The physiology of a vegan

One of the most pervasive myths surrounding veganism is the belief that humans are naturally meant to eat meat – that we are evolutionarily adapted to eat and thrive on dead flesh. The evidence presented in these pages knocks this myth firmly on its head. Human beings belong to the primate family and the primate family is essentially a vegetarian one. Our closest living relatives such as chimpanzees and gorillas live on a diet of foods overwhelmingly derived from plants, and we ignore our evolutionary past at our peril. Indeed we are already seeing the dangers of dismissing what evolutionary studies show us we should be eating – plants, not animals – with the growing epidemics of killer diseases such as cancer, heart disease, obesity and diabetes which are now occurring in almost every corner of the planet.

At a glance

Compare human physiology to carnivores, omnivores and herbivores in the table below and then click on the headings beneath the table to explore in more detail.

Physiology	Carnivore	Omnivore	Herbivore	Human
Cheek muscles (Herring and Herring, 1974; Fehrenbach and Herring, 2013)	Reduced to allow wide mouth gape	Reduced to allow wide mouth gape	Well-developed to aid chewing	Well-developed to aid chewing
Jaw motion (Schwenk, 2000; Feldhamer, 2007)	Slicing; minimal side-to-side motion	Slicing; minimal side-to-side motion	No slicing; good side-to-side, front-to-back motion	No slicing; good side-to-side, front-to-back motion
Mouth opening vs head size (Herring and Herring, 1974)	Large	Large	Small	Small
Teeth: incisors (front teeth) (Hillson, 2005; Feldhamer, 2007)	Short and pointed	Short and pointed	Broad, flattened and spade-like	Broad, flattened and spade-like
Teeth: canines (Hillson, 2005)	Long, sharp and curved	Long, sharp and curved	Dull and short or long (for defence), or none	Short and blunted
Teeth: molars (back teeth) (Ungar, 2015)	Sharp, jagged, and blade-shaped	Sharp blades and/or flattened	Flattened to crush and grind food	Flattened to crush and grind food
Chewing (Ungar, 2015)	Hardly any; swallows food whole	Hardly any or simple crushing	Extensive chewing necessary	Extensive chewing necessary
Saliva (Tucker and Miletich, 2010; Boehlke et al., 2015)	Mostly mucous saliva, not large volume No enzyme amylase to pre-digest carbohydrates	Seromucous (mixed) saliva Amylase in the saliva to start carbohydrate digestion	Mostly serous (watery) saliva, larger volume Alkaline saliva; amylase to start carbohydrate digestion	Mostly serous (watery) saliva, larger volume Alkaline saliva; amylase to start carbohydrate digestion
Colon (Kararli, 1995)	Simple, short, smooth	Varied	Long, complex, often sacculated	Long and sacculated
Nails	Claws	Claws or sharp hooves	Hooves or soft nails	Soft nails
Body cooling technique (Willmer et al., 2009)	Panting	Panting	Sweating	Sweating

References

Boehlke C, Zierau O, Hannig C. 2015. Salivary amylase – The enzyme of unspecialized euryphagous animals. Archives of Oral Biology. 8 (60) 1162-1176.
Fehrenbach MJ and Herring SW. 2013. Illustrated anatomy of the head and neck. Elsevier Health Sciences
Feldhamer GA. 2007. Mammalogy: adaptation, diversity, ecology. JHU Press.
Herring SW and Herring SE. 1974. The Superficial Masseter and Gape in Mammals. The American Naturalist. 962 (108) 561-576.
Hillson S. 2005. Teeth. Cambridge University Press.
Kararli TT. 1995. Comparison of the gastrointestinal anatomy, physiology, and biochemistry of humans and commonly used laboratory animals. Biopharmaceutics and Drug Disposition. 16 (5) 351-80.
Linzey DW. 2000. Vertebrate Biology. McGraw-Hill Science/Engineering/Math.
Schwenk K. 2000. Feeding: form, function and evolution in tetrapod vertebrates. Academic Press.
Tucker AS and Miletich I, eds. 2010. Salivary glands: development, adaptations and disease. Vol. 14. Karger Medical and Scientific Publishers.
Ungar PS. 2015. Mammalian dental function and wear: A review. Biosurface and Biotribology. 1 (1) 25-41.
Willmer P, Stone G, Johnston I. 2009. Environmental Physiology of Animals. John Wiley & Sons.

