Carnivore, Herbivore, Omnivore: What Diet Does To An Animal’s Body

Animals are usually sorted into three groups, and the categories are useful as long as you know where they leak.

• Carnivores eat animals. Obligate carnivores, like cats, cannot do anything else.
• Herbivores eat plants, and need elaborate machinery to do it.
• Omnivores eat both, and are generalists by design.

The leaks are worth knowing. The giant panda is in the carnivore order, has a carnivore’s short gut, and eats bamboo almost exclusively, which it digests badly and compensates for by eating for most of the day. The polar bear is a near-obligate carnivore while the brown bear it evolved from is a thorough omnivore. And plenty of animals switch: sparrow adults eat seeds while their chicks are fed almost entirely on insects, which is why killing sparrows for eating grain backfires so badly.

This is the fact that makes sense of all the anatomy: no vertebrate can digest cellulose. Not cows, not horses, not us. Nobody makes the enzyme.

Every plant-eating animal solves this by keeping microbes that can, and essentially renting out gut space to bacteria in exchange for the energy they release. The difference between herbivores is mostly where the rented space is.

• Foregut fermenters, like cattle, sheep, giraffes and the alpaca, ferment before the stomach. Cattle have four chambers, alpacas three. They bring food back up and chew it again, which is what chewing the cud is.
• Hindgut fermenters, like horses, donkeys, elephants and rabbits, ferment after the stomach in an enlarged caecum and colon. It is less efficient per mouthful but allows faster throughput, so these animals eat more and digest it less thoroughly.

Some hindgut fermenters have an additional trick that sounds unpleasant and is essential. A hare produces two kinds of droppings, soft green ones and hard dry ones, and eats the soft ones. The soft pellets come from the fermentation chamber and are full of nutrients the bacteria made, so eating them is effectively a second pass through the gut.

The tortoise’s famous slowness comes from the same place. Fiber is low in energy, so an animal living on it has little energy to spare, and moving slowly is the sensible response.

Meat is easy to digest and dense in energy, so a carnivore’s gut is short and simple, often only three or four times its body length against ten to twenty for a herbivore.

The teeth tell the same story. Carnivores have carnassials, blade-like cheek teeth that shear past each other like scissors, and jaws that hinge for a powerful vertical bite with almost no sideways movement. Herbivores have broad flat molars and jaws that move side to side for grinding.

Many animals have no teeth suited to chewing at all. Dolphins have simple identical cones purely for gripping and swallow prey whole. Spiders and scorpions cannot eat solids, so they inject digestive fluid and drink the liquefied contents.

The cost of specialization is that it is nearly irreversible. Cats lost the genes for making taurine and several vitamins because, eating meat, they never needed them, and those capabilities are not coming back. A specialist is extremely good at one thing and cannot switch when that thing becomes scarce.

An omnivore is good at nothing in particular and adequate at most things, and that turns out to be a powerful strategy.

The anatomy is middling by design: a gut of intermediate length, teeth of several kinds in one mouth, and digestive enzymes for a wide range of foods. An omnivore cannot extract as much from grass as a cow or as much from meat as a cat, but it can eat both and switch when one runs short.

It also needs a better brain. A specialist can rely on instinct, because its food is predictable. A generalist has to learn what is edible, remember where and when it was found, and decide whether something unfamiliar is worth trying, which is expensive but flexible.

That is why so many of the animals people consider clever are omnivores. Crows, raccoons, pigs, pigeons, bears, cockatoos and ourselves. It is also why they do so well around people: a diet of whatever is available is well suited to a world full of discarded food.

A few diets are narrow enough to deserve their own mention.

The dugong eats seagrass and essentially nothing else, which means its entire distribution is decided by where seagrass grows. The koala eats eucalyptus, which is toxic to most animals. Cuckoos eat hairy caterpillars other birds refuse, then periodically shed the stomach lining with the hairs in it. Anteaters and the aardvark eat ants and termites and have lost their teeth entirely.

Specialization of this kind works beautifully until the one food source is disturbed, which is why narrow specialists are so heavily represented among threatened species and generalists among the animals thriving in cities.

Most feeding mistakes come from treating an animal’s diet as a preference rather than a mechanism.

Cats are obligate carnivores and cannot be made vegetarian safely. Rabbits need unlimited hay rather than pellets and carrots, because their gut is built for continuous fiber, and a rabbit that stops eating fiber can go into gut shutdown within hours. Tortoises fed fruit and lettuce develop lumpy, pyramided shells and kidney damage. Donkeys and alpacas are so efficient on poor feed that rich pasture makes them dangerously fat.

In each case the correct diet looks mean to someone used to a different animal, and in each case it is what the gut was built for.

You can browse the profiles on this site by diet, which is a useful way to compare how differently animals have solved the same problem: carnivores, herbivores and omnivores.