One of the ideas I've expressed in my Creature Culture posts is that I want all the denizens of my world to live in a reasonably realistic way. Moreover, I don't want all of these creatures to live the same way. It seems highly unlikely that all of my sentient species could evolve while all occupying the same environmental niche. One of the ways to vary this is by varying the diet of these creatures. The three basic kinds of animal diet are: carnivore, herbivore, and omnivore. How these physical systems have evolved to make different body plans work is an essential element of the beautiful variation in our world.
Carnivores
This category includes: cats, seals, eagles, snakes, salmon, etc. Creatures who eat almost solely animals are called obligate carnivores (including carnivorous plants). Carnivores can either be predators who hunt or ambush their meals, or they can be scavengers. These animals may also specialize, eating one specific type of prey (fish, insects, birds).
Digestive systems tend to be short in these animals, making them less able (or unable) to break down tough plant fibers. They are monogastric (the stomach has one chamber), beginning digestion as soon as the food enters the mouth..
These animals tend to have pointed teeth to aid in tearing apart prey. Consequently, even if they had the digestive capacity, they may not be equipped to chew or even grip the plants (have you watched a cat eating grass?).
Because carnivores are either hunters or opportunistic feeders, they may gorge themselves at each meal (African lions ingesting up to 90lbs in one meal), in case the next one is hard to find. It's estimated that a mountain lion kills a deer (preferred prey) every 9-14 days.
Herbivores
Herbivores typically have flat teeth, and mouths designed to grind and rasp. Contained within this category are not just cows and goats (which are ruminants), but many types of insects and birds, koalas, beavers, monkeys and so on. It is theorized that the larger the herbivore, the less it needs to consume each day, relative to its size. Beef cows will eat from 1.8-2.0% of their weight per day. A 26lb year-old beaver may eat 1.5lbs of aspen per day to maintain itself (5.7% of its body weight), or 3.6lbs for maximal growth (14%).
Ruminants in particular are fun for a couple of reasons. Their teeth continuously grow, making the constant wear caused by chewing tough plants a non-issue. The second adaptation is the four-chambered stomach which allows them to break down plant fibers. The first two chambers use fermentation to start breaking down the plant fibers. Solids are consolidated and spit back up to be broken down further mechanically (chewing the cud) before re-swallowing and eventual absorption into the system.
Omnivores
The category is defined by those animals who eat from a wide variety of sources (plant, animal, insect, fungus, algae, whatever). This term is a difficult one, since many "carnivores" include some plants as a percentage of their diets. Additionally, herbivores can digest meat, they just don't usually eat it in the wild (like how my brother used to feed his pet iguana dog food for protein). Many species have preferences one way or another, but will switch as opportunity or extreme permits/requires. Some species also eat different types of material at different life stages, making it all more complex (did you think nature was simple?).
Certain omnivores (like humans) show their adaptation in their dentistry, having teeth suitable for both tearing and grinding. Otherwise, there are few distinguishing characteristics for this category (which was, like many other terms, invented to help us put things into neat boxes). Omnivores tend to be most notable for their lack of specialization. This jack-of-all-trades nature allows omnivores to better adapt on the fly to the situation at hand.
There are so many different ways that animals have found to sustain themselves, it seems a shame to make all sentient Fantasy creatures act essentially human. I'm no vegetarian, but why aren't there some intelligent grazers out there? I've read a fair number of SciFi carnivores, but relatively few in Fantasy realms who still go out hunting regularly (except for all the damn wolf companions). Sure, the intelligent ones will learn to supplement their diets with the other end of the culinary spectrum, but it's time for putting a little more effort behind our creature creation. It really doesn't take that much work and it can yield some really fun nuggets for character development.
ps: Be careful when reading biology articles that mention evolution. They often seem to get their developmental ideas the wrong way around. Just me being picky.
http://beef.unl.edu/cattleproduction/forageconsumed-day
https://kb.osu.edu/dspace/bitstream/handle/1811/5317/V67N04_242.pdf;jsessionid=AF7CC778DC47C943FDE9DBE3D885C5EF?sequence=1
http://www.aws.vcn.com/mountain_lion_fact_sheet.html
http://www.audubonmagazine.org/articles/climate/arctic-omnivore-s-dilemma-0
http://www.examiner.com/article/what-exactly-is-an-obligate-carnivore
http://animals.io9.com/how-do-red-and-giant-pandas-coexist-the-secret-is-in-t-1561988329/+rtgonzalez
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