All animals are heterotrophic, meaning they must eat other organisms, living or dead, to acquire organic nutrients. A large percentage of an animal's life is occupied with acquiring food. Almost every living species is eaten by something else, but food varies in its spatial distribution, seasonal availability, predictability, how well hidden or easily detected it is, how much competition for it exists, and whether or not it can resist being eaten. Consequently, animals have a variety of feeding strategies to meet these challenges.
Some animals are food generalists (euryphagous); that is, they eat a wide variety of foods. Coyotes, opossums, and humans are good examples. Others are food specialists (stenophagous), feeding on a narrow range of foods. For example, the Everglades kite (a small hawk) feeds on just one species of snail, and many feather mites can survive on just one species of bird.
Behavioral ecologists who study feeding strategies are often concerned with theories of optimal foraging. Obviously, animals must gain more energy from their food than they expend in searching for it, capturing it, and consuming it. In addition to energy, they must acquire specific nutrients, such as certain salts, which provide no energy but are crucial for survival. Thus, theories of feeding are concerned with such issues as food choice, prey switching, sensory mechanisms for recognizing and locating food, optimal search strategies, overcoming the defenses of food organisms, and how to compromise between finding food and not carelessly falling prey to some other hunter.
Following are some of the basic methods that animals use to acquire food. Many animals use mixed strategies, shifting from one method to another as different kinds of food become available, or using combinations of methods simultaneously.
Grazers crop grasses and other ground plants on land or scrape algae and other organisms from surfaces in the water. They include animals as diverse as snails, grasshoppers, geese, rodents, kangaroos, and hoofed mammals. Grass and algae are palatable foods that offer little or no resistance to being eaten, but are adapted to survive grazing and quickly replace the lost biomass. A disadvantage of such food, however, is that it is nutrient poor. Grazers therefore must consume a large quantity of it and spend a larger percentage of their time eating than predators do. While eating, they are vulnerable to attack. To eat without being eaten requires alertness and quick escape responses. Grazing mammals tend to form herds: There is safety in numbers, and the abundance of grass supports the high population density of grazing herds.
Terrestrial browsers nip foliage from trees and shrubs. They include caterpillars, tortoises, grouse, giraffes, goats, antelopes, deer, pandas, koalas, and monkeys. In aquatic habitats, browsers feed on algae, aquatic plants, and corals, and include sea slugs, sea urchins, parrot fish, ducks, and manatees. Browsers depend on food that is less abundant and widespread than grass, so they tend to form smaller groups or to be solitary and secretive.
Eating Nectar, Fruits, Pollen, and Seeds
Plants provide an abundance of food other than foliage, some of it for the purpose of rewarding animals. Sweet nectar rewards bees, flies, moths, butterflies, and bats that spread pollen from one flower to another, and sugary fruits entice birds, monkeys, fruit bats, bears, elephants, and humans to eat them and spread the indigestible seeds throughout the countryside. Pollen and seeds, being a plant's reproductive capital, are not meant to be eaten, but many bees, flies, and beetles nevertheless consume pollen, while birds, squirrels, and harvester ants take their toll on the seed crop.
Fruit bats consume both fruit and flowers. They normally suck on the flowers and fruit, then swallow the nectar or juice and spit out the rest. Because fruit bats disperse seeds and pollinate the flowers of many plants, many of the fruits and vegetables we eat every day would not exist without these bats.
Some animals burrow into their food, eating a tunnel as they go. These include many herbivores such as bark beetles, fly and moth larvae called leaf miners, and wood-boring termites. In the sea, unusual clams and crustaceans called shipworms and gribbles, respectively, burrow through wooden piers and ships, causing enormous destruction. Earthworms and many marine worms burrow in soil and sediment, eating indiscriminately as they go, digesting the organic matter and defecating the indigestible sand and other particles. Burrowing animals not only have the benefit of being surrounded by food, but also are less exposed to predators.
Filter-feeding is a common strategy in aquatic habitats, especially the ocean. It uses anatomical devices that act as strainers to remove small food items from the water. Sessile filter-feeders, such as barnacles, oysters, fanworms, brachiopods, and tunicates sit in one place, pumping sea water and straining plankton from it. Other filter-feeders are mobile. Herring swim with their mouths open, letting water flow through the gill rakers, which strain small particles of food from it. Flamingoes take in mouthfuls of water and mud, then force the water through the fringed edges of their bills, which serve as strainers that retain food such as brine shrimp, aquatic insects, and plankton in the mouth. Small and even microscopic food in the water may not seem very abundant, yet the largest animals on Earth—the basking sharks, whale sharks, manta rays, and baleen whales, including the largest species alive today, the great blue whale—nourish themselves entirely in this way. Filter-feeding is more common in the ocean than in fresh water, because plankton is less concentrated in fresh water.
