October 25, 2020

Anatomical Differences – Herbivores

There are many different types of herbivores [Wikipedia]. Many eat a variety of plants, while some stick to one type of plant. Those who stick to one type of plant have their own special classifications. For example, animals that eat primarily fruit are called frugivores, according to Encyclopedia Britannica. Fruit bats and flying foxes are examples of frugivores. And, animals and insects that eat mostly leaves — such as pandas, caterpillars, giraffes or koalas — are called folivores.

Herbivores evolved differently. Their teeth function to rupture cell walls to release nutrients that would otherwise pass through the gut undigested, and to fragment tough, fibrous plant parts to increase surface area available for digestive enzymes to act on. Herbivores often have more square-shaped teeth with broad but complex biting surfaces – planes cut by rows of low crests or ridges connecting small cusps. Vegetation is ground or milled between opposing teeth when lowers slide along uppers in the direction opposite the orientation of the crests. For cows and sheep, those crests run anteroposterior, and horizontal movement during mastication is buccolingual. For many rodents, crests run buccolingually, and the horizontal movement is posterior to anterior, or propalinal. The herbivores anatomical features are adapted to process carbohydrates and other nutrients produced by plants.

Anatomical features common to herbivores confirm their adaptation for a plant-based diet.

Ruminant Herbivore Four-Chambered Stomach with Large Rumen
  • Long Digestive Tracts, up to 10 times their body length are needed due to the relative difficulty with which plant foods are broken down. Herbivores have significant longer and more elaborate guts than do carnivores.
  • Square and Flat Molars, providing an ideal grinding surface to crush and grind plants. A lower jaw with pronounced sideways motion facilitates the grinding motion need to chew plants.
  • Carbohydrate-Digesting Enzymes in the Saliva, amalyze is a digestive enzyme in saliva that helps in digesting carbohydrates. Herbivores methodically chew their food to ensure the thorough mixing with amylase.

The digestive systems of carnivores and herbivores are very different. Carnivores typically have only one stomach chamber and a simple digestive system. Herbivores can often have several stomach chambers and a much longer digestive tract. Herbivores with multiple stomach chambers — such as camels, deer, sheep, giraffes and cattle — are called ruminants.

No-Ruminant Herbivore - Simple stomach, large cecum.

This is because plant cells are tough, and the cellulose that makes up their cell walls is difficult to digest. Digestion starts with the teeth. Herbivores have large, flat teeth that grind up plant materials. In contrast, carnivores have mostly sharp, pointed teeth that are used for tearing flesh. Once the plant materials are chewed, special bacteria in the gut of an herbivore and the longer digestive tract break down the plant material.

Food for thought: There is no animal or mammal on the planet, visible to the naked eye, that is able to digest (and for our purposes, “digest” will mean reduce to the point nutrients can be extracted) plant materials without help. Not even the herbivore can do this.

Let’s contextualize the role and importance of digestive enzymes and bacteria for starch and other plant materials in herbivores. The digestive enzyme amylase is required to begin the breakdown of the plant materials (carbohydrates) in the mouth – that very first bite. This is required since the metabolism of plant materials (carbohydrates) takes a long time. Not even the herbivore’s digestive system is efficient enough to digest all plant material when the process starts in the mouth with saliva. The herbivorous cow has both enzymes and plant eating bacteria in his saliva. This means that at first bite, digestion of plant materials is well underway – not just reducing the size of food particles through chewing, but actual digestion of plant materials at the cellular level as food particles are coated with plant cell reducing agents. Despite this jump-start, the cow has 4 stomach chambers​1​ – think of them as fermentation chambers – where massive amounts of bacteria feed on ever reduced food particles​2​. Even then, it’s not enough, ergo the cow’s ability to regurgitate stomach contents back to the mouth for further chewing / particle reduction (the term is rumination) which is then swallowed back down into the fermentation chambers. Ruminants regurgitate food and re-chew it to help with the digestive process. This regurgitated food is called cud. After swallowing the cud, it travels to the second stomach chamber. After it softens, the cud goes back up to be chewed again and then goes down to the third chamber. This keeps happening until the cud has made its way through all of the stomach chambers. Enzymatic and bacterial action continue through the intestines right up to the point that ingested plant material becomes a cow patty on the ground.

Via ruminal fermentation, cows are eating a diet that consists of about 70 to 80% fat, mostly saturated, 20 to 30% protein, and virtually zero carbohydrates. And yet, we expect our dogs and cats to thrive on a diet consisting of between 28% to 55% of carbs!

We can therefore safely assume that our species “dog” does not have the ability to process plant materials, when compared to herbivores. No flat teeth, no multiple chambers, no amalyse in the saliva, no bacteria to break down plant materials.

Carnivores, such as cats and dogs, cannot use fiber as an energy source. Carnivores have a much simpler digestive system. But that is not an issue, because wild carnivores eat nutrient-rich prey, typically herbivores, which provide the carnivore with a HIGH FAT, LOW CARB diet. The cow, the gorilla, and the lion all eat a HIGH FAT, moderate protein, LOW CARB diet.

References and Research

  1. 1.
    Bradford A. Herbivores: Facts About Plant Eaters. Live Science. Published 2016. https://www.livescience.com/53452-herbivores.html
  2. 2.
    Understanding the Ruminant Animal’s Digestive System. The Cattle Site. Published 2009. http://www.thecattlesite.com/articles/2095/understanding-the-ruminant-animals-digestive-system/