It is often assumed that dogs are carnivores like the wolf. However, one of the significant changes that occurred with domestication was in the genes involved in digestion. One of these that is especially significant is in the gene coding for pancreatic amylase (AMY2B), which breaks down starch into the sugar maltose in the small intestine. Related carnivores like the wolf, coyote, and golden jackal have two copies of the AMY2B gene. Most dogs, however, have more than this, and in some cases many-fold more. These duplicate copies of a gene are called “copy number variations”, and they can occur during DNA replication. In the case of the amylase gene, dogs with more copies of the gene produce more of the enzyme amylase, which allows them to digest starch more efficiently.
In this chart, the dogs with two or very few copies of the gene include the most primitive breeds and some of the spitzes (Dingo, Greenland Dog, Siberian Husky, Pug, Laika, etc). Among breeds that typically average more than 12 copies of the gene are the English Springer Spaniel, German Shepherd, Rottweiler, Border Collie, and whippet. The breed with the most AMY2B copies determined so far is the Tazi, with an average of about 18 and some individuals with more than 20.
How would dogs benefit from extra copies of a gene for starch digestion? Dogs followed humans around the globe and lived where they did, and what was available to eat locally would be reflected in the diet of both. One of the key innovations that allowed expansion of humans across Asia and into Europe was agriculture – the ability to keep livestock and to grow crops for food.
Dogs were used to guard and manage livestock, but the animals were how shepherds made their living and were extremely valuable. They were not the food source for the shepherd and his dogs. Instead, they were used as a renewable resource, providing milk on a regular basis and reproducing yearly to produce the income of the next season. Eating the livestock would be self-defeating, like spending the principal in a savings account instead of just living on the funds produced by earned interest. Instead, the dogs and their shepherds lived primarily on their renewable resources, grain and milk, perhaps with an occasional wild animal for meat, so both had diets that were high in starch. Dogs with more copies of the AMY2B gene would be more efficient at digesting starch, and if this was a major component of the diet it would drive selection for an increase in the number of copies of the gene.
What is the evidence to support this scenario? A survey of dogs from around the world found that dogs from “agrarian” areas (red) had more copies of the AMY2B gene than dogs from “non-agrarian” areas where plant crops were difficult or seasonally impossible to grow and comprised less of the diet of both humans and dogs (blue). You can see from the map below, on which the dash lines mark the approximate extension of prehistoric agriculture, that from the ancestral number of copies of the gene (two) in wolves and other related carnivores, the number of copies increased as much as 11 fold (to 22 in both the Tazi and the New Guinea Singing dog) in dogs from agrarian regions. This held not just for the “breed” dogs, but also for the native village dogs in those areas. The dogs with the fewest copies of the AMY2B gene were from areas where the climate was not suitable for large-scale agriculture, and fish and meat likely formed a greater proportion of the diet (e.g., Greenland dog, Siberian Husky).
So one of the ways in which dogs were changed by their association with humans in most of the world was in their diet. Hunting is time consuming, energy intensive, and dangerous, and the ability to grow crops as a reliable source of food is one of the key events that facilitated the development of human civilization. The common vision of dogs as strict carnivores that get tossed a nice meaty bone for dinner each night is not reflected in what we know about their history and biology. Dogs have adapted physiologically to a diet high in starch, and in much of the world today the diet of working dogs still consists of boiled grain and milk products like whey or cheese.
So, what about all the hype about raw meat diets and grain-free kibble? They are fads, based on ideas instead of science. Clearly, dogs have adaptations for digestion of starch, just like we do, and they also love meat – again, just like we do. But grains have been a major component of the dog’s diet for thousands of years and the notion that the “natural” diet of the dog is a big, meaty bone is a myth. If anybody tries to convince you otherwise, demand to see some scientific evidence. You have the scientific sources available here that convincingly demonstrate otherwise.
REFERENCES
Arendt M, KM Cairns, JWO Ballard, P Savolainen, & E Axelsson. Diet adaptation in dog reflects spread of prehistoric agriculture. Heredity 2016: 1-6.
Marciniak S & GH Perry. 2017. Harnessing ancient genomes to study the history of human adaptation. Nature Reviews Genetics 18: 659-674. doi:10.1038/nrg.2017.65
Pennisi E. 2016. How farming changed the dog. Science 360(6391): doi:10.1126/science.aal0353
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