What do herbivores eat?

Many farm animals are vegetarian. Most of their time they put green plants in their mouths (or were such - for example straw), chew them and swallow them. But do they really feed on the green plants?

Dror Bar-Nir

The distinction between herbivores (herbivores - from the Latin words herba, which means grass, and vorus-, which means nourish, eat) and carnivores, the meat eaters (carnivores - from the Latin word carne, which means meat) is famous. To remind you, we, along with the bears (and other animals), belong to a third group known as Omnivores (from the Latin word Omne which means everything).

We also know the herbivores from our immediate surroundings. Many farm animals are vegetarian - cattle, sheep, horses. When we look at them, we do see that most of the time they put green plants in their mouths (or were such - for example straw, hay), chew them and swallow them. But do they really feed on the green plants?

Cellulose joints and other sugars that make up the plant wall

Our digestive system secretes enzymes (the first of which - for example amylase, the enzyme that breaks down starch - are already secreted in the mouth), which break down food components that cannot be absorbed as they are. The substances that we managed to break down are absorbed through the walls of the intestines into the bloodstream, and from there reach the cells of our body.

If we extract the digestive enzymes and provide them in the test tube with the optimal conditions for their activity, and put animal food into the test tube when it is ground (instead of being chewed), the food will break down into its components in the test tube. If we introduce plants into such a system, most of the plant matter will not be digested, but will remain in the system undigested.

If we do this with the enzymes of a cow's digestive system, we will put plant pulp in a test tube and check the result, even there we will find out, perhaps to our surprise, that most of the plant material does not decompose.

The main components of plant food are cellulose, hemicelluloses - also known as cellulose-linking polysaccharides, and various pectins. These three are the main components of plant cell walls. These are energy-rich substances, which almost no animals are able to break down and utilize (with few exceptions). Those that break down the large plant mass are several species of microorganisms - bacteria, protozoans and fungi. These microorganisms secrete decomposing enzymes into the environment, absorb the decomposition products and feed on them.

Another important polysaccharide, found mainly in seeds and tubers, is starch. Unlike the polysaccharides that make up the wall, omnivores are able to break down starch; But in the digestive system of the herbivores it is mainly digested by microorganisms.

What happens in mammals?

Among the herbivorous mammals, two important groups can be noted, according to the structure of their digestive system.

ruminants The name of these animals in English is derived from the name of a special section, huge in volume (in a cow, about 150-100 liters; in a sheep, about 6 liters) where the food passes (and returns from it to the mouth for further grinding - raising the rumen) before it reaches the stomach. real", a section known as the digestive system (rumen). The digester gets a lot of saliva, which contains only salts (and no digestive enzymes), including bicarbonate (NaHCO3). In the solvent, the pH is kept close to neutral (6.5), the temperature remains constant, around 39ºC, and the environment is airless.

In the fermenter, the plant food stays for about 12-9 hours, in the company of a wide variety of about 1011 microorganisms per gram. Certain microorganisms (see box) break down the polysaccharides that enter the solvent into smaller sugars - disaccharides and monosaccharides. Other microorganisms ferment the small sugars into organic acids: acetic acid, butyric acid and propionic acid. These acids are absorbed into the bloodstream directly from the solvent, and are actually the main source of energy for these herbivores.

Other products of the fermentation processes in the digester are the gases carbon dioxide and methane, which are emitted from the digester in hiccups (on the importance of methane-producing archaea in the digestive systems of herbivores and their contribution to the greenhouse effect, see this section, issue 107). But energy alone is not enough: the herbivores need amino acids, vitamins and other substances to build their body components. These ingredients are provided by the microorganisms, the excess of which (since they multiply in the solvent and the space there is limited) reaches the "real", acidic stomach, and is digested there and in the intestines.

The other non-ruminant herbivores (horses, rabbits) also have special sections for digesting polysaccharides; In rabbits it is mainly the cecum, while in the horse it is mainly the large intestine, but also the cecum. Also in these sections, as in the digester, there is a large pool of microorganisms that break down the plant polysaccharides. But unlike in ruminants, these sections are located after the stomach and small intestines, so the excess bacteria are excreted with the feces, and are not digested.

Although the herbivores put plants in their digestive tracts, they actually feed on the decomposition products of those plants
A peek inside

To study the microbiology and biochemistry of the nutrition in the digester, researchers cut a section from the outside into the digester, and install a kind of tube with a special lid that allows physical access to the digester. This surgical device is called a fistula. Using the fistula, you can check the composition of food and microorganisms at any time.

The cellulose in the solvent is broken down by the enzyme cellulase. Bacteria of the species Fibrobacter succinogenes, for example, contain the enzyme in their periplasmic space (between the inner membrane and the outer membrane), so they must adhere to their substrate. Bacteria of the species Ruminicoccus albus, on the other hand, secrete the enzyme outside the bacterial cell.

The hemicelluloses are broken down by Butyrivibrio fibrisolvens bacteria, using the enzyme hemicellulase.

The pectins are broken down by Lachnospira multiparus bacteria using the pectinase enzyme.

The lignin is broken down by bacteria from the actinomycetes group, with the help of the enzyme lignin peroxidase.

