What the Genomic Signature Says about the Domestic Dogs Ability to Digest Starch

     The latest craze in canine diets is the prey model formula.  If one is looking to improve the dental hygiene and therefore the overall health of one’s dog, then adding raw meaty bones to the diet is a must.   Dogs have alkaline mouths.  This makes them five times more likely to develop periodontal disease than humans.  When food sticks to the teeth the immune system views the food as a foreign body and releases white blood cells which in turn stimulate enzymes to destroy  not only the foreign bodies clinging to the teeth but gum tissue.  This leads to bone and eventually tooth loss.  Chewing on raw meaty bones helps dogs dislodge foreign matter from the teeth and strengthens surrounding musculature. 

     The Prey Model Diet for dogs; however, is built upon a fallacy.   It is based upon the diet of wolves which genetically are the domestic dog’s closest relative.  It is a carnivorous diet.  It avoids plant materials claiming that dentition and the digestive system of dogs indicate that they have not evolved to eat plant based materials.  Is this in fact the case?

      One of the areas the Prey Model Diet points to when claiming that dogs are strictly meat and bone eaters is the dentition.   It is surmised that the canine teeth which are long and pointed indicate that dogs are carnivores.  Are teeth alone, especially the canines, a good indication of diet?  The teeth to the right are a case in point.  Is this a carnivore or an herbivore? 

If you guessed carnivore you are wrong.  This skull shows the dentition of the Gelada baboon which eats grass.  In one study which measured the strength of canines in primates, it was found that the canines of primates were as strong as that of carnivores.  Most primates eat plant based diets.  This of course is a simplification as the dentition does tell one a great deal about diet and species, but one cannot determine what an animal can and cannot process by simply looking at the  canines. 

      If one looks at a cousin of man, Neanderthal, one sees that they possessed dentition very similar to our own; however, scientists have determined that Neanderthals were almost

exclusively meat eaters.  How could they and Homo sapiens effectively eat meat when neither have carnivore dentition?  Both Neanderthals and Homo sapiens chewed in a rotating type fashion which allows the effective break down of fibrous tissue.  However unlike Neanderthal, humans consumed a wide variety of food stuffs.  Although this is certainly not the only reason Neanderthals are now extinct while Homo sapiens flourish, by limiting themselves to one food source Neanderthals  limited their options when that source became scarce.

     This is the key for survival among wild canines as well.  The more varied the diet the more likely the species will survive to reproduce.  If we look at the diet of wild canines we find:

               

Species	Diet
Wolves	Carnivore, will consume plant materials
Dingo	Omnivore
Coyote	Omnivore
Ethiopian Wolf	Carnivore, will consume plant materials
Black Backed Jackal	Omnivore
Golden Jackal	Omnivore
Side-stripped Jackal	Omnivore
African Wild Dog	Carnivore
Dhole	Omnivore
Crab Eating Fox	Omnivore
Short Eared Dog	Carnivore, will consume fruit
Culpeo	Carnivore, will consume plant materials
Darwin’s Fox	Omnivore
Hoary Fox	Insectivore, will eat small rodents and plant materials
Pampas Fox	Omnivore
South American Gray Fox	Omnivore
Sechura Fox	Omnivore
Maned Wolf	Omnivore
Arctic Fox	Omnivore
Bush Dog	Carnivore
Red Fox	Omnivore
Swift Fox	Omnivore
Ruppell’s Fox	Omnivore
Kit Fox	Omnivore
Blandford’s Fox	Omnivore
Cape Fox	Omnivore
Corsac Fox	Omnivore
Bengel Fox	Omnivore
Pale Fox	Omnivore
Tibetan Fox	Omnivore
Gray Fox	Omnivore
Bat Eared Fox	Insectivore
Raccoon Dog	Omnivore
Island Fox	Omnivore
Results	Omnivore  26 species Insectivore  2 species Carnivore  6 species  of these  only 2 appear to be true carnivores, meaning they consume only meat and bone.

      This listing of canine species demonstrates that the vast majority of them are omnivores excepting Wild Dogs and Bush Dogs.  Gray Wolves which share a common ancestor with domestic dogs are primarily carnivores but have been observed consuming plant materials including fruit, vegetation and they also raid human garbage where available.  To base a domestic dog’s diet on a prey model diet based solely on their connection with wolves is erroneous.  Looking at the fossil record scientists now believe that dogs may have accompanied humans when they lived among Neanderthals in what is now Europe during the Pleistocene period.   At a site in Predmosti in the Czech Republic remains of dogs and man were found dating to 29,000 years ago. 

