An angry question is sent by a reader frustrated by counting calories and chewing celery: How is it that cows that eat straw, grass and other leaves and foliage that are so recommended for diets are so fat?
If we mean the cows living in the barn today, then part of the responsibility for the body dimensions is ours: eA person has cultivated breeds with a large body mass for meat and milk and insists on feeding the cows a different feed than what they are adapted to. But even without human intervention the cattle is a fairly large mammal. Observing the animal world shows a paradox: those who eat grains and seeds high in carbohydrates and oil are the small rodents, those whose food is dates, figs, berries and other sweet fruits are the agile and compact bats, while those who carry their oils heavily, such as cows, feed on grass with no caloric value.
The answer to being thin is also paradoxical: you need a lot of calories to be thin. In mammals, most of the energy is wasted on maintaining body temperature. We lose heat to the environment through the skin and the smaller the animal, the more skin area it has in relation to its weight. The mouse has to eat a lot relative to its weight to compensate for the heat loss and therefore must eat high calorie food. This is why there are no birds among the herbivores: those who want to fly cannot get fat and therefore must consume a lot of calories in relation to their weight. The cow, on the other hand, loses (per unit of weight) much less heat and can therefore afford low-calorie food. But why are the green salads so low in calories? The grass and hay is made up mainly of cellulose (cellulose) which is the material from which paper is also made. It is surprising to discover that the paper or the wood that have no caloric value for us are built from chains of the same sugar that makes up the starch in the flour or the potato. The calorimeter (a device that measures the emission of heat from burning food, for example) will not show much difference between hay and candies of the same weight. But, unlike candy, hay will pass through our digestive tract without being absorbed.
Why can't we digest cellulose?
Our digestive system is powerless against cellulose because we do not have the necessary enzymes to break down these chains into the simple sugars that make them up. Cellulose is a polymer that evolution developed as a structural material: a stable skeleton that is required for stability throughout the life of the plant. The sugars are bound together in densely packed fibers and glued together by another stable polymer: lignin. A dedicated and complex system is required to break down this tough material into its building blocks. Thus, even though almost half of the carbon absorbed by plants is found in cellulose fibers and even though genes coding for enzymes that break down cellulose were an active part of the ancestral genome common to all animals, the direct utilization of cellulose as food is the property of a few creatures that have specialized in this, mainly unicellular and fungi. Other creatures give up the cellulose and it is excreted as it entered or outsource the project, meaning they create symbiotic systems with bacteria.
To adapt the digestive tract to accommodate bacteria, anatomical adjustments are required. The panda, a predator that evolved to eat bamboo, did not develop such an adaptation and therefore does not digest most of the organic material in the canes it eats (and therefore requires more eating). The cow and other ruminants have two stomachs adapted to function as an incubator for cellulolytic bacteria. The cow swallows the grass and after the digestion has started, she brings it up again in her mouth, continuing to chew to break the fibers well and make the pulp more comfortable for bacteria. The cellulose pulp is indeed broken down by the bacteria into sugar, but most of the sugar is consumed by the microscopic friends themselves and the cow has to make do with the leftovers.
The intestinal bacteria of the cow "steal" all the sugar
A herbivorous animal needs to contain a large volume of grass in its stomach to obtain the necessary energy and a large stomach needs a large body around it. That's why small animals that feed on cellulose are rare, so among the million or so insect species that infest the earth less than a hundred digest cellulose, the well-known representatives of that group are the termites, some species of which are one of the only animals in which the intestine secretes enzymes that break down cellulose, meaning they are not required to host bacteria for this purpose. Cellulose is, as mentioned, the most common source of carbohydrates and there is an advantage for those who can use it: it turns out, for example, that a species of hammerhead shark (Petition Tiburo : Sphyrna tiburo) succeeds in extracting energy from the cellulose of seaweed which he digs up from the bottom. The evolutionary origin of this strange ability of a predator seems to be in symbiosis with chitin-degrading bacteria, a hard-to-digest polysaccharide that builds the shells of crustaceans, which are an important food component.
Reptiles that do not maintain a constant body temperature require less energy expenditure on heating and therefore can afford a plant diet even with smaller body sizes than ruminants among mammals. But even among the reptiles, the vegetarian diet is usually preserved for the body livers in the group: the iguanas and the turtles. Similar to cows, iguanas and large vegetarian turtles such as the green sea turtle allocate space in their digestive tract for cellulose-dissolving bacteria, and it is likely that this is how the vegetarian dinosaurs were fed as well. Studies of dinosaur footprints and nesting sites show that vegetarian dinosaurs tended to group together, like modern herbivores in herds, some explain the initial motivation for herds in the need to receive cellulosic gut bacteria from neighbors since the dinosaur grows young just like a calf is born with a sterile digestive system and needs faecal remains to populate the system in essential bacteria.
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More of the topic in Hayadan:
4 תגובות
Salt from the bottom and not from your salt
Salt from the bottom and not from your salt
D.H.
It is interesting to check whether the cows were dairy cows.
If the cows are milked, it is a process that takes a lot of energy from the cow.
Although I know that buffalo milk is used, it does not seem to me that it produces milk in the amount that the cow produces.
I don't think there is another mammal that produces milk like a cow relative to its size (I assume a whale produces more milk)
While in Thailand I noticed an interesting phenomenon: in the farmers' yards I saw small barns with a cow or two and a buffalo or two. Everyone eats the same food from the same manger, and yet - the cows are thin and their bones stand out and the buffalo are round and chubby...
There is probably a difference in metabolism between these two closely related species, the domestic cattle (Bos taurus) and the water buffalo (Bubalus bubalis) and it is interesting if they have studied it.