To: psychoceramics@zikzak.net
Subject: psychoceramics: The Comparative Anatomy of Eating
From: Ernie Karhu <ekarhu @ suneast.East.Sun.COM>
Date: Tue, 26 Mar 1996 08:38:09 -0500
Sender: owner-psychoceramics
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