Canine teeth were found with holes drilled through them indicating they were worn as ornaments and the remains of one animal was discovered with a bone place between its teeth shortly after death. [i] Thousands of years therefore separate domestic dogs from their nearest canine companion the Gray Wolf.  How have domestic dogs changed during this period?

      One area that changed at the genome level is the dog’s ability to digest starches.  Natural selection says that biological variation comes about through competition.  Favorable traits allow an organism to survive and reproduce.  These traits are inherited.  Overtime favorable traits passed on to offspring become common in that population.  This leads to later generations becoming distinct from their ancestors.  Has this happened with dogs in the area of digesting starches?

      A group of scientist recently identified “3.8 million genetic variants used to identify 36 genomic regions” that they feel are “targets for selection during dog domestication” in the gene sequencing of dogs and wolves.  They identified ten genes which they felt were associated with the dog’s ability to process starches as compared to wolves.  The scientists used pooled DNA from 12 wolves from differing locations on the planet.  They looked at the genetic material of 60 dogs from 14 different breeds.  They identified “3,786,655 putative[ii] single nucleotide polymorphisms[iii] (SNPs) in the combined dog and wolf data.  1,770,909 (46.8%) of which were only segregating in the dog pools.”  The scientists among other things looked for areas in the genes which had changed between the wolf and the dog.

     They discovered 10 genes which related to starch and fat metabolism.  They “proposed that genetic variants within these genes may have been selected to aid adaptation from a mainly carnivorous diet to a more starch rich diet during dog domestication.”  Humans begin digestion of food in the mouth through chewing.  Human saliva carries the enzyme amylase which begins the breakdown of starch.  Prey model diet supporters often claim that because the dog produces no amylase in their saliva, they cannot digest starches.  Dogs do not chew their food.  Digestion for dogs begins in the stomach and dogs produce amylase in their pancreas to aid in the digestion of starches.

     Somewhere in the distance past dogs began to duplicate the pancreatic amylase gene.  When the scientists looked at the gene in 136 dogs and 36 wolves they found that there was a 7.4 fold average increase in the expression in the dog as compared to the wolf.   Dogs can much more readily produce the amylase enzyme than can wolves.  Maltase-glucoamylase, a brush border membrane enzyme, also plays a role in the final steps of the digestion of starch.  When the scientists looked for differences between wolves and dogs in this area they found that dogs showed an increase in changing maltose to glucose when compared with wolves.  The scientists finally looked to see if the converted glucose was being absorbed through the membrane of the small intestine more efficiently by dogs than by wolves.  What they found was that only one of 19 wolves carried the inherited  gene sequence which allows the processed starch to be absorbed through the luminal plasma membrane while all dogs tested carried the haplotype[iv] with 63 being homozygous[v] and eight heterozygous[vi].    The scientist concluded “ we have presented evidence that dog domestication was accompanied by selection at three genes with key roles in starch digestion.”[vii]

     What does this mean for the diet of domestic dogs?  If one is looking for a diet based on wild canines than the evidence indicates that one would choose an omnivorous diet.  This diet allows for variety including fruits and plants as well as raw meat and bones.  Canines unlike felines taste sweets.  It is one of the reasons chocolate appeals so much to dogs and can lead to unintentional poisoning and even death.  Dogs can and do enjoy berries, water melon, carrots and many sweet tasting plant based products just as we enjoy them.  But more importantly they can digest them.  They have the enzymes needed to digest, absorb, and utilize starches.  Thousands of years separate dogs and wolves from their common ancestor.  Dogs adapted to living with man and sharing his diet and this adaptation is present in their genes today.

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[i] Pat Shipman, “Do the Eyes Have It?: Dog Domestication May Have Helped Humans Thrive While Neandertals Declined,” www.americanscientist.org, accessed April 25, 2013.

[ii] A gene is putative when no action has been assigned to it.

[iii] Genetic variation in a DNA sequence occurs when a single nucleotide in a genome changes. This is called a mutation.  Successful mutations are those that enhance an animal’s ability to survive and reproduce.

[iv] Haplotype are genes which are close together on the same chromosome but are less likely to combine

[v] Homozygous means having two identical genes at corresponding locus (locations) on homologus chromosomes.

[vi] Heterozygous means having differing genes at one or more corresponding locus on a chromosome.

[vii] Erik Axelsson, Abhirami Ratnakumar, Maja-Louise Arendt, Khurram Maqbool, Matthew Webster, Michele Perloski, Olof Iberg, Jon Armemo, Ake Hedhammar & Kerstin Lindblad-Toh, “The Genomic Signature of Dog Domestication Reveals Adaptation to a Starch-rich Diet,” www.nature.com, Accessed April 15, 2013.