[682] in Vegetarian_Support_Group
omnivore vs. herbivore anatomy
daemon@ATHENA.MIT.EDU (Laura C. Dilley)
Tue Oct 17 17:54:29 1995
To: vsg@MIT.EDU
Date: Tue, 17 Oct 1995 17:51:56 EDT
From: "Laura C. Dilley" <elsiedee@MIT.EDU>
For a change of pace, thought I would post something I found off a Web
site (sorry, can't remember where).
If you think of humans as being natural omnivores (or even
carnivores!) this might change your mind.
-Laura
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The Comparative Anatomy of Eating
by Milton R. Mills, M.D.
Humans are most often described as "omnivores". This classification is
based on the "observation" that humans generally eat a wide variety of
plant and animal foods. However, culture, custom and training are
confounding variables when looking at human dietary practices. Thus,
"observation" is not the best technique to use when trying to identify
the most "natural" diet for humans. While most humans are clearly
"behavioral" omnivores, the question still remains as to whether
humans are anatomically suited for a diet that includes animal as well
as plant foods.
A better and more objective technique is to look at human anatomy and
physiology. Mammals are anatomically and physiologically adapted to
procure and consume particular kinds of diets. (It is common practice
when examining fossils of extinct mammals to examine anatomical
features to deduce the animal's probable diet.) Therefore, we can
look at mammalian carnivores, herbivores (plant-eaters) and omnivores
to see which anatomical and physiological features are associated with
each kind of diet. Then we can look at human anatomy and physiology to
see in which group we belong.
Oral Cavity
Carnivores have a wide mouth opening in relation to their head size.
This confers obvious advantages in developing the forces used in
seizing, killing and dismembering prey. Facial musculature is reduced
since these muscles would hinder a wide gape, and play no part in the
animal's preparation of food for swallowing. In all mammalian
carnivores, the jaw joint is a simple hinge joint lying in the same
plane as the teeth. This type of joint is extremely stable and acts
as the pivot point for the "lever arms" formed by the upper and lower
jaws. The primary muscle used for operating the jaw in carnivores is
the temporalis muscle. This muscle is so massive in carnivores that it
accounts for most of the bulk of the sides of the head (when you pet a
dog, you are petting its temporalis muscles). The "angle" of the
mandible (lower jaw) in carnivores is small. This is because the
muscles (masseter and pterygoids) that attach there are of minor
importance in these animals. The lower jaw of carnivores cannot move
forward, and has very limited side-to-side motion. When the jaw of a
carnivore closes, the blade-shaped cheek molars slide past each other
to give a slicing motion that is very effective for shearing meat off
bone.
The teeth of a carnivore are discretely spaced so as not to trap
stringy debris. The incisors are short, pointed and prong-like and are
used for grasping and shredding. The canines are greatly elongated
and dagger-like for stabbing, tearing and killing prey. The molars
(carnassials) are flattened and triangular with jagged edges such that
they function like serrated-edged blades. Because of the hinge-type
joint, when a carnivore closes its jaw, the cheek teeth come together
in a back-to-front fashion giving a smooth cutting motion like the
blades on a pair of shears.
The saliva of carnivorous animals does not contain digestive enzymes.
When eating, a mammalian carnivore gorges itself rapidly and does not
chew its food. Since proteolytic (protein-digesting) enzymes cannot be
liberated in the mouth due to the danger of autodigestion (damaging
the oral cavity), carnivores do not need to mix their food with
saliva; they simply bite off huge chunks of meat and swallow them
whole.
According to evolutionary theory, the anatomical features consistent
with an herbivorous diet represent a more recently derived condition
than that of the carnivore. Herbivorous mammals have well-developed
facial musculature, fleshy lips, a relatively small opening into the
oral cavity and a thickened, muscular tongue. The lips aid in the
movement of food into the mouth and, along with the facial (cheek)
musculature and tongue, assist in the chewing of food. In herbivores,
the jaw joint has moved to position above the plane of the teeth.
Although this type of joint is less stable than the hinge-type joint
of the carnivore, it is much more mobile and allows the complex jaw
motions needed when chewing plant foods. Additionally, this type of
jaw joint allows the upper and lower cheek teeth to come together
along the length of the jaw more or less at once when the mouth is
closed in order to form grinding platforms. (This type of joint is so
important to a plant-eating animal, that it is believed to have
evolved at least 15 different times in various plant-eating mammalian
species.) The angle of the mandible has expanded to provide a broad
area of attachment for the well-developed masseter and pterygoid
muscles (these are the major muscles of chewing in plant-eating
animals). The temporalis muscle is small and of minor importance. The
masseter and pterygoid muscles hold the mandible in a sling-like
arrangement and swing the jaw from side-to-side. Accordingly, the
lower jaw of plant-eating mammals has a pronounced sideways motion
when eating. This lateral movement is necessary for the grinding
motion of chewing.
