Animal Diets: Herbivores, Carnivores, and Omnivores

Animal diets are classified by what an animal eats: herbivores eat plants, carnivores eat other animals and omnivores eat both. In brief: a rabbit is a herbivore, a lion is a carnivore and a bear is an omnivore. In a food relationship, the animal that hunts is the predator and the animal that is hunted is the prey.

Diet shapes an animal’s teeth, digestive system, senses and behavior. This guide explains the three main diet types, how teeth and digestion match the food, the roles of predators and prey, how to work out the diet of an animal from its skull and the mistakes people often make when naming diets.

What are animal diets?

All animals are heterotrophs, so they must obtain energy and nutrients by eating other organisms. The type of food an animal eats is its diet. Diet is a feature of the species that has developed together with the animal’s body.

The terms herbivore, carnivore and omnivore describe the main category of food. They are useful labels, but real diets are more varied. Many herbivores occasionally eat insects, and many carnivores sometimes eat fruit or grass.

What are herbivores?

Herbivores feed on plants and plant parts such as leaves, grass, seeds, fruit, nectar and roots. Cows, horses, elephants, rabbits, caterpillars and many birds are herbivores.

Plant material is difficult to digest because plant cell walls contain cellulose. Herbivores have adaptations to deal with it:

• Teeth: Flat molars grind plant food. Many herbivores have broad incisors for cutting grass, and rodents have chisel-like incisors that keep growing.
• Long digestive system: A long intestine gives time to absorb nutrients.
• Gut microbes: Bacteria and other microorganisms in the gut break down cellulose. Cows, sheep and deer have a four-part stomach and chew the cud, which means they bring food back to the mouth for more chewing.
• Eyes on the sides: Many herbivores see a wide area, which helps them to spot predators.

What are carnivores?

Carnivores eat mainly animal tissue. Lions, wolves, eagles, frogs, sharks and spiders are carnivores. Meat is rich in protein and energy, so carnivores usually need to eat less often, but they spend effort finding and catching prey.

• Teeth: Sharp canines stab and hold prey. Special cheek teeth called carnassials shear flesh.
• Short digestive system: Meat is easier to digest than plants, so the intestine is shorter.
• Forward-facing eyes: They give good depth perception, which is useful for judging distance when attacking.
• Claws, speed and venom: Many have tools for catching and killing prey.

What are omnivores?

Omnivores eat both plants and animals. Humans, bears, pigs, crows, raccoons and chimpanzees are omnivores. Their flexible diet helps them survive in places where the available food changes with the seasons.

Their bodies often have a mix of features. Humans have incisors for cutting, canines for tearing and molars for grinding. The digestive tract is intermediate in length. Omnivores can often adapt quickly to new foods, which is one reason why species such as rats and crows live successfully in cities.

Diet types compared

What are predators and prey?

A predator is an animal that hunts and kills other animals for food. The animal it catches is its prey. A carnivore, such as a hawk, is a predator, and so are some omnivores. Not all carnivores are predators, because some scavenge dead animals, as vultures do.

The two groups influence each other. Prey species develop defences such as speed, camouflage, warning colors and group living. Predators develop better senses and hunting methods. This long-term relationship is a type of evolutionary arms race.

Predators also help ecosystems. By eating weak or sick individuals and limiting numbers of herbivores, they influence plant growth and the balance of food webs.

Worked example: reading a diet from teeth

Teeth are among the best clues to diet. Imagine three skulls, labelled A, B and C, and the following observations.

1. Skull A: Large, flat molars with ridges, wide front incisors and a gap between the front and back teeth. The eye sockets face sideways.
2. Skull B: Long, pointed canines, sharp-edged cheek teeth and eye sockets that face forward.
3. Skull C: Small canines, flat molars and cutting incisors, with eye sockets that face forward.

Skull A fits a herbivore, because grinding teeth and side eyes match a diet of plants and a need to watch for predators. Skull B fits a carnivore: stabbing and shearing teeth suit meat and forward eyes suit hunting. Skull C has a mix of tooth types, which suggests an omnivore. The evidence is strong, but not certain. A panda, for example, has carnivore ancestry and eats mostly bamboo, so teeth alone do not always give the full picture.

How did scientists study diets?

Early naturalists inferred diets from teeth and from direct observation. The French anatomist Georges Cuvier, around 1800, argued that the parts of an animal’s body fit together so well that one bone or tooth can suggest its whole structure and way of life, and this idea guided fossil studies.

Today researchers examine stomach contents, droppings, chemical signatures in bones and DNA in feces. Some of these methods show diets that observation cannot, such as what a shy night-time hunter eats.

Why do animal diets matter in practice?

Farmers feed livestock according to digestive needs. Cattle are fed grass, hay and silage, while chickens, as omnivores, receive grain and protein. Pet owners must choose food that matches the animal. A rabbit given only meat or a cat given a plant-only diet would suffer from serious health problems.

Conservation planners need to know what animals eat when they restore habitats or reintroduce species. The reintroduction of wolves in Yellowstone National Park, for instance, was followed by changes in the numbers and behavior of elk. The size of the wider effect on vegetation is still discussed by scientists.

Which mistakes are common?

Mistake 1: Assuming that all carnivores are predators

Scavengers like vultures eat meat but do not kill their food. Predator describes a hunting relationship, not just a diet.

Mistake 2: Calling any animal that eats insects a carnivore

Insects are animals, so an animal that eats insects is a carnivore. A bird that eats seeds and insects is an omnivore.

Mistake 3: Believing that omnivores are less specialized

They are adapted to flexible feeding. Their mixed teeth and gut are an adaptation, not a lack of one.

Mistake 4: Using diet labels as rigid categories

Many species change diet with the season or age. Tadpoles often feed on algae, while adult frogs catch insects.