Diets

Carnivores mainly or exclusively live on a diet of animal tissue. They can be predators or scavengers or both. Herbivores mainly or exclusively eat plant matter. Omnivores eat both.

Humans (Homo sapiens) are certainly capable of consuming both animal and plant foods. Whether we are morphologically and physiologically more adapted to consume one or the other, however, is something we will explore here. See for yourself whether we have more traits in common with carnivores, omnivores, or herbivores.

Figure 1 illustrates that diets are often more varied and that it is hard to draw sharp lines. For example, we here use the term herbivore to describe an animal whose diet mainly or entirely consists of plant matter; however, within that group of herbivores is a huge variety of preferred foods. While some animals nibble on soft fruits, others almost exclusively eat grass or mature leaves.

Similarly, the term omnivore encompasses animals that truly eat both animals and plant foods in roughly equal amounts, but also those who mainly live on fruits, nuts, and seeds, and only occasionally eat small animals if the opportunity presents itself.

Nature is complex, and there are exceptions to every rule. One excellent example is the panda bear. Although pandas are phylogenetically and morphologically in the order Carnivora, they certainly have a herbivorous diet.

Fig. 1

References:

Hiiemae, Karen M. “Feeding in Mammals”. In: Schwenk, Kurt, ed. Feeding: form, function and evolution in tetrapod vertebrates. Academic Press, 2000.

Buccinator muscle and gape

The mouth opening, or gape, in mammals serves a variety of purposes: breathing, vocalisation, prehension of food, but also agonistic display. The mammalian jaw system has adapted to each species’ behaviour and diet. All mammals have in common that the frontal, prehensile part of the jaw is covered by the lips and the premolar and molar teeth (used for chopping up and chewing food) are covered by the cheeks. The cheek muscle, musculus buccinator (Fig. 1), assists the tongue in placing and keeping food where it needs to be while we chew.¹

In herbivores, the mouth opening is generally small compared to the size of the head (Fig. 2). The buccinator is usually well-developed, since tough, fibrous plant matter requires thorough chewing.² The cheeks are thus more spacious to temporarily store the food that is being chewed.

Carnivores have a less-developed buccinator and smaller cheeks, since they tend to slice off bite-sized chunks of meat which they swallow whole. In contrast to herbivores, the buccinator is more u-shaped in carnivores and borders the margin of the upper and lower jaw. This allows them a wider gape angle (Figs. 2, 3). As most carnivores are predators, they require a wide gape to apprehend their prey.² The carnivoran jaw system – unlike that of herbivores – has adapted to achieve great bite forces at wide gape angles.³

What about humans?

The human buccinator muscle is more similar to that of herbivores (Fig. 1). Our gape angle is significantly smaller than that of carnivores (Fig. 2).

Fig. 1: The buccinator muscles in a typical carnivore (dog), herbivore (horse), and a human. The u-shape of the carnivores’ buccinator allows them a wider gape.

Fig. 2: Gape angle of several carnivores, omnivores, herbivores, and humans. Although there are some exceptions (like the hippopotamus), herbivores tend to have a smaller gape than carnivores. Humans are also on the lower end of the gape spectrum.

⁴

Fig. 3: A yawning lioness (photograph by Ramesh Ratwatte). Go look in the mirror and open your mouth as wide as you possibly can. It’s not quite the same, is it?

References:

Fehrenbach, Margaret J., and Susan W. Herring. Illustrated anatomy of the head and neck. Elsevier Health Sciences, 2013.
Galton, Peter M. “The cheeks of ornithischian dinosaurs.” Lethaia 6.1 (1973): 67–89.
Bourke, Jason, et al. “Effects of gape and tooth position on bite force and skull stress in the dingo (Canis lupus dingo) using a 3-dimensional finite element approach.” PLoS One 3.5 (2008): e2200.
Herring, Susan W., and Stephen E. Herring. “The superficial masseter and gape in mammals.” American Naturalist (1974): 561–576.

