without ptyalin (salivary amylase)acid saliva without ptyalin (salivary amylase)alkaline saliva with ptyalin (salivary amylase)
acid urineacid urinealkaline urine
renal secretion of uricaserenal secretion of uricasenot secrete uricase
strong hydrochloric acidstrong hydrochloric acidweak hydrochloric acid
does not require fiber to stimulate peristalsisdoes not require fiber to stimulate peristalsisrequire fiber to stimulate peristalsis
metabolize large amount of cholesterol and vitamin Ametabolize large amount of cholesterol and vitamin Ametabolize small amount of cholesterol and vitamin A
sweat glands in the paws; gasp to cool the bloodsweat glands in the whole bodysweat glands in whole body
intestine from 1.5 to 3 times the body lengthintestine 3 times the body lengthintestine 20 times body length
colon short smooth alkalinecolon short smooth alkalinecolon long complex acid
not metabolizing cellulosenot metabolizing cellulosemetabolize cellulose
complete digestion 2 to 4 hourscomplete digestion 6 to 10 hourscomplete digestion 24 to 48 hours

It should be emphasized, and be very clear to our readers, that comparative anatomy and comparative physiology are legitimate, useful, and important tools for scientific inquiry and research. In particular, the following are relevant:

  • Comparative anatomy and physiology are used, legitimately, in biology, paleo-anthropology [Wikipedia], and other fields.
  • It is not the intent of this essay to criticize, minimize, or unfairly criticize the valuable, and importance, disciplines of comparative anatomy and physiology.

In our minds, this classification simply helps define which foods the animal is actually adapted to eat, enabling us to resolve the concept of “biologically, species appropriate, real food diets” as applied to the species or animal. WikiBooks discuss some of these topics in greater detail in book article titled “Anatomy and Physiology of Animals” [WikiBooks].

The second part of our hypothesis resolves around the concept that the species “dog” has lived in the shadow of the “wolf” for far too long. It is true that the “dog” is a direct descendant of the grey wolf (Canis lupus), with evidence that lots of different wolves fed into the dog gene pool over the millennia. In the course of dog domestication, however, their behavior, morphology and physique has changed, and differences among dog breeds are indeed astonishing.

The species “dog” is the most phenotypically (observable physical or biochemical characteristics of an organism, as determined by both genetic makeup and environmental influences) diverse mammal, and more than 400 breeds are recognized today (depending on which body you reference). The difference in cranial and skeletal proportions among the “dog” breeds exceeds that among wild Canids, or their wild cousins, and considerable differences in behavior and physiology are also evident. These many different breeds were created along the centuries, being adapted to purposes such as herding, guarding, hunting or just companion pets. All these breeds show a wide range of phenotypes unmatched by any other species. For example, the Great Dane is up to 100 times heavier than the Chihuahua, and the Greyhound is eight times larger than the Pekingese​3​.

Imagine if future paleontologists were to find Chihuahua remains in the fossil record: this animal would appear to have little in common with wolves. Even though dog breeds differ drastically in their appearance and other characteristics, most of their genomes are still very much alike. Comparing different breeds​4​, most of their genomes indeed show only little differentiation. In other words, Chihuahuas and Great Danes are overall very similar to one another​5​, but no longer a “wolf”, perhaps more a “wolf-like” Canids. Street dogs are a vivid illustration of this point – they show how the distinct gene pools of dog breeds can rapidly mix once the restrictions of breeding are removed. Moscow’s famous feral dogs have existed separate from purebred pets for at east 150 years now, and in South Africa, the Africanis [Africanis] breed has become very popular. There is also our special case, the African Wild Dog [Wikipedia], which can easily be confused with Africanis breeds, but yet, is not directly related to the species “dog”.

Genetically, dogs have cognitive and brain function differences from wolves and their other wild cousins. However, where the genetic divergence of the “dog” and “wolf” took place remains controversial. There is genomic signature in domestic dogs that suggests mutation in intestinal starch digesting enzymes​6​ , potentially allowing dogs to “better” digest carbohydrates than their wild ancestors. But is this just true for dogs? Is this mutation an evolutionary mutation, or perhaps by design for survival? Besides the evolutionary time span, there also is much dispute about what genetic similarities (or differences) mean. Morphologically, dogs and wolves share a number of traits because they all belong to the species “Canids”. Generically, they are 99.8% similar in their mtDNA, which supports the notion that dogs were domesticated from wolves. Wolves and coyotes on the other hand are only 96% similar, or stated differently, they differ genetically by about 4%. But dogs and foxes only differ with 0.5%. For example, we differ by about 2% genetically from chimpanzees – but huuman males and females also differ genetically by about 2% (men have 2% less DNA). Can we learn about humans by studying chimpanzees? Can we learn about men by studying women? In other words, is 2% a little or a lot?

(Phylogenetic relationships and divergence times among six carnivore species. Wayne et al. 1997; Wayne and Ostrander 1999; Wayne and Vila 2001) ((The scale bar is in million years (Ma))

Before you try to answer that, think about this: genetically humans differ by only about 15% from rabbits, and about 11% from dogs. We think it’s reasonable to say that a rabbit is more than seven times as different from a human female than a human male is, but on the other hand, is it (or is it not) seven times more different from us as a chimpanzee is?

In the end, we still don’t really know, and this doesn’t matter. Apparently, very small genetic differences can lead to vastly differing behavior. Each species will behave in its own way, regardless of its genetic similarity to some other species. For a longer or shorter time, the species “dog” has been living in its own ecological niche and has become adapted to that niche. No matter what it started out as, and no matter when it stopped being a whatever else it was, the species “dog” is now a dog as far as we are concerned.

We explore some of these anatomical differences in greater depth.

References and Research

  1. 1.
    Dr. Whitson’s Bio Lab Review. Northern Kentucky University. https://www.nku.edu/~whitsonma/Bio120LSite/Bio120LReviews/Bio120LAnimalRev.html
  2. 2.
    The Comparative Anatomy of Eating, Milton R. Mills, M.D. Animals Deserve Absolute Protection Today and Tomorrow (adaptt.org). http://adaptt.org/archive/Mills%20The%20Comparative%20Anatomy%20of%20Eating1.pdf
  3. 3.
    Middleton RP, Lacroix S, Scott-Boyer M-P, et al. Metabolic Differences between Dogs of Different Body Sizes. Journal of Nutrition and Metabolism. Published online 2017:1-11. doi:10.1155/2017/4535710
  4. 4.
    Parker HG. Genomic analyses of modern dog breeds. Mamm Genome. Published online January 10, 2012:19-27. doi:10.1007/s00335-011-9387-6
  5. 5.
    Doggie Diversity. Arizona State University. https://askabiologist.asu.edu/plosable/dna-dogs
  6. 6.
    Reiter T, Jagoda E, Capellini TD. Dietary Variation and Evolution of Gene Copy Number among Dog Breeds. Barendse W, ed. PLoS ONE. Published online February 10, 2016:e0148899. doi:10.1371/journal.pone.0148899

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