The dentition of herbivores is quite varied depending on the kind of
vegetation a particular species is adapted to eat. Although these
animals differ in the types and numbers of teeth they posses, the
various kinds of teeth when present, share common structural features.
The incisors are broad, flattened and spade-like. Canines may be small
as in horses, prominent as in hippos, pigs and some primates (these
are thought to be used for defense) or absent altogether. The molars,
in general, are squared and flattened on top to provide a grinding
surface. The molars cannot vertically slide past one another in a
shearing/slicing motion, but they do horizontally slide across one
another to crush and grind. The surface features of the molars vary
depending on the type of plant material the animal eats. The teeth of
herbivorous animals are closely grouped so that the incisors form an
efficient cropping/biting mechanism, and the upper and lower molars
form extended platforms for crushing and grinding. The "walled-in"
oral cavity has a lot of potential space that is realized during
eating.
These animals carefully and methodically chew their food, pushing the
food back and forth into the grinding teeth with the tongue and cheek
muscles. This thorough process is necessary to mechanically disrupt
plant cell walls in order to release the digestible intracellular
contents and ensure thorough mixing of this material with their
saliva. This is important because the saliva of plant-eating mammals
often contains carbohydrate-digesting enzymes which begin breaking
down food molecules while the food is still in the mouth.
Stomach and Small Intestine
Striking differences between carnivores and herbivores are seen in
these organs. Carnivores have a capacious simple (single-chambered)
stomach. The stomach volume of a carnivore represents 60-70% of the
total capacity of the digestive system. Because meat is relatively
easily digested, their small intestines (where absorption of food
molecules takes place) are short -- about three to five or six times
the body length. Since these animals average a kill only about once a
week, a large stomach volume is advantageous because it allows the
animals to quickly gorge themselves when eating, taking in as much
meat as possible at one time which can then be digested later while
resting. Additionally, the ability of the carnivore stomach to secrete
hydrochloric acid is exceptional. Carnivores are able to keep their
gastric pH down around 1-2 even with food present. This is necessary
to facilitate protein breakdown and to kill the abundant dangerous
bacteria often found in decaying flesh foods.
Because of the relative difficulty with which various kinds of plant
foods are broken down (due to large amounts of indigestible fibers),
herbivores have significantly longer and in some cases, far more
elaborate guts than carnivores. Herbivorous animals that consume
plants containing a high proportion of cellulose must "ferment"
(digest by bacterial enzyme action) their food to obtain the nutrient
value. They are classified as either "ruminants" (foregut fermenters)
or hindgut fermenters. The ruminants are the plant-eating animals with
the celebrated multiple-chambered stomachs. Herbivorous animals that
eat a diet of relatively soft vegetation do not need a
multiple-chambered stomach. They typically have a simple stomach, and
a long small intestine. These animals ferment the difficult-to-digest
fibrous portions of their diets in their hindguts (colons). Many of
these herbivores increase the sophistication and efficiency of their
GI tracts by including carbohydrate-digesting enzymes in their saliva.
A multiple-stomach fermentation process in an animal which consumed a
diet of soft, pulpy vegetation would be energetically wasteful.
Nutrients and calories would be consumed by the fermenting bacteria
and protozoa before reaching the small intestine for absorption. The
small intestine of plant-eating animals tends to be very long (greater
than 10 times body length) to allow adequate time and space for
absorption of the nutrients.
Colon
The large intestine (colon) of carnivores is simple and very short, as
its only purposes are to absorb salt and water. It is approximately
the same diameter as the small intestine and, consequently, has a
limited capacity to function as a reservoir. The colon is short and
non-pouched. The muscle is distributed throughout the wall, giving the
colon a smooth cylindrical appearance. Although a bacterial population
is present in the colon of carnivores, its activities are essentially
putrefactive.
In herbivorous animals, the large intestine tends to be a highly
specialized organ involved in water and electrolyte absorption,
vitamin production and absorption, and/or fermentation of fibrous
plant materials. The colons of herbivores are usually wider than their
small intestine and are relatively long. In some plant-eating mammals,
the colon has a pouched appearance due to the arrangement of the
muscle fibers in the intestinal wall. Additionally, in some herbivores
the cecum (the first section of the colon) is quite large and serves
as the primary or accessory fermentation site.
What About Omnivores?
One would expect an omnivore to show anatomical features which equip
it to eat both animal and plant foods. According to evolutionary
theory, carnivore gut structure is more primitive than herbivorous
adaptations. Thus, an omnivore might be expected to be a carnivore
which shows some gastrointestinal tract adaptations to an herbivorous
diet.