Jaws

A distinguishing feature between carnivores and herbivores is the posterior end of the jaw, the so-called angle (Fig. 1). It is the boney bit you can feel just below your ears. In carnivores and most omnivores, the angle is quite small. In herbivores and humans, it is expanded and convex.

Fig. 1: Skulls of a carnivore (dog), herbivore (horse), and a human. Dashed lines mark the level of the tooth row. Crosshairs mark the jaw joint. In carnivores, the jaw joint lies on the same level as the tooth row, which results in a scissor-like jaw motion; horizontal movement of the mandible (lower jaw) is, however, not possible. In herbivores and humans, the jaw joint lies above the tooth row, allowing for side-to-side and backward-and-forward jaw movement.

The size of the angle has to do with those groups’ differing relative proportions of the two main jaw-closing muscles (adductors): musculus temporalis and musculus masseter (Fig.2).

The masseter muscle attaches at the zygomatic arch and the outer side of the lower jaw. It is usually larger and stronger than the temporalis in herbivores to allow side-to-side motion of the jaw for grinding fibrous plant matter. In carnivores, the temporalis muscle is larger than the masseter to pull the lower jaw up with great speed and power. The masseter is relatively small in carnivores and mostly aids in stabilising the jaws during closing. Consequently, the angle – the part of the jaw where the masseter attaches – is small or even absent, but enlarged in typical herbivores.

Fig. 2: The two main jaw-closing muscles (adductors), masseter and temporalis, in dogs, horses, and humans.

Figure 3 gives an overview of the relative proportions of masseter and temporalis in several mammals, including humans.

Fig. 3: Jaw-closing muscles of several carnivores (red), omnivores (orange), herbivores (green), and humans (black). Note how much larger the temporalis muscle (dark grey) is in carnivores and omnivores compared to herbivores. Although the temporalis is slightly larger than the masseter muscle in humans, the distribution is nearly identical to that of the dromedary – an avid herbivore. The third adductor, the pterygoid, merely assists in the fine control of jaw movement, but does not add significantly to the bite force.

What about humans?

Humans – like herbivores – have an expanded, convex angle.
Their jow joint – like that of herbivores – lies above the tooth row, allowing for side-to-side and backward-and-forward jaw movement.
The temporalis muscle is slightly larger than the masseter muscle, and the latter is not as well developed as that of most herbivores. However, this is also observed in some herbivores, e.g., the dromedary.

References:

Schwenk, Kurt. Feeding: form, function and evolution in tetrapod vertebrates. Academic Press, 2000.
Turnbull, William D. Mammalian masticatory apparatus. Vol. 1088. Field Museum of Natural History, 1970.

Jaw motion and mastication

Most carnivores are predators who hunt, kill, and butcher their prey.¹ These predators kill either with a single, strong penetrating bite (e.g., mustelids and felids) or several more shallow bites (e.g., canids and hyaenids). Due to the morphology of their jaws and adductors (Fig. 1), they bite with a chopping (up and down) motion. Their jaw joint is on the same level as the tooth row, which allows for a scissor-like jaw motion, but makes horizontal movement (side to side, backwards and forwards) impossible.²

Once the prey is rendered immobile, carnivores use their sharp canine and carnassial teeth to tear and slice flesh into chunks which they swallow whole.¹ Some carnivores, like dogs, have large, robust molars, which allow them to crush bones, something cats are not capable of.¹

Although the dentition of omnivores (e.g., bears) is in general similar to that of carnivores, their carnassials are not as developed as those of true carnivores.³ Omnivore dentition is not well adapted to process tough and fibrous plant matter.¹ Hence, their food is either swallowed in whole chunks or simply crushed, but not thoroughly ground and chewed.

