Don’t be fooled: dogs are not omnivores!
Oddly enough, I have only been heckled once in my life and that was during a talk to a Lincolnshire dog club about ten years ago. I was explaining about the history and taxonomy of the dog, and had just said: ‘Dogs are carnivores, as evidenced by their anatomy and digestive system.’ When an extremely irate man stood up and shouted: ‘No, they aren’t, there are omnivores, as evidenced by the fact that they will eat almost anything you feed them.’ I suggested he let me finish and then he could ask a question, and he said he couldn’t see the point since I was clearly an idiot (an opinion shared by a surprising number of people) and stomped out, slamming the door behind him.
Diet, whether human or canine, is an extremely emotional subject as anyone who has witnessed (or participated in) a discussion between, say, a vegan and an enthusiastic meat eater can confirm. What we eat isn’t just a scientific question, it is an emotional question, too. Our choice of diet is one of our core beliefs like our religion (or lack of it) or our politics (or lack of them). No one argues that a healthy diet for a human is going to consist of a nourishing balance of carbohydrates, protein, fats, vitamins, minerals, fibre and water. Nor that the amount required will depend on our size, metabolism and activity levels. The row starts when one person says that humans have, in essence, the same anatomy as a herbivore and someone else say, yes, maybe we did once, but that has all changed over time, because we have evolved.
Either way, the really important point is that not all animals have the same dietary requirements. Indeed, there is huge variation in the foods consumed by different animals. Some animals, such as lions or tigers, are obligate carnivores. They only eat prey, which provides all their nutritional requirements. Others, one thinks of cows and sheep, are obligate herbivores, and obtain all of their nutrition from vegetable matter: plants, their seeds, roots and fruit. And then there are omnivores, adapted to eating a combination of animals and plants.
A fascinating and little-known 2019 study, Evolution of diet across the animal tree of life by Cristian Roman-Palacios, Joshua P. Scholl and John J. Weins, investigates this topic further. The scientists involved summarised their research as follows:
‘What an animal eats is a fundamental aspect of its biology, but the evolution of diet has not been studied across animal phylogeny. Here, we performed a large-scale phylogenetic analysis to address three unresolved questions about the evolution of animal diets. (i) Are diets conserved across animal phylogeny? (ii) Does diet influence rates of species proliferation (diversification) among animal phyla? (iii) What was the ancestral diet of animals and major animal clades? We analyzed diet data for 1087 taxa, proportionally sampled among animal phyla based on the relative species richness of phyla. Our survey suggests that across animals, carnivory is most common (∼63%), herbivory less common (∼32%), and omnivory relatively rare (∼3%). Despite considerable controversy over whether ecological traits are conserved or labile, we found strong conservatism in diet over extraordinarily deep timescales. We found that diet is unrelated to rates of species diversification across animal phyla, contrasting with previous studies showing that herbivory increased diversification within some important groups (e.g., crustaceans, insects, and mammals). Finally, we estimated that the ancestor of all animals was most likely carnivorous, as were many major phyla (e.g., arthropods, molluscs, and chordates). Remarkably, our results suggest that many carnivorous species living today may have maintained this diet through a continuous series of carnivorous ancestors for >800 million years.’
In plain English? The researchers studied the diets of more than 1000 species over a period of around 800 million years using a technique known as phylogenetic analysis – essentially the study of evolutionary relationships between species and how they have evolved, together and independently, over time. The results of this was compiled with information about each species’ diet. It transpired that carnivorous diets were most common: 63% of the animal groups studied were exclusive meat eaters. Herbivorous diets were the second most common, around 32% of all animal groups. Omnivores, on the other hand, were rare, making up just 3% of all the animal groups studied. (As an aside, this comes to 98%, leaving me to wonder what the missing 2% survived on.)
The team also considered how long each animal group had been eating the way they did now (were lions always carnivores?) and how has their diet evolved. It transpires that species change their diets infrequently and slowly – maintaining the identical diet over, literally, millions and millions of years. Not surprisingly, this has allowed different species to adapt better to their optimum diet. A really good example of this is the shape and size of some species’ teeth have changed over the millennia. Carnivores, by the way, generally have large, sharp teeth – perfect for shredding and ripping off flesh. Herbivores have flat molar teeth for crushing and grinding – perfect for plant matter. Omnivores? They have a combination of the two types, allowing them to eat both plants and meat.
Why are there so many carnivores compared to herbivores and omnivores? No one really knows. And, in a way, maybe it isn’t that relevant. What’s important is that every species eats an appropriate diet – a diet that ensures optimum health. Before we look at what a dog’s natural diet is, let’s consider humans. Whatever you may think about humans eating meat, there can be no argument that we have not yet adapted to heavily processed foods, which are high in fat, salt and sugar. These types of foods were not available, even to our recent ancestors, and we haven’t evolved to eat them. It is almost certainly the cause of the surge in so-called lifestyle diseases such as obesity, heart disease and type 2 diabetes. What I am saying is that if a species changes its diet too quickly it will not thrive.
So, back the thorny question of whether dogs are carnivores or omnivores. The most convincing argument that dogs are not omnivores is to be found in another piece if research, published in 2015, and titled: Dietary nutrient profiles of wild wolves: insights for optimal dog nutrition? To summarise the paper:
‘Domestic dogs diverged from grey wolves between 13,000 and 17,000 years ago when food waste from human settlements provided a new niche. Compared to the carnivorous cat, modern-day dogs differ in several digestive and metabolic traits that appear to be more associated with omnivorous such as man, pigs and rats. This has led to the classification of dogs as omnivores, but the origin of these ‘omnivorous’ traits has, hitherto, been left unexplained. We discuss the foraging ecology of wild wolves and calculate the nutrient profiles of fifty diets reported in the literature. Data on the feeding ecology of wolves indicate that wolves are true carnivores consuming a negligible amount of vegetable matter. Wolves can experience prolonged times of famine during low prey availability while, after a successful hunt, the intake of foods and nutrients can be excessive. As a result of a ‘feast and famine’ lifestyle, wolves need to cope with a highly variable nutrient intake requiring an adaptable metabolism, which is still functional in our modern-day dogs. The nutritive characteristics of commercial foods differ in several aspects from the dog’s closest free-living ancestor in terms of dietary nutrient profile and this may pose physiological and metabolic challenges.’
In other words, dogs and wolves are virtually one and the same (they can interbreed, for example) and wolves are carnivores. Due to the feast or famine nature of a wolf’s lifestyle it has an adaptable metabolism and this explains why dogs can survive (if not thrive) on modern, processed dog food despite the fact that it contains very little meat and bone. It is the wolf’s adaptable metabolism, therefore, that is allowing dog food manufacturers to falsely claim that dogs are omnivores. A claim that allows them to sell dog food made from cheap, poor quality, highly processed and inappropriate ingredients… but don’t get me started!
There are, of course, lots of other arguments that point towards dogs being carnivores from the nature of their teeth to their digestive system and from their physiology to what genuinely ‘wild’ dogs choose to eat (not kibble!). Arguments that are supported by the first bit of research I quoted, which points out that species change their diets infrequently and slowly. Given that dogs are carnivores, I don’t think there can be any doubt about what we should be feeding them: a raw food diet made up largely of meat and bone.