MadSci Network: Other

Re: Are humans Vegetarians or Omnivores?

Date: Tue May 23 14:18:31 2000
Posted By: Thomas M. Greiner, Assistant Professor of Anatomy / Physical Anthropology
Area of science: Other
ID: 957941229.Ot

Are humans vegetarians or omnivores?

Those terms are not strict biological designations, and so the question 
needs to be rephrased a bit before it can be answered. The confusion stems 
from the use of the word "carnivore" to mean "meat eater" - instead the 
word carnivorous should be used. To be biologically strict here, a 
carnivore is an order of mammal (recognized by the presence of the 
carnassial tooth, among other things) that includes cats (felids), dogs 
(canids), bears (ursids), and a number of other mammalian families. 
Carnivores are not strictly meat eaters - most will eat some type of plants 
as a part (sometimes even the main part) of their diet. 

The term "vegetarian" is strictly a human construct. Vegetarians shun 
animal products for food, usually for religious or ethical reasons. Even 
still, there are degrees of vegetarian. Some vegetarians will never 
consciously eat any food that comes from an animal. Other will consume 
dairy products (an animal food even though it isn't meat). Others will 
allow themselves to eat eggs (the pre-chicken, so to speak), or fish. 
Vegetarianism is more of a dietary philosophy then an ecological food 
preference, so it's hard to speak of it in biological terms.

Biologists rarely (never in my experience) categorize non-carnivorous 
animals into one "plant eating" group. That is because different dietary 
specializations are required to eat different types of plants. Thus, you 
will hear of herbivores (specialized to eat shoots and growing tips), 
folivores (specialized to eat leaves) and frugivores (specialized to eat 
fruit) among other adaptations. Even within these broad groupings there are 
further specializations, such as grazers that eat grass and browsers that 
eat shrubs (both are types of herbivore). And even among the "plant eating" 
animals there is no animal that I am aware of that will not consume some 
type of animal protein when given the opportunity.

An omnivore is an animal that will draw its food from all aspects of the 
ecosystem (plants, animals - whatever). As I've already pointed out, you 
could claim that nearly every animal is omnivorous since a purely meat 
eater or purely plant eater is very rare. Nonetheless, you can look at the 
animal's anatomy to look at how they are adapted to process food. 

First you can look at the teeth. 

Molars are broad flat teeth that are useful in grinding up tough fibrous 
material - such as plants. Animals that are primarily plant eaters have 
very large molars with six pairs (three uppers and three lowers) on each 
side. Animals that do not specialize in eating plants tend to have a 
reduced number of molars. Humans are equipped with six pairs of molars. 
Although they are not very large, this would suggest that humans have the 
ability to process fibrous plant food.

Premolars (bicuspids) are the slicing teeth. Mammals originally had eight 
pairs of premolars, although most mammals alive today have fewer. Premolars 
are the primary teeth used by meat eating specialists, and are frequently 
missing in animals that specialize in plant foods. Humans have four pairs 
of premolars, which suggests that humans have the ability to process animal 

Incisors are grasping teeth, that change shape depending upon how they are 
used. The broad flat surfaces of human incisors is most associated with 
animals that specialize in eating fruit.

So, if we just look at our teeth - humans are clearly built to be 
omnivorous. But, of course there is more data. We can look at how nutrients 
are processes and absorbed in the body.

Meat and fruit are high quality foods that are not difficult to assimilate. 
Animals that specialize in these types of food tend to have a short 
digestive tract, with a very short large intestine. Plant foods can be 
nutritious, but take longer to absorb. Therefore, animals that specialize 
in plant eating tend to have long and elaborate digestive tracts. Humans 
are clearly intermediate here. We have a long large intestine (more common 
in plant eaters), but we lack the elaborations that would allow us to 
digest and assimilate nutrients from high fiber plant foods (such as 
grass or leaves). So, again, the human digestive tract can be used to argue 
that we are omnivorous.

Finally, you need to look at nutritional requirements. There are some 
B-complex vitamins that are available only by eating other animals. The 
human body requires this nutrient, but does not synthesize it the way some 
other animals do. Therefore, if humans truly ate no animal foods, and had 
no artificial vitamin supplements, they would sicken and die. In nature, 
there are no true "human vegetarians."

Humans are omnivores. The order of mammals that includes humans (the 
primates) are all omnivores. To be sure, the modern American diet includes 
a lot more meat than is healthy. And the human animal can be very healthy 
by being a lot more vegetarian. But to never eat meat is both unnatural and 

Finally, you ask about my credentials to answer this question. Well, I am 
employed as an anatomy professor (and am therefore a specialist in human 
anatomy). I teach at a college that specializes in training health care 
workers (so I am familiar with issues of human health an nutrition). 
Finally, I earned by Ph.D. in Physical Anthropology (which sort of makes me 
an expert in how humans adapt both biologically and culturally to the 

Some References relevant to this questions:

Harding, RSO & Teleki, G (1981) Omnivorous Primates. Columbia University 
Press: New York.

Romer, AS & Parsons, TS (1986) The Vertebrate Body. Saunders College 
Publishing: New York

Oxnard, C. (1987) Fossils, Teeth and Sex. University of Washington Press: 

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