Herbivores feed on a broad range of plant foods, ranging from fruits, nuts and seeds to grass, leaves, and even tree bark. Extensive chewing is necessary to process such tough plant tissue. Their jaw, adductor, and tooth morphology is adapted to their herbivorous diet. Their canine teeth are reduced or even absent. Their molars, however, are capable of crushing and grinding tough and fibrous plant matter in a mortar-and-pestle fashion.⁴ The location of the jaw joint above the plane of the tooth row (Fig. 1) allows for side-to-side and back-to-front motion of the mandible (lower jaw) and, therefore, extensive chewing. (Picture a cow chewing grass.)

What about humans?

In humans – like in herbivores – the jaw joint is also located above the level of the tooth row. (Try moving your mandible from side to side and backwards and forwards.)

We also tend to chew our food quite thoroughly. (Please do not try swallowing whole chunks of food like a carnivore!)

Fig. 1: The jaw-closing muscles (adductors), masseter and temporalis, in a carnivore (dog), a herbivore (horse), and a human. The masseter is usually larger and stronger than the temporalis in herbivores to allow side-to-side motion of the jaw for grinding fibrous plant matter. In carnivores, the temporalis is larger than the masseter to pull the lower jaw up with great speed and power. The masseter is relatively small in carnivores and mostly aids in stabilising the jaws during closing.

References:

Feldhamer, George A. Mammalogy: adaptation, diversity, ecology. JHU Press, 2007.
Schwenk, Kurt. Feeding: form, function and evolution in tetrapod vertebrates. Academic Press, 2000.
Hillson, Simon. Teeth. Cambridge University Press, 2005.
Hiiemae, Karen M. “Feeding in Mammals”. In: Schwenk, Kurt, ed. Feeding: form, function and evolution in tetrapod vertebrates. Academic Press, 2000.

Teeth

One of the most distinguishing features between mammals with different diets are their teeth (Fig. 1). The dentition of a species has adapted to process a particular, preferred diet.

Carnivores, for example, have very prominent, sharp canine teeth used to apprehend, kill, and butcher their prey. Another set of characteristic carnivore teeth are the carnassials, which are the lower first molar and the upper fourth premolar. These sharp, blade-like teeth shear against one another like a pair of scissors and are used to slice off chunks of flesh.¹

Carnivoran incisors are relatively small compared to the rest of the dentition, with pointed, close-packed crowns that form a comb-like structure which the animals use to groom their fur.¹

Omnivores, such as bears, generally have a similar dentition to that of carnivores, with sharp, well-developed canines and small, pointed incisors (Fig. 1). However, their carnassial teeth are not as well-developed as those of strict carnivores.¹ Their teeth are not suited to process tough, fibrous plant foods, which is why they tend to either swallow their food in large, bite-sized chunks or crushed, but not thoroughly ground and chewed.²

An excellent example of dentition adaptation to diet within the same genus is that of the polar bear (Ursus maritimus). The polar bear is thought to have evolved between 700,000 and 150,000 years ago – which, in evolutionary terms, is rather recent – from coastal populations of the brown bear (Ursus arctos).³ However, while the brown bear is omnivorous, only occasionally eating meat, the polar bear is a strict carnivore, living on a diet of mainly seal flesh and blubber. As a result, the polar bear has reduced molars and premolars compared to omnivorous bears, since he does not require them to grind fibrous plant foods.⁴ However, he also does not have the sharp, well-developed carnassials of a true carnivore.⁴

Fig. 1: Upper and lower permanent dentitions of a typical carnivore (dog), omnivore (bear), herbivore (deer), and a human.

Herbivores generally do not have prominent canine teeth. Exceptions are species that have large canines that they use for sexual display or agonistic behaviour. Such is the case with gorillas or male musk deer (Fig. 2), for example.

Unlike carnivores, herbivores have broad, rather flat premolar and molar teeth, which close in perfect occlusion and which they use to thoroughly grind their food in a mortar-and-pestle fashion.⁵ Plant matter requires extensive chewing in order to break down tough cellulose cell walls.