Suspension and Deposit Feeding
Another form of small particulate food in aquatic habitats is the steady "rain" of organic matter that settles to the bottom: living and dead plankton and bits of dead animal, plant, and algal tissue. Suspension feeders pick this material from the water as it falls and deposit feeders consume it after it settles on the bottom. Many sea anemones, corals, marine worms, and crinoids, for example, spread out an array of tentacles and capture whatever settles on them. Other worms, some bivalves, brittle stars, and sea cucumbers spread sticky palps, arms, or tentacles over the substrate , picking up the organic matter that has settled there. The feeding arms or tentacles of many of these animals have ciliated , sticky grooves. Food becomes caught in mucus, and cilia steadily propel the mucus strand toward the mouth. Sea cucumbers, however, reach out and pick up sediment on their sticky tentacles, then draw the tentacles into their mouths and remove the food, like licking jam off one's fingers.
Predators are animals that depend on killing other animals outright. Since the other animals have evolved defenses against predation—hard shells, toxins, the ability to fight back, or simply running or flying away—predators have evolved a wide range of strategies for capturing their prey. Some hunt in packs (wolves), some collaborate to ambush prey (lions), some are stalkers (solitary cats), some use lures to attract unsuspecting prey (snapping turtles and angler fish), some employ camouflage so their prey does not notice them until it is too late (praying mantids), and some use snares (spiders, jellyfish).
Symbionts are animals that live in a close physical relationship with another animal, the host, from which they benefit. Unlike predators, symbionts do not benefit from the death of their hosts; ideally, they steal food or consume host tissue at a rate that the host can tolerate, allowing the host to survive. Symbiosis includes mutually beneficial relationships (mutualism); relationships in which one partner benefits, typically by stealing food from the host or eating its tissues, but the host is neither benefited nor harmed (commensalism); and relationships in which the host is harmed, usually because the symbiont consumes nutrients or tissue faster than the host can replace it (parasitism). The host is often both food and shelter for its symbiont.
Finally, and fortunately for the planet's "hygiene," many animals belong to a community of scavengers that feed on organic refuse such as manure (dung beetles, flies), leaf litter (snails, millipedes, earthworms), and dead animals (blowflies, vultures, hyenas, storks). The family name of the vultures, Cathartidae, is from the Greek katharos, meaning "to cleanse." Disgusting as some people may find their habits, we would be infinitely more disgusted with an environment from which such scavengers were lacking.
see also Ecosystem; Herbivory and Plant Defenses; Ocean Ecosystems: Hard Bottoms; Ocean Ecosystems: Open Ocean; Ocean Ecosystems: Soft Bottoms; Parasitic Diseases; Predation and Defense; Protozoan Diseases; Symbiosis
Kenneth S. Saladin
Alcock, John. Animal Behavior, 7th ed. Sunderland, MA: Sinauer Associates, Inc., 2001.
Owen, Jennifer. Feeding Strategy. Chicago: University of Chicago Press, 1980.
All animals must eat to live. Animals obtain energy and essential nutrients from the things that they eat. Since animals cannot harvest energy directly from the sun, they obtain energy to grow, move, and reproduce by consuming other organisms. Herbivores obtain solar energy by eating plants. Carnivores prey on other animals. Omnivores, such as humans, obtain energy by eating both plants and other animals.
Some animals consume enormous quantities of food with little nutritional value. Grazers, for example, eat large quantities of grass and other plants that have a low energy content and nutritional value. These animals have evolved special adaptations, such as two stomachs, that allow them to obtain sufficient energy from grass.
Some predators have developed special adaptations that allow them to consume large quantities of food at one time. Large snakes may catch and consume prey only three or four times a year. Lions and other large cats have adaptations that allow them to gorge on large amounts of meat at one time, then go without eating for several days or weeks.
Other predators take a different approach. The blue whale (Balaenoptera musculus ), the largest animal that has ever lived, primarily feeds on small, shrimp-like crustaceans called krill. Vast quantities of krill must be consumed for the whale to obtain sufficient energy and nutrients. A blue whale may consume four tons of krill per day. The whale shark (Rhincodon typus ), the largest living fish, feeds mainly on tiny plankton, anchovies, and sardines that it filters from the water.
Feeding and searching for food take up most of an animal's time. Foraging behavior requires some sort of system for distinguishing food from nonfood and for recognizing desirable food. Omnivores that consume a variety of different foods must learn which foods are good to eat. Sampling is risky, because many possible food items can be poisonous.
