To lay some ecological ground rules I'll start by stating that every calorie of energy on this planet was originally converted from sunlight by plants (or algae). So even if you are eating meat, the calories you ingest came from the plants that animal ate that had converted energy from the sun using chlorophyll in a process known as photosynthesis. So all animals need plants and we benefit from their ability to convert sunlight to an energy form we can use. The problem is, plants don't normally want to be eaten (there are exceptions but that's in another chapter). Plants don't have a brain to think about it, but it's not hard to imagine that a plant that is indigestible (so avoided by animals) would probably produce more seeds that grow to adult forms versus one that is digestible. So in each generation of plants, a mutation that makes them more indigestible will probably become more common and more and more plants will become difficult to digest. Give plants a few 100 million years and they are pretty good at preserving their energy and nutrients. Case in point…as I sit surrounded by millions of calories that I can't digest. Herbivores have to keep up with these changes and mutations that make them more efficient at digesting plants will become more common over time as well. The chief defense in most of the plant kingdom is a tough cell wall known as cellulose. Many animals, certainly you and I, do not have a digestive system that can cut through most plant species cellulose walls to get at the glucose (energy) and nutrients. I could chew all the acorns I wanted and not get enough energy to survive because of the cellulose. The deer seemingly has no problem, but actually the cellulose is indigestible to them as well so they need help.
Deer and the animals I listed above are known as ruminants, which are even toed, hoofed animals with a specialized digestive system, specifically a 4 chambered stomach that allows them to obtain energy from indigestible plant matter. There are just over 150 species of ruminants and it includes wild and domestic species including deer, sheep, goats, cattle, camels, giraffes, yaks, llamas, pronghorn, and the antelopes. They actually don't do the digesting, the quadrillion (that's 15 zeros) of bacteria and protozoa that live in the 4 chambered stomach complete the breakdown of cellulose because they have the ability to break it down. So by providing the microbes a place to live with the proper conditions, the deer get the energy released from the cellulose and hemicellulose in the form of volatile fatty acids (vfas) and from the bacteria themselves. To take advantage of this “deal” they have evolved a complex 4 chambered stomach with conditions almost perfect for the bacteria and protozoa which results in a superior fermentation chamber. A little different than your favorite winery, but similar in nature (excuse the pun).
The 4 chambered stomach has a chamber for each of the following functions, a sorting and filtering chamber for the chewed up vegetation, a fermentation chamber, a water absorption chamber, and a gastric stomach that is not as acidic as you and I have but serves somewhat the same function. The honey combed sorting chamber (ever eaten menudo?) or reticulum is the largest, and the series of folds and villi make sure that only small particles pass through to the bacteria. Larger particles are passed back up to the mouth to be rechewed, hence rumination or cud chewing. The next section, the rumen houses the bacteria. The animal must ensure that conditions are correct for the bacteria which require a warm temperature, a lot of water (saliva), and the opposite pH of our stomach. To keep the area moist and buffered, every ruminant produces an enormous amount of saliva, a cow is estimated to produce 100 – 150 liters a day. Once the bacteria do their job and the particles are small enough, they move into the Omassum where the ruminant tries to get as much water back from both the saliva and any water the plant had in it as it can before the mostly digested food continues on to the gastric stomach, the small intestine, and finally the large intestine. From an energy standpoint, the 3 byproducts of fermentation are the volatile fatty acids, the water soluble vitamins produced by the bacteria, and the superior proteins contained in the bacteria themselves. The vfa's are absorbed into the blood through the rumen wall, most of the vitamins and proteins a little later in the small intestine. As the foods continue to move along the digestive tract some microbes move along with it and food value and nutrients continue to be extracted throughout the small intestine. The large intestine is the chamber where the all the water possible is extracted before the feces is released. So by creating a stomach that has ideal conditions for a microbial population, the whitetail I’m watching can ingest things and get food value from items neither of us can properly digest.
Ruminants aren’t the only animals that take advantage of this symbiotic relationship however, most herbivores do since plants have built up defenses for over 500 million years. Other mammals such as rodents, birds, and even a couple of our lizard species have digestive chambers that evolved to house bacteria so the cellulose and hemi cellulose that protects plant energy and nutrients can be extracted. Most don’t have near the efficient system as the ruminant however. A couple other herbivores I can see as I wait for the deer to move are cottontail rabbits and Arizona gray squirrels. The rabbit doesn't have a 4 chambered stomach, but it does have a cecum located at the end of the small intestine. It's located the same area as our appendix. It's a large chamber (up to 60% of the digestive tract) and it's a most important part of the tract. The cecum is a fermentation chamber that has the same function as the rumen in the whitetail. The rabbit absorbs vfa's produced by the bacteria as they process the grass the rabbit ate. But the big difference between the two species is that the cecum is located at the end of the small intestine, not before it. Because of that, all the proteins and vitamins absorbed by ruminants are not absorbed by rabbits or other herbivores known as “hind gut fermenters”. The rabbit has a special adaptation that other animals do not. It's not appetizing to think about, but it works well. Rabbits actually produce 2 types of feces, a hard fibrous feces and a soft moist one known as cecotropes. To absorb the vitamins and proteins that the whitetail absorbs in the small intestine, rabbits eat the moist soft feces and then the proteins and minerals can be absorbed in the small intestine. Other species will eat their own feces as well, which is known as copraphagy. (Sometimes I need to tell my students to get the copraphagic grin off of their face!) The gray squirrel has a digestive system more like ours. A highly acidic gastric stomach, a long small intestine, and a cecum that is not that large. Their bacteria population is not large enough to digest the cellulose found in many plant parts and species thus their diet is more selective. Without softer berries, nuts, fungi (mushrooms) etc., they would starve. They are better at processing cellulose than you and I, but not that much better. You and I, we need to stick to our soft green beans, corn, lettuce, etc., all plants that have a weak cellulose wall if we want to access that energy. If you want to add Brussels sprouts to that list and eat them, fine. I think I'd rather try acorns.