This is exactly the situation we find in the Bear, Raccoon and certain
members of the Canine families. (This discussion will be limited to
bears because they are, in general, representative of the anatomical
omnivores.) Bears are classified as carnivores but are classic
anatomical omnivores. Although they eat some animal foods, bears are
primarily herbivorous with 70-80% of their diet comprised of plant
foods. (The one exception is the Polar bear which lives in the frozen,
vegetation poor arctic and feeds primarily on seal blubber.) Bears
cannot digest fibrous vegetation well, and therefore, are highly
selective feeders. Their diet is dominated by primarily succulent lent
herbage, tubers and berries. Many scientists believe the reason bears
hibernate is because their chief food (succulent vegetation) not
available in the cold northern winters. (Interestingly, Polar bears
hibernate during the summer months when seals are unavailable.)
In general, bears exhibit anatomical features consistent with a
carnivorous diet. The jaw joint of bears is in the same plane as the
molar teeth. The temporalis muscle is massive, and the angle of the
mandible is small corresponding to the limited role the pterygoid and
masseter muscles play in operating the jaw. The small intestine is
short ( less than five times body length) like that of the pure
carnivores, and the colon is simple, smooth and short. The most
prominent adaptation to an herbivorous diet in bears (and other
"anatomical" omnivores) is the modification of their dentition. Bears
retain the peg-like incisors, large canines and shearing premolars of
a carnivore; but the molars have become squared with rounded cusps for
crushing and grinding. Bears have not, however, adopted the flattened,
blunt nails seen in most herbivores and retain the elongated, pointed
claws of a carnivore.
An animal which captures, kills and eats prey must have the physical
equipment which makes predation practical and efficient. Since bears
include significant amounts of meat in their diet, they must retain
the anatomical features that permit them to capture and kill prey
animals. Hence, bears have a jaw structure, musculature and dentition
which enable them to develop and apply the forces necessary to kill
and dismember prey even though the majority of their diet is comprised
of plant foods. Although an herbivore-style jaw joint (above the
plane of the teeth) is a far more efficient joint for crushing and
grinding vegetation and would potentially allow bears to exploit a
wider range of plant foods in their diet, it is a much weaker joint
than the hinge-style carnivore joint. The herbivore-style jaw joint is
relatively easily dislocated and would not hold up well under the
stresses of subduing struggling prey and/or crushing bones (nor would
it allow the wide gape carnivores need). In the wild, an animal with a
dislocated jaw would either soon starve to death or be eaten by
something else and would, therefore, be selected against. A given
species cannot adopt the weaker but more mobile and efficient
herbivore-style joint until it has committed to an essentially
plant-food diet test it risk jaw dislocation, death and ultimately,
extinction.
What About Me?
The human gastrointestinal tract features the anatomical modifications
consistent with an herbivorous diet. Humans have muscular lips and a
small opening into the oral cavity. Many of the so-called "muscles of
expression" are actually the muscles used in chewing. The muscular and
agile tongue essential for eating, has adapted to use in speech and
other things. The mandibular joint is flattened by a cartilaginous
plate and is located well above the plane of the teeth. The temporalis
muscle is reduced. The characteristic "square jaw" of adult males
reflects the expanded angular process of the mandible and the enlarged
masseter/pterygoid muscle group. The human mandible can move forward
to engage the incisors, and side-to-side to crush and grind.
Human teeth are also similar to those found in other herbivores with
the exception of the canines (the canines of some of the apes are
elongated and are thought to be used for display and/or defense). Our
teeth are rather large and usually abut against one another. The
incisors are flat and spade-like, useful for peeling, snipping and
biting relatively soft materials. The canines are neither serrated nor
conical, but are flattened, blunt and small and function Like
incisors. The premolars and molars are squarish, flattened and
nodular, and used for crushing, grinding and pulping noncoarse foods.
Human saliva contains the carbohydrate-digesting enzyme, salivary
amylase. This enzyme is responsible for the majority of starch
digestion. The esophagus is narrow and suited to small, soft balls of
thoroughly chewed food. Eating quickly, attempting to swallow a large
amount of food or swallowing fibrous and/or poorly chewed food (meat
is the most frequent culprit) often results in choking in humans.
Man's stomach is single-chambered, but only moderately acidic.
(Clinically, a person presenting with a gastric pH less than 4-5 when
there is food in the stomach is cause for concern.) The stomach volume
represents about 21-27% of the total volume of the human GI tract. The
stomach serves as a mixing and storage chamber, mixing and liquefying
ingested foodstuffs and regulating their entry into the small
intestine. The human small intestine is long, averaging from 10 to 11
times the body length. (Our small intestine averages 22 to 30 feet in
length. Human body size is measured from the top of the head to end of
the spine and averages between two to three feet in length in
normal-sized individuals.)