The vast majority of non-mammal herbivores also depend on microorganisms in their digestive system for food digestion. A tangible demonstration of the matter was given by the vegetarian iguanas in the Galapagos Islands, which began to die en masse after relatively mild oil pollution (in 2001). In the examination, undigested algae was found in their stomachs, which remained after the oil damaged the population of microorganisms in the joints of the plants in their intestines (see "Beneficial and Harmful"), causing the iguanas to die, in fact, of starvation.

The termites and the lignin decomposers

Another plant material that herbivores are unable to digest is lignin. Lignin is a huge phenolic polymer, which gives plants their rigidity (woodiness). Most herbivores don't touch it at all. But nevertheless, there are creatures that feed on it (and thus also recycle it). The most familiar example to us is the termites, the social insects that devour the trees from the inside. Even termites cannot break down lignin, and aerial bacteria from the actinomycetes group, from the genera Streptomyces and Thermospora, perform this job for them. The primary decomposition products of the lignin are decomposed by other bacteria, usually under non-aerial conditions.

To sum up: the herbivores do ingest plants into their digestive tracts, but they actually feed on the decomposition products of those plants in the metabolism of different microorganisms and these microorganisms themselves. Can we call them (at least for migratory birds) "bacteria eaters"?

What happens on Passover?

The population of microorganisms in the digester is adapted to the cow's diet. This is particularly noticeable in the dairy industry, when a few weeks before Passover, kosher drivers replace the mixture that the cows receive (which contains wheat, barley and other types of forbidden grains, which are classified as chametz) with "kosher for Passover" mixtures, based on corn and sorghum kernels.

Following the exchange, the cows suffer from problems in the digestive system, mainly manifested by diarrhea, and in some the milk yield decreases. In extreme cases (which are not relevant for Passover), for example when cows are transferred at once from a grazing diet (which is based almost exclusively on breaking down the components of the wall) to a grain diet (mainly starch), bacteria that ferment starch into lactic acid (such as the bacterium Streptococcus bovis) "take over" the population of the digesting bacteria (from 107 bacteria per milliliter in the grazing condition to 1010 bacteria per milliliter). Due to their activity, the pH of the solvent drops, and most of the other bacteria do not survive. This acidosis can be fatal to the cow. A gradual transition (lasting a few days) from a grazing diet to a grain diet allows other, slower starch-degrading bacteria, such as Ruminobacter amylophilus, to multiply while maintaining a neutral pH, and the fatal outcome is avoided.

Raising cows on a high-starch diet causes a decrease in the pH (increase in acidity) of the intestinal contents, which is after digestion. The increase in acidity allows disease-causing strains of Escherichia coli, which are relatively resistant to acid, to thrive, and eventually reach us through the meat industry.

On agricultural ants, a harmful fungus and the hidden drug suppliers

Many species of ants (the most studied species is the ant Atta sexdens) in the tropical regions of Central America have been growing mushrooms in their nests for 50 million years. The mushrooms are of different species, but they all belong to the Lepiotaceae family. The ants eliminate all competitors of the fungus and provide it with its food - plant leaves that are toxic to the other herbivores in the forest. The fungus grows inside the nest, and the ants themselves feed on the parts of the fungus and its sugar secretions. When a queen ant leaves to found a new colony, she carries a piece of mushroom in her mouth that will form the basis of "agriculture" in the new nest.

When the fungus is grown in the laboratory, separately from the ants, the same aggressive mold of the genus Escovopsis always develops on the mushrooms (taken from dozens of nests). This mold is not observed in the active nests, and the researchers wondered how the ants manage to remove it. The solution to the mystery was found when the content of a white substance, found on the back of the ants in the nest, was investigated. It turned out that the white substance is a dense colony of bacteria from the actinomycetes group, of the genus Streptomyces. These bacteria secrete an antibiotic substance that specifically damages the aggressive mold, thus protecting the fungus that feeds the ants.

If so, it turns out that ants were ahead of us both in agriculture and in the use of antibiotics by about 50 million years.

The honey guide - and the hidden partners

The two species of African birds of the genus Indicator, called in Hebrew "obile" (because of the transport action, in English Honeyguides, I. indicator in the north of the African continent and I. minor in the south; from the woodpecker family), are known for their beauty and their love for the products of wild beehives. These birds lead humans, honey badgers (and even bears, until their extinction in Africa) towards the hives. After the "movers" dismantle the hives and rob their contents, the birds feed on the remains (which for a small bird are certainly sufficient).

for further reading

The activity of cellulolytic bacteria in ruminants, Dr. Yehoshua Miron and Prof. Daniel Ben Gedaliah, "Synthesis" 9, pp. 33-32, June 1995.

One of the "energetic" products produced in beehives, besides honey, is beeswax, which is an ester of a fatty acid and a long-chain alcohol. Birds do not have suitable enzymes to break down the wax, so they cannot use it without the bacteria Micrococcus cerolyticus and the yeast Candida albicans, which live in their intestines and break down the wax into its components available to the bird.

Dr. Dror Bar-Nir teaches microbiology and cell biology at the Open University

From: Galileo Magazine, April 2008