Rodent Eating Behavior, Nutrition & Digestion
Note: This post is rough. It grew out of another post because I know I've addressed some of these issues before but can't really find a post to link to. So I made this and will add, tweak, etc. as need or desire arises.
For those who are not aware, I spent a few years in my youth doing research for for one of the most evil of entities in all the world -- a pharmaceutical company. My job was a mix of half analytical chemistry (to be able to quantify the levels of drugs in blood and tissues, and identify the way in which drugs are broken down and excreted) and hands-on metabolism/physiology work involving designing and conducting experiments to characterize the metabolism of new drug candidates. For the latter I have picked up, held, been bitten by, injected, gavaged, euthanized, decapitated, performed surgical procedures on, monitored, bled and dissected more mice and rats than I care to recollect. As such, I logged many hours of observation, and I've seen the insides of countless of these critters as well. I held the dubious unofficial records for getting the most blood from a mouse vena cava, and a decapitated rat ... but have no trophy to prove it ;-)
Years later, I moved to and lived for over a decade in a home built on former farmland. I cohabitate with domestic felines. As such, it was virtually impossible to keep the home rodent free (forget the garage!), but the kitties did help keep them to skulking in the basement and away from where the humans live and eat most of the time. Still, you get a feel for what wild and semi wild rodents do to survive (and judging from the size of some, thrive!), and every now and then, especially when I'd go upstairs to get something out of the storage boxes, I'd find evidence of activity including the hulls of birdseed we stored in our breezeway and boxes of soap mysteriously missing any soap in them.
In the laboratory, most of our mice and rats were kept in appropriately sized cages resembling this one. Animals were ordered by a central department and housed there until we needed them, then usually brought to the lab for a period to acclimate. Most of the studies were single-day dose, sample, and sacrifice in nature. We used a standard chow, you can see the pellets here, and they got fresh (tap) water in a bottle. Usually several animals were housed per cage.
During the day they mostly slept together in one corner of the cage, but every now and again one or another would move around, clean itself, nibble at the food, etc. The rodents eat all day and night, but as nocturnal animals so so much more at night. They appear to sort of snack mostly, and if memory serves at all, they don't "drink with meals" but generally separate eating and drinking. As you can see, this sort of lab cage set up is far from a natural environment for the rodent. There isn't a whole lot of room to move around, and (unless restricted) they are presented with an abundant supply of food 24/7 for which they need to exert a minimum of effort to consume. Cages for longer term studies may be larger and include some sort of wheel for exercise, but we are still talking confines that are a way far cry from "the wild".
As for my occasional unwelcome house guests, one good thing about them is that you rarely saw a trace of them during the day, but if one got into the house proper, you would hear something and see the kitties on alert about the appliances and baseboard heat pipe holes at night. There is nowhere near the constant feeding going on, though even in the cleanest of households there would usually be some food (remember the soap!) available to a hungry mouse.
As just discussed, unless there's some specific feeding schedule, need for fasting, etc., going on, lab rodents are basically "fed" around the clock. This is usually with some sort of uniform (homogeneous) pellets of food. To me, these somewhat resemble fuel pellets for those stoves or mini-sawdust logs for the fireplace, minus the waxy binder. Generally, no nutrition -- macro or micro -- is provided in the water supplied.
Such a feeding protocol differs from "real rodent life" in a very meaningful way other than the general availability...
Every single bite of food this animal takes, from the moment it is weaned until it meets its end, will be a nearly perfectly "balanced" one of formulated nutrient content.
Think about that! Even prisoners sometimes get a holiday meal that differs from the every day fare, and even cultures that consume a few relatively monotonous staples still have different "condiment" foods for variety. Furthermore, all chows are fortified with a standardized vitamin and mineral mix so that regardless of what they're made of every bite is the same in micronutrition as well. In no particular order:
- Verifiable content
- Ability to control micronutrient intake across a wide variety of macronutrient ranges and variety of ingredients.
- Ablility to control macronutrients
- Relative ease of measuring total intake
- A "whole food" that includes each of all three macros plus fiber.
- No choice. Unless the animal dislikes a feed so much it doesn't eat it, the animal has no choice but to eat all components of chow
- Usually solid, separating all/most water from food alters intake
- Micronutrients don't vary with intake of mixed diet
- Ingredients often not "natural" food sources for a rodent
- Often highly processed single sources of various components: e.g. casein for protein, soybean or coconut oil or lard for fat, starch and/or sugar for carbs.
*A Note to Self to Discuss Weaning and Breeding Location.
While rodents may be "natural" omnivores, their native diet differs from humans. Their digestive systems differ as well in that they have a functioning cecum. I remember the first time I opened a mouse and saw what appeared to be a second weird looking stomach. Later I learned it was not a stomach at all, but the cecum. I don't want to gross you out, so I'll link to this image. It could be quite large compared to everything else if the mouse had eaten somewhat recently.
This digestive morphology is indicative of a hindgut fermenter. The functioning cecum contains gut flora capable of extracting calories from otherwise undigestible fiber. The contribution of this to meeting energy needs is greater than fermentation in monogastrics lacking a functioning cecum (e.g. humans), but less efficient than the ruminant. From my link:
Rodents and omnivores
Many rodents are either partly or wholly herbivorous. Their generally small size means that they have high metabolic energy requirements but little physical capacity for retentive digestion of vegetative matter. Therefore, ingestion of plant material is generally restricted to high energy sections of the plant such as fruit, nectar and pollen or seeds; or to sections that are more easily digestible such as growing tips, seedlings and flowers. This selectivity is also practiced by omnivorous animals such as bears, pigs, possums and humans.
This digestive difference is often overlooked when extrapolating rodent studies to humans. When feed contains starch and sugar instead of ground cereal grains, there is generally a higher "feed efficiency". However the differences in digestible energy may differ in different ways compared to how the human digestive system deals with fiber.