Fig. 2: Although an avid herbivore, the male musk deer has long, sharp canine teeth which he uses for agonistic display and in combat with other males (Images: a. from [1]; b. by Nick Usik).

Fig. 3: Schematic of the dentition of a dog and a human.

⁶

What about humans?

Unlike carnivores, humans have spatulate incisors and reduced, rather blunt canines (Fig. 3). The canines are similar in size to the adjacent incisors and premolars. Our premolars and molars are broad and close in perfect occlusion – ideal for grinding and chewing those fruits, vegetables, and grains. All in all, our dentition has more characteristics in common with that of an herbivore.

References:

Hillson, Simon. Teeth. Cambridge University Press, 2005.
Feldhamer, George A. Mammalogy: adaptation, diversity, ecology. JHU Press, 2007.
Slater, Graham J., et al. “Biomechanical consequences of rapid evolution in the polar bear lineage.” PloS One 5.11 (2010): e13870.
Sacco, Tyson, and Blaire Van Valkenburgh. “Ecomorphological indicators of feeding behaviour in the bears (Carnivora: Ursidae).” Journal of Zoology 263.1 (2004): 41–54.
Hiiemae, Karen M. “Feeding in Mammals”. In: Schwenk, Kurt, ed. Feeding: form, function and evolution in tetrapod vertebrates. Academic Press, 2000.
Collins, A. Animal Kingdom: Comparative Anatomy. www.slideshare.net/veterinaria_urp/animal-kingdom-comparative-anatomy

Saliva

Different dietary habits of mammalian groups are reflected in the size and importance of the three major salivary glands (the parotid, sublingual, and submandibular glands¹ (Fig. 1) and the viscosity of the saliva they produce.

Fig. 1: Human salivary glands.

The paratoid glands generally produce a serous (watery) saliva. The glands are larger, more developed in herbivores and humans than in carnivores.¹ This is due to the fact that the plant matter that herbivores, especially browsers and grazers, consume is often quite dry and requires extensive chewing before swallowing.¹

The saliva secreted by the submandibular glands of carnivores is mucous, while that of herbivores, rodents and humans is mostly serous.¹ The mucous saliva of carnivores is helpful as they tend to swallow chunks of meat whole or after minimal chewing. In omnivores, the submandibular glands produce mixed (seromucous) saliva.

What about humans?

With large, well-developed parotid glands and submandibular glands that produce a mostly serous saliva, humans share more traits with herbivores.

References:

Tucker, Abigail S., and Isabelle Miletich, eds. Salivary glands: development, adaptations and disease. Vol. 14. Karger Medical and Scientific Publishers, 2010.
Blausen.com staff. “Blausen gallery 2014”. Wikiversity Journal of Medicine (2014).

Digestive tract

The morphology of mammals’ digestive tracts reflects their evolutionary adaption to different diets (Fig. 1). The digestive tract of herbivorous mammals is generally much longer than that of carnivores.¹ The increased length – especially of the small intestine – allows for more time for the cellulose of plant cell walls to be broken down by microorganisms.

Fig. 1: Digestive systems of various mammals with different diets. Note how much longer the digestive tracts (especially the small intestine) of herbivores are compared to that of carnivores.

Fig. 2: The human digestive tract. Its length is around seven times that of the body length.

What about humans?

Although the human gastrointestinal tract is not as long as that of herbivores such as horses, cattle, or sheep, it is certainly not as short as that of carnivores such as lions or dogs (Tab. 1). In fact, humans and elephants have the same ratio of digestive tract length to body length.

It is important to note that there are exceptions to every rule. This is especially true when it comes to highly specialised feeders. The sloth and the panda, for example, both herbivorous, have digestive tracts that are only 3.3 and 4.5 times as long as the body length, respectively.² On the other hand, seals and dolphins, both carnivores, have digestive tracts that are up to 30 times as long as their body length² – longer than that of ruminant herbivores such cattle and sheep (Tab. 1).

