But, to become milk, of course, the grass has to pass through the stomach. The ruminant stomach is a fine example of the kind of cooperative symbiosis we wrote about on 7/21. We all learn as children that ruminants -- hooved animals like cows, goats, and sheep -- “have four stomachs,” which allow these animals to subsist pretty much on cellulose alone. Cellulose is a very poor source of nutrients, and it requires a lot of time and energy to consume. Cows, for example, have to eat just about around the clock.
While the idea of four stomachs suggests four separate but equal digesting pots, in fact ruminants are more like us than we imagined as children, as they actually have only one stomach, with either three or four chambers, depending on the species--the rumen-reticulum, the omasum and the abomasum. This division allows these animals to digest plant food, or cellulose, that comprises most of their diet--despite the urban myth about goats eating everything from tin cans to nuts and bolts. Here's a public domain figure of a ruminant digestive system.
Cellulose is impossible for people to digest, as we have no way to break it down into usable nutrients, but the complicated ruminant digestive system allows cows, sheep, deer, camels, llamas, goats and so on to do so. Ruminants chew their food briefly and then swallow it, whereupon it enters the first chamber of the stomach, the rumen-reticulum. Here it’s mixed with saliva--enormous quantities of saliva; cows make something like 150 liters of it everyday, which explains why they drool so much, and why they have to drink so much--and then it’s divided into liquid and solid layers.
The solids form the cud, which the animal regurgitates around 500 times a day and slowly chews to break down the cellulose particles, in a process called “ruminating” (from the Latin, ruminare--could it really be because cows seem so thoughtful when they chew their cud?). When the animal swallows the semi-digested material, it is returned to the rumen-reticulum where the fibers are further broken down and fermented by many kinds of bacteria found in great numbers in the gut. It is primarily the carbohydrates that ferment (in fact, if you feed a ruminant enough sugar, she will get drunk), and the nutritive products of this fermentation are absorbed here, through the wall of the rumen-reticulum.
Fermentation produces an enormous amount of methane gas, which the ruminant releases by belching or farting. Anything that interferes with the release of this gas, causing “bloat,” is serious and can be life threatening; if a cow or a goat spends too much time lying on its side, it can literally bloat and die because there’s no easy exit for the methane in that position. From time to time, when someone with not much experience is bottle feeding the kids on the farm, they’ll miss the signs that one of them has had enough (bulging stomach), and will let the kid drink too much. The poor animal can be dead of bloat within hours. We worry about this when we feed babies on Jen and Melvin's farm, but so far we’ve been lucky. We’ve seen it happen, though, and it’s not pretty.
Like most animals, including humans, ruminants need help doing some things that are vital to their survival. Ruminants can’t digest grass without the help of the abundant micro-organisms in their gut. Although the symbiosis may not be good for each individual bacterium, because many of them get digested in the animal’s abomasum (the stomach’s final chamber), it’s good for bacterial species overall, because they absolutely thrive in the forestomach. Another example of the centrality of cooperation in life. This is not cooperation in the social intentional sense, such as a group project or a company, but it is social group behavior in which the parties not only benefit but depend on each other.
The liquid layer in the rumen-reticulum containing the broken down fibers of the re-chewed and re-swallowed cud gets passed to the omasum where the liquid and metabolites from breakdown of the bacteria are absorbed into the blood stream. The semi-digested bolus that remains, which is primarily protein at this point, is passed to the abomasum, the final stomach chamber, and then on to the intestine where the nutrients are absorbed, in a way similar to that of any single-stomached animal. In an adult ruminant, the abomasum is about 10% of the stomach. (Here's a picture of feta cheese that Jennifer is making.)
The rumen and reticulum of young ruminants like goats are undeveloped and non-functional until the animal begins to feed on grass or hay or grain at about two months of age. Before then, the abomasum is much the largest compartment so that the stomach functions more like that of a “monogastric,” or single-stomached animal. The milk the young animal drinks passes directly from the esophagus to the abomasum where it is digested. As the animal begins to add dry food to its diet, the rumen becomes populated with bacteria that are in the food or the air, and these colonizing bacteria actually stimulate the development of the goat’s rumen and reticulum, which allows the growing animal to begin to ruminate, digesting its food through fermentation. The development of these various sequential pouches along the gut is another example of cooperation, between cells, signaling each other and inducing growth and differentiation at appropriate places along the gut tube.
Some non-ruminants can digest cellulose, too, but not with a forestomach, as they don’t have one. These “monogastric herbivores” have the same kind of simple stomach we have, but have a huge large intestine, akin to the ruminant rumen, where their diet of cellulose ferments. Horses, rabbits and other rodents, porcupines, beavers, elephants, rhinoceroses, and pigs are all examples of hindgut fermenters. This is but another example of the many ways of evolutionary success, even to similar ends.
Although all these animals consume and digest cellulose with the aid of microbes, hindgut fermenters, unlike ruminants, can’t take advantage of all the microbial protein they break down in their gut because they can’t absorb it. Most nutrients are absorbed in the small intestine, but with fermentation happening beyond that, in the large intestine, these animals aren’t able to absorb the amino acids that are the result of the breakdown of bacteria there and so they are expelled in their feces. This in fact means that their feces are loaded with protein and nitrogen and so on, and a number of these animals actually reclaim these nutrients by consuming their own feces. It makes a great meal for insects, too.
From front end to back, cooperation of various kinds, in various ways, allows these mammals to consume cellulose. In a sense, too, they are cooperating with the grasses in the fields, as it stimulates their growth and proliferation, too. These days, of course, the farmer cooperates by cutting the field grass, making hay so they goats can survive the winter.
And, in another nice example of cooperation between "us" and "them", goats give really good massages. (Thank you for the photos, Jennifer!)