Herbivores cannot digest grass, at least not on their own. In reality, most herbivorous vertebrates use a bacteria that lives inside their intestines, which is what actually digests this type of food. In general, animals do not have enzymes that can hydrolyze cellulose, so we need other organisms to do it for us. Another example is the human microbiome itself, the set of bacteria that live in our body and without which we would not be alive. In the same way, the microbiome uses our own organism to survive.
In biology, this type of association is called symbiosis, a persistent biological interaction between organisms of different species. For it to occur, both species provide a number of advantages that allow their survival.
The key to microbial associations with animal organisms is balance. It is a favorable interaction as long as the body remains in good health. Without bacteria, the body would not survive, but an imbalance in the balance can lead to some conditions.
Although there are microbes populating the entire body, most of them exist in the digestive system, and that is where they perform most of their functions. It is estimated that there are about 40 billion microbes in the human body, representing approximately 3% of human weight.
Interestingly, in addition to not having enzymes that digest cellulose, humans are not prepared to digest foods such as potatoes or legumes. To do this, we have to subject them to some processes that change their chemical composition, such as cooking.
In the case of herbivorous animals, bacteria that digest cellulose are found in their digestive tracts. The group of ruminant herbivores, for example, carry out the ingestion process in two phases: first, chewing; and then they regurgitate the food to re-soak it in saliva. Once this is done, they pass into the digestive tract and are treated by the microbiota that the animals harbor inside. Your saliva, like your urine, is alkaline. Saliva also contributes to maintaining a correct balance of microbial activity in its access to the gastric system.
Their type of diet also means that herbivores tend to have a series of characteristics that are very different from those of carnivorous mammals: flat teeth and molars, to crush rather than tear; elongated mouth, and are devoid of claws.