The human colon demonstrates the pouched structure peculiar to
herbivores. The distensible large intestine is larger in cross-section
than the small intestine, and is relatively long. Man's colon is
responsible for water and electrolyte absorption and vitamin
production and absorption. There is also extensive bacterial
fermentation of fibrous plant materials, with the production and
absorption of significant amounts of food energy (volatile short-chain
fatty acids) depending upon the fiber content of the diet. The extent
to which the fermentation and absorption of metabolites takes place in
the human colon has only recently begun to be investigated.
In conclusion, we see that human beings have the gastrointestinal
tract structure of a "committed" herbivore. Humankind does not show
the mixed structural features one expects and finds in anatomical
omnivores such as bears and raccoons. Thus, from comparing the
gastrointestinal tract of humans to that of carnivores, herbivores and
omnivores we must conclude that humankind's GI tract is designed for a
purely plant-food diet.
Summary
Facial Muscles
CARNIVORE: Reduced to allow wide mouth gape
HERBIVORE: Well-developed
OMNIVORE: Reduced
HUMAN: Well-developed
Jaw Type
CARNIVORE: Angle not expanded
HERBIVORE: Expanded angle
OMNIVORE: Angle not expanded
HUMAN: Expanded angle
Jaw Joint Location
CARNIVORE: On same plane as molar teeth
HERBIVORE: Above the plane of the molars
OMNIVORE: On same plane as molar teeth
HUMAN: Above the plane of the molars
Jaw Motion
CARNIVORE: Shearing; minimal side-to-side motion
HERBIVORE: No shear; good side-to-side, front-to-back
OMNIVORE: Shearing; minimal side-to-side
HUMAN: No shear; good side-to-side, front-to-back
Major Jaw Muscles
CARNIVORE: Temporalis
HERBIVORE: Masseter and pterygoids
OMNIVORE: Temporalis
HUMAN: Masseter and pterygoids
Mouth Opening vs. Head Size
CARNIVORE: Large
HERBIVORE: Small
OMNIVORE: Large
HUMAN: Small
Teeth: Incisors
CARNIVORE: Short and pointed
HERBIVORE: Broad, flattened and spade shaped
OMNIVORE: Short and pointed
HUMAN: Broad, flattened and spade shaped
Teeth: Canines
CARNIVORE: Long, sharp and curved
HERBIVORE: Dull and short or long (for defense), or none
OMNIVORE: Long, sharp and curved
HUMAN: Short and blunted
Teeth: Molars
CARNIVORE: Sharp, jagged and blade shaped
HERBIVORE: Flattened with cusps vs complex surface
OMNIVORE: Sharp blades and/or flattened
HUMAN: Flattened with nodular cusps
Chewing
CARNIVORE: None; swallows food whole
HERBIVORE: Extensive chewing necessary
OMNIVORE: Swallows food whole and/or simple crushing
HUMAN: Extensive chewing necessary
Saliva
CARNIVORE: No digestive enzymes
HERBIVORE: Carbohydrate digesting enzymes
OMNIVORE: No digestive enzymes
HUMAN: Carbohydrate digesting enzymes
Stomach Type
CARNIVORE: Simple
HERBIVORE: Simple or multiple chambers
OMNIVORE: Simple
HUMAN: Simple
Stomach Acidity
CARNIVORE: Less than or equal to pH 1 with food in stomach
HERBIVORE: pH 4 to 5 with food in stomach
OMNIVORE: Less than or equal to pH 1 with food in stomach
HUMAN: pH 4 to 5 with food in stomach
Stomach Capacity
CARNIVORE: 60% to 70% of total volume of digestive tract
HERBIVORE: Less than 30% of total volume of digestive tract
OMNIVORE: 60% to 70% of total volume of digestive tract
HUMAN: 21% to 27% of total volume of digestive tract
Length of Small Intestine
CARNIVORE: 3 to 6 times body length
HERBIVORE: 10 to more than 12 times body length
OMNIVORE: 4 to 6 times body length
HUMAN: 10 to 11 times body length
Colon
CARNIVORE: Simple, short and smooth
HERBIVORE: Long, complex; may be sacculated
OMNIVORE: Simple, short and smooth
HUMAN: Long, sacculated
Liver
CARNIVORE: Can detoxify vitamin A
HERBIVORE: Cannot detoxify vitamin A
OMNIVORE: Can detoxify vitamin A
HUMAN: Cannot detoxify vitamin A
Kidney
CARNIVORE: Extremely concentrated urine
HERBIVORE: Moderately concentrated urine
OMNIVORE: Extremely concentrated urine
HUMAN: Moderately concentrated urine
Nails
CARNIVORE: Sharp claws
HERBIVORE: Flattened nails or blunt hooves
OMNIVORE: Sharp claws
HUMAN: Flattened nails
Contributed by Doug Percival
