Because the term omnivore is vague, it is not surprising that authors often differ in their classifications of the dietary status of animals. Pilbeam(9) describes apes as very broadly "herbivores", as do Yerkes and Yerkes(4), whereas Maier(2) says primates should be considered to be "omnivores". Opinions on how to classify primates in general, and chimpanzees, our closest genetic relatives, seem to be at varience.
In order to classify digestive systems, Chivers has performed some of the most extensive study of mammal digestive system anatomy, yet his research on humans is inconclusive. Summing up in 'Diet and Guts'(1) he states that human gut anatomy is characteristic of meat-eating, or some other rapidly digested foods. However, his plots show the human digestive anatomy is at the edgeof the "carnivore" cluster. Even more critically, at the centre of the "carnivore" cluster is Cebus capucinus (the white-fronted capuchin). According to The Pictorial Guide To The Living Primates, Cebus capucinus eats 95 types of fruit that make up 65% of its diet, while leaves make up 15%. The remainder of the diet consists of berries, nuts, seeds, shoots, buds, flowers, gums, bark and animal matter including insects, in that order. Cebus capucinus is primarily a foli-frugivore, not a carnivore, but they have also been called omnivores, because they consume a range of animal matter. Cebus capucinus is rumoured to have a digestive system somewhat like humans, and Chivers chart demonstrates some similarity, but his carnivore data set seems to be misleading, and is not objectively defined. We should be careful with using concepts such as "similarities", because they are subjectively formed.
Of the few examples of other species categorised as omnivores, none seem to closely resemble humans in their anatomy, unless perhaps the chimpanzee is to be categorised as an omnivore. However, the chimpanzee is often described as a frugivore, or foli-frugivore although others call it an omnivore and as we shall see, human anatomy is distinct from the chimps. Presently, there is no precise system for classifying a species diet based on either its anatomy or behaviour. Anatomical observations can be misleading, for example Milton(7) points out that the panda bear is said to have a digestive system that resembles a carnivore(p. 14), yet it normally eats a herbivorous diet - is it an omnivore? Furthermore within the order Carnivora species that all share anatomical traits (by definition) have diets that vary from pure carnivory, through omnivory to frugivory(p. 14).
Chimpanzees favour a high fruit diet when fruit is in season but diversify, and may include more foliage and animal matter when fruit supply is sparse. Furthermore, chimpanzees also have food cultures and procure foods using primitive "tools" (not technology), so that as with humans, their diet may not reflect their adaptations as much as local habitat, traditions and learnt behaviour to deal with food shortages. We might accept this as a good example of omnivores, but even rabbits engage in mild cannibalism, and many other "herbivores" are known to eat their placentas. Herbivorous species will also eat animal matter under captive conditions, and most "herbivores" will ingest incidental insect matter along with foliage, but this doesn’t seem to allow us to call them omnivores.
It seems that an opportunity to eat nutritious food is not passed over by wild animals, even when it involves a herbivore consuming its own placenta. On this basis then, we might conclude that all mammals are "omnivores" - however a catchall definition is not really a category and such broad categorisation would allow similarities to be passed off as equivalents. Clearly, there are issues of frequency, quantity and type of animal matter consumed. These need quantifying before a species can be called an omnivore in the biological sense. Such clarification needs to separate animals that infrequently eat flesh in small amounts, or under unnatural conditions due to domestication or unusual environmental pressures, from those that eat animal foods more uniformly, and tolerate such a diet without detriment. Some might suggest that the practice of varying the diet under environmental pressure is what sets omnivores apart from carnivores and herbivores, and that such a behaviour is evidence of omnivory.
Having studied a significant amount of literature, it becomes obvious that as yet academics have not produced the kind of systematic quantified and widely agreed definitions found in and expected of a precise science. There are significant inconsistencies in classifying primate diets.
Note: In this article, 'animal matter' usually implies vertibrates, but may include insects.