Differences in digestive system between herbivores, carnivores and omnivores
Within the animal kingdom, three distinctions can be made based on the diet: herbivores, carnivores and omnivores. The word ‘-vores’ stems from the Latin word -vorus: eater. Besides this, the words ‘herbi’, ‘carni’ and ‘omni’ stem from the Latin words:
herba: plant caro: meat omnis: all
In general, all animal species, from fish to birds, can be placed in a category. While some species are easily placed within a group (i.e. zebra), others may be more difficult as their diet is closely related to two groups (i.e. dwarf mongoose). In addition to this, it can also be the case that a young animal is herbivorous while it gets more omnivorous as the animal grows older.
The body adapts itself to digest food as efficiently as possible. For this reason, physiological differences occur between herbivores, carnivores and omnivores, which can mainly be seen in the digestive system. In general, animal material is considered as easily digestible while plant material is seen as difficult to digest. Therefore, the digestive system of herbivores is most complex while that of carnivores is least complex. Compared to terrestrial animals, birds are an exception in terms of digestive system (read more).
Adaptions in digestive system
The primary function of the digestive system is to break down food and moving the nutrients towards cells in the body. In turn, they can use these nutrients for building tissues or energy production. In order to do this efficiently, adaptations have occurred within the digestive system of herbivores, carnivores and omnivores. The biggest differences can be seen in the teeth, the stomach and intestines.
The digestive system start at the mouth, where the teeth grind the food. The teeth have adapted through evolution based on the food that is being eaten. For example: as a result of a plant-based diet, herbivores have large flat molars that help grind fiber-rich material. In comparison, the teeth of carnivores has well-developed incisors and canines; which are used for catching prey and tearing meat. Omnivores have mixed teeth compared to herbivores and carnivores, which can be used for both grinding as well as tearing. In this way, each animal has teeth that are specialized for their specific natural diet (see image). As a result, it is often possible to identify an animal just by looking at the skull and the placement of the teeth.
Source adjusted from: https://drbillspetnutrition.com/carnivores-omnivores-herbivores/
After the teeth, food passes through the oesophagus into the stomach. Here the food is stored, kneaded and mixed with gastric juices that aid in further digestion. The stomach can differ significantly between carnivores, herbivores and omnivores.
Carnivores have a relatively simple stomach because the material that they consume is digested more easily compared to plant material. In herbivores, the stomach is more complex because the energy in plant material is difficult to access. Therefore, a special mechanism has evolved to 'release' this energy: fermentation. During the fermentation process, bacteria break down difficult-to-digest material, after which the nutrients can be further absorbed by the body. Fermentation can occur in two places: the stomach and the large intestine. Stomach fermentation mainly occurs in ruminants (such as giraffes or deer) and some monkey species. Although fermentation in the stomach is rare in omnivores, their stomach is better able to process plant material than a carnivore stomach. Intestinal fermentation takes place in the large intestine of animals with one stomach compartment (non-ruminants), such as most herbivores, and many omnivores also have functions of this.
After the stomach, food is gradually moved to the small intestine, where it is largely hydrolysed and absorbed. Here, the biggest differences can be seen in the length of the intestine. This provides information about the time the food slurry is in the gut to be absorbed. Easily digestible material takes less time to be absorbed and therefore the gut is also shorter, as in carnivores. The opposite is true for hard-to-digest material, such as in omnivores and herbivores.
After the small intestine comes the large intestine. This is where resorption and fermentation takes place. This form of fermentation takes place in carnivores, omnivores and herbivores with fermentation in the stomach. However, it is a lot better developed in herbivores with intestinal fermentation. The image below shows how in kangaroos (stomach fermentation) the stomach is better developed than the large intestine. The pony (intestinal fermentation) shows how the large intestine is considerably better developed than the stomach. Because fermentation in the gut is less efficient than fermentation in the stomach, these animals have to consume larger amounts of food to meet their energy needs.
Source adjusted from: https://www.sciencedirect.com/science/article/pii/S1095643308009914
The digestive system of herbivores is more complex compared to carnivores and omnivores, enabling them to consume difficult-to-digest material, such as plants. An omnivore's digestive system has adapted to be very flexible, and therefore they can consume a wide range food items. Carnivores have the simplest digestive system, which makes them less flexible in terms of the diversity of their food. In addition, they are more dependent on a specific diet because otherwise nutritional deficiencies can arise.