On your last trip to the country, you probably came across cows peacefully chewing a few blades of grass by the side of the road. A question may have crossed your mind: why can cattle feed on grass when I can’t? And the mystery thickens: what about horses, rabbits and other herbivores? What do they have that we don’t? Today, we plunge into the heart of a very vegetal enigma!
There is a catch…
To survive, the body needs nutrients. These are essential substances that are provided by the diet. Among the main ones are carbohydrates or “carbohydrates”. There are 2 types of carbohydrates:
Digestible carbohydrates (starch, sugars…). Enzymes in the digestive tract can break them down. They are therefore easily absorbed.
Dietary fibers. That’s where it gets stuck: digestive enzymes are not able to break them down. Only certain micro-organisms can (partly).
But grass contains large amounts of fiber… So if you want to digest it and eat it, you have to find another strategy. There are two main types of grass: ruminants and non-ruminant herbivores.
The strategy of cows and other ruminants
This group includes many species: cattle, sheep, goats, giraffes, deer, antelopes… There are also pseudo-ruminants that have a rather similar approach: hippos, camels, dromedaries, llamas, alpacas, hamsters, kangaroos, wallabies…
- A very special tactic
Their stomach is gigantic. It is composed of 4 pockets (2 or 3 for the pseudo-ruminants). The last one is more or less the equivalent of the human stomach. On the other hand, the first 3 are pre-stomachs filled with micro-organisms (bacteria, fungi, protozoa…) degrading the fibers (by fermentation).
To make it easier for them, ruminants “ruminate”: they swallow large quantities of grass quickly with hardly any chewing. And from time to time they regurgitate a part of it, chew it again to break it down further, then raid it. It’s a bit like eating your breakfast at full speed at 8am, then decide to regurgitate it later in the morning to chew it a little better!
- Advantages and disadvantages
By degrading these feeds that cannot be used by humans, microbes provide ruminants with a source of energy, high quality proteins, and vitamins that are then absorbed and distributed throughout the body. In fact, these microbes are the one and only natural source of vitamin B12, which is why vegetarians who do not eat any animal products must take supplements.
However, these microbes also take advantage of this to produce greenhouse gases that will then be eructated. Yes, contrary to popular belief, it’s not the cow’s farts that pollute, it’s their burps!
The strategy of non-ruminant herbivores
These animals do not have a “super stomach”. So they opted for various B plans. But they are less efficient. As a result, these herbivores have to feed on better quality plants. Let’s look at some of these approaches.
- The horse, elephant, and rhinoceros
Their tactic: an overdeveloped colon! This is where the many fiber-degrading microorganisms are located. The mechanism (microbial fermentation) is quite similar to that found in bovine pre-stomachs.
However, the absorption of vitamins and microbial proteins is not optimal in the colon. It is much better in the small intestine. The latter is located after the pre-stomachs of cows, which can therefore absorb the microbial products produced upstream. However, it is located before the large intestine of horses, which cannot benefit from it as much.
- Rabbits and some rodents (guinea pigs, chinchilla…)
Their tactic: an overdeveloped cecum! The cecum is the first part of the large intestine, just before the colon. In rabbits and some rodents, it is very large compared to their size and it is there that the many fermenting microbes are located.
These animals also have the sweet peculiarity of eating part of their excrement (caecotrophy). We saw earlier that the horse cannot benefit from all the microbial products created in its large intestine. Well, rabbits have found a solution: they re-ingest their droppings! The droppings go back to the beginning of the digestive system, reach the small intestine, and thus bring the many nutrients that would otherwise not be assimilated.
We human beings have no overdeveloped stomach, no overdeveloped colon, and no overdeveloped cecum. Our stomach has only one pouch, our large intestine is of medium size and we do not eat our feces (don’t try it, we do not make cecotrophs anyway, the excrement is suitable for re-ingestion!). Therefore, our anatomy and physiology do not allow us to feed in the same way as herbivorous animals.
However, eating fiber remains essential for humans, because even if we assimilate it very poorly, it stimulates the motor skills of our digestive tract. There, you have been told the essential, you are now pros of comparative digestion!