Dr John P Wright
Updated: 8 June 2006
The deceptively simple task of buying pet food can be a daunting task. From the commercial breaks on television to the shelves at the supermarket we are bombarded with advice on the correct food for our furry feathered friends. Does it matter what we feed them? Why not scraps for dogs and cats, old crusts for birds and fish, and left over muesli for the gerbil. The answer we would all give is “It is not what is best for them”. Dry pellet food is designed to supply all their dietary requirements but have our pets really had much input to this conclusion and might they not well prefer the left over mutton stew.
While dogs will eat almost anything cats are very much more particular as they are almost exclusively meat eaters. Unlike humans and dogs they are not able to manufacture all the proteins they need so they have to ingest all their requirements. Mammals as a group however have similar digestive processes. Two notable exceptions are the hindgut fermenters, being horses and elephants, and the ruminants, cattle and sheep. The former are heavily reliant on their colonic micro flora to digest the vegetable cellulose fibre that they feed on. The gaseous products of this metabolism are then absorbed and utilized by the colon. The process may be likened to a personal compost heap. Ruminants on the other hand have a different approach. They store the vegetable material in their rumens and then move it back for re-mastication and oral salivary digestion. Both of these systems of wet digestion have major weight implications which would not be viable for flying birds.
Birds have a digestive system modelled on that of the dinosaurs. They have a crop in the lower oesophagus which simply stores food followed by a stomach (proventricus) that adds acid and enzymes. The next stage is grinding by the gizzard with or without gravel to assist the process. Once the food leaves the gizzard the nutrients are absorbed in the small bowel and any residue passed out via the cloacae. The colon is usually rudimentary although assisted by one or two caeca that house the micro flora fermenters. Some seed eaters have larger colons but the biggest is found in our ostriches. These large birds also boast a bladder which is probably another reason why they do not fly. In fact if ostriches flew the dropping of ballast would pose serious problems for us earthlings. The other great non flying bird, the emu, has a different approach it has the normal rudimentary avian colon but a very well developed foregut system, a sort of ruminant of the bird world.
It is clear from the above that each species has its own digestive methods and to extrapolate from one species’ eating pattern to the next is not always possible. As the digestive system varies so does the menu. Most species and in particular mammals are omnivores to greater or lesser extent. Even some buck such as the “dik dik” eat carrion. When it comes to primates this theme continues. At first sight it appears that some are strict vegetarians. Gorillas eat bamboo shoots and chimpanzee’s fruit. Closer inspection however shows that both consume insects and animal protein. An archetypical primate is probably the baboon who eats whatever it can lay its hands on, insects, fruit, grass seeds, meat and roots. A close second would be Homo sapiens who eats everything the baboon eats plus fish and even bird nests. This wide variety of food sources has ensured the survival of both of these primates in a wide range of habitats. Man however took more control and added hunting to the gathering. Over the last 2 million years man has slowly become more dependent on meat. The enlargement in the incisor teeth with the passage of time supports this increase in meat consumption.
Further evidence on the diet of man comes from the digestive system. We have a simple stomach without crop, gizzard or rumen. We have high acid output but not as high as true carnivores such as dogs and vultures. We have no foregut adaptations for a vegetarian diet. Our colons are neither rudimentary as in seed eating birds nor massive as in herbivores such as horses and rabbits.
In about 6000 BC we learnt to farm animals with beef and mutton appearing in our diets. In retrospect this was probably a serious mistake. Wild game has a 10% unsaturated fat content in their muscles where as beef has 20% saturated fat. The consumption of dairy beyond infancy added further stress to our own fat deposits. The final blow to ruddy good health was the belief that it was immoral to eat animal products. This is best characterised by the call to abstain from “Anything with a mother”. We seem to have lost the plot.
Human physiology has been designed over a few million years to consume a basically very simple diet of meat which we can catch, grains and vegetables which we can store and fruit in season. We need to differentiate staples from fun food. Staple human food is the meat (fresh on the hoof), cereals (storable for a year) and roots (potato etc which we can harvest over a long time). Fun food is the seasonal food that we have to consume immediately, for example fruit and broccoli. The use of refrigeration, genetic engineering and rapid air transport is distorting the basic human diet.
If we are not careful we will soon be eating pellets ourselves and race horses will be on steak and chips.