Are Humans Omnivores? HTML version
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.
The digestive system in humans is dominated by intestines that are relatively larger than in other primates, and a colon that
is relatively smaller. Overall, the human digestive system is also a less significant portion of the body when compared to
other primates. According to Milton(7) the human small intestine makes up greater than 56% of the total gut, whereas the
colon makes up only 17 to 23%. However, in all other apes the colon makes up greater than 45% of the total, and the
intestines from 14 to 29%. This corroborates Chivers findings, and demonstrates that the human digestive anatomy is in a
class distinct from the other apes. This being the case is logical to look outside of the apes for a species that might better
match our digestive anatomy, perhaps at monkeys, birds or bats.
In general, larger primates including all of the great apes are foli-frugivores but eat some animal matter, and the smaller are
usually faunivores (Tarsius sp.) that may also eat fruit (e.g. Galagoides demidoff). Amongst the primates only Callithrix
humeralifer (tassel-eared marmoset) and Ateles paniscus (black spider monkey) eat more than 80% of their diet as fruit
(11), with the remainder coming mainly from gum or foliage respectively and then a small percentage from animal matter.
The tassel-eared marmoset is almost totally frugivorous, in that the gums that make up 17% of its diet are also chemically
similar to fruits in being primarily a source of carbohydrate. The remaining 0.5% of feeding time is spent on ingesting
small insects. Strong frugivory is therefore found in only a couple of species out of the 234 known primate species.
A study of the literature on functional anatomy reveals that foliage is digested mainly in highly sacculated stomachs or
haustrated colons. These adaptations dramatically increase the gut volume for a given length, thus slowing digestion down
so that bacterial fermentation can occur. Humans also have haustrated colons, but the degree is not as great as in the great
apes. The foods which digest mainly in the intestines are animal matter and fruits, which can be broken down speedily
compared to leaves, due to lack of the indigestible cell walls found in foliage.
Chivers work omits birds and bats, so also omits any highly frugivorous species. It is only amongst birds and bats that we
encounter animals that live exclusively on fruits such as the totally frugivorous pteropodid bats. It's worth noting that fruits