Treeshrew (Ptilocercus) – O	125
Quoll (Dasyrus) – C	200
Lion (Panthera leo) – C	300
Tarsier (Tarsius) – C	470
Guenon (Cercopithecus) – O	500
Dog/Wolf (Canis) – C	500
Platypus (Ornithorhynchus) – C	520
*Human (Homo sapiens)*	700
African elephant (Loxodonta) – H	700
Bear (Ursus) – O	800
Rock hyrax (Procavia) – H	900
Kangaroo/Wallaby (Macropus) – H	1000
Orangutan (Pongo) – O	1000
Horse (Equus) – H	1000
Cattle (Bos) – H	2000
Sheep (Ovis) – H	2300

Tab. 1: Length of digestive tract as percentage of body length (measured from mouth to anus) for some carnivores (C), omnivores (O), herbivores (H), and humans (bold)²

References:

Linzey, Donald W. Vertebrate Biology. McGraw-Hill Science/Engineering/Math, 2000.
Starck, Dietrich. Vergleichende Anatomie der Wirbeltiere auf evolutionsbiologischer Grundlage. Band 3: Organe des aktiven Bewegungsapparates, der Koordination, der Umweltbeziehung, des Stoffwechsels und der Fortpflanzung, 1982.

Colon

The role of diet in colon morphology becomes clear when considering that even within the same order of mammals, differences can occur. Such is the case within the order of primates: more carnivorous primates have simple and smooth-walled colons, while frugivorous primates have an elongated, sacculated colon.¹

Absorption of water, sodium, and other minerals, but also production and absorption of volatile fatty acids, takes place in the colon.² Furthermore, the colon harbours the largest population of microorganisms in the digestive tract. These bacteria produce essential vitamins; for example, vitamin B₁ (thiamine), vitamin B₂ (riboflavin), vitamin B₁₂ (cobalamin), and vitamin K, which are then absorbed by the colon.

Colon morphology varies between species with different diets. Carnivores generally have short and smooth colons (Fig. 1). Herbivores, on the other hand, tend to have long and sacculated colons.¹²³⁴ Colonic morphology in omnivores is highly variable. Whereas some have short and smooth colons similar to those of carnivores (e.g., rats), others (e.g., pigs) have either long or medium-sized colons, which are also sacculated.²

What about humans?

The human colon, like that of herbivores, is long and sacculated.²

Fig. 1: Digestive tracts of carnivores and herbivores (ruminant and non-ruminant.

⁴

Fig. 2: Anatomy of the human colon.

⁵

References:

Chivers, David J., and Claude Marcel Hladik. “Morphology of the gastrointestinal tract in primates: comparisons with other mammals in relation to diet.” Journal of Morphology 166.3 (1980): 337–386.
Kararli, Tugrul T. “Comparison of the gastrointestinal anatomy, physiology, and biochemistry of humans and commonly used laboratory animals.” Biopharmaceutics & drug disposition 16.5 (1995): 351–380.
Alvarez, Walter C. “An Introduction to Gastro-Enterology.” Paul B. Hoeber, Inc., New York (1940).
Feldhamer, George A. Mammalogy: adaptation, diversity, ecology. JHU Press, 2007.
Blausen.com staff. “Blausen gallery 2014”. Wikiversity Journal of Medicine (2014).

Body Cooling

Most mammals regulate their body temperature through evaporative heat loss. There are two main methods of doing this: sweating and panting. Whereas most herbivores have sweat glands spread out over the body surface, sweating being their predominant body cooling technique, nearly all carnivores lack these glands and pant in order to cool down (Fig. 1).¹

What about humans?

We humans – like most herbivores – regulate our body temperature through sweating (Fig. 2).

Fig. 1: Carnivores, like this dog, cool their body by panting, i.e., rapid breathing through the mouth (Image: Jenny Di Leo).

Fig. 2: Humans and most herbivores sweat from specific glands spread across the body surface, e.g., in the armpits (Image: Corbis).

References:

Willmer, Pat, Graham Stone, and Ian Johnston. Environmental Physiology of Animals. John Wiley & Sons. 2009.

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