Rats need to consume forty different kinds of material including water, amino acids, fatty acids, vitamins, and minerals. The food available at any given time may lack some essential part, requiring the rat to spend extra time searching for foods that contain the missing element. In one study, rats were provided with a variety of foods, including yeast, a source of B-complex vitamins. When the yeast was removed from their diets, they immediately began to consume feces, another good source of B-vitamins. When yeast was reintroduced, they stopped consuming feces.
Humans are the ultimate foragers, regularly consuming an amazing variety of different foods. Yet people are generally cautious about trying new foods. Human cultures are easily identified by the food they eat. Food choices embody the accumulated wisdom of a culture about what is good to eat. However, humans do not seem to make food choices based on the need for essential vitamins and minerals. For example, a craving for fresh fruit does not appear to develop in people suffering from vitamin C deficiency. British sailors had to be required to eat limes in order to counteract scurvy caused by vitamin C deficiency. Humans also crave and eat far more fat and salt than is necessary for good health.
Foraging behaviors are learned and are adapted to an animal's lifestyle. Nervous systems are organized to enable animals to make associations between toxic effects and foods ingested hours earlier. Most people have experienced a strong aversion to the kind of food consumed right before an illness that caused nausea and vomiting, even when the food had nothing to do with the illness.
Grazers and Herbivores
Herbivores consume many different parts of plants. This group of animals can be subdivided into folivores (eaters of leaves), frugivores (eaters of fruit), and granivores (seed eaters).
Caterpillars are probably the most familiar example of folivores. Caterpillars must eat large quantities of leaves in order to store enough fat and protein to undergo metamorphosis. However, some plants have developed a defense against caterpillars. In Finland, caterpillars of the moth Oporinia autumnata eat leaves of the birch tree. Caterpillars that feed on leaves from trees that were heavily damaged the previous year grow more slowly than caterpillars that feed on lightly damaged trees. Apparently the birch tree has developed chemicals that make the damaged leaves hard to digest.
Species of milkweed have evolved a highly toxic sap that prevents most caterpillars from consuming their leaves. However, the caterpillar of the monarch butterfly has evolved a tolerance for the toxin in milkweed sap and readily consumes it. Toxins from the sap are incorporated into the tissue of the caterpillars, making the caterpillars unpalatable to most predators. However, certain species of orioles have evolved a tolerance for this toxin and readily consume the insect.
Mammalian grazers generally consume a variety of different plants during a day. Only a few, such as the koala of Australia, eat only one type. Large species of grazers tend to be less selective than smaller species. In East Africa, communities of grazing animals have developed a commensal relationship. Elephants and buffalo first eat the tall, coarse grasses and then move on. They are able to consume large quantities of this low-nutrition food source. Zebras follow along behind the elephants, reducing the plant biomass even more. The zebras are followed by still smaller wildebeest, who select among the lower-growing plants that remain after the zebras have fed. Finally the smallest grazers such as Thompson's gazelles are able to reach the young, protein-rich sprouts of grass missed by the wildebeest.
Filter feeders are found in many different animal phyla, including brachiopods, mollusks, various worms, and chordates. They are indiscriminate eaters. They feed on prey much smaller than themselves that are suspended in water or air. Each filter feeder has some sort of apparatus with which it filters prey from the medium of air or water. The structure of this apparatus determines the size of prey. Filter feeders also have an apparatus for moving the air or water relative to the filter. This can be done by moving the filter. Barnacles wave feather-like fronds through the water, periodically drawing in whatever happens to have been caught. Other animals, such as oysters, pump water through a filter, thus moving the medium. Sometimes the filter feeder is passive and depends on wave action to move the medium. More often the filter feeder expends energy to move the medium through the filter. For example, the blue whale and the whale shark swim through the ocean with gaping mouths. The blue whale is able to gulp an enormous quantity of water because its throat has special pleated folds that allow it to expand to several times its original volume. Then the whale closes its mouth, forcing the water back through a specialized structure known as baleen. The whale shark has special structures in its gills that filter small organisms and other debris from the water passing through the gills. Sponges have specialized cells with flagella that keep a constant flow of water through the sponge. Once the energy has been expended to capture the prey, the filter feeder rarely rejects it, thus being indiscriminate eaters.
Carnivores are animals that eat other animals. Carnivores can be found in most of the animal phyla. Some mollusks prey on other mollusks; praying mantids eat other insects and anything else they can catch; hawks and eagles eat small mammals, large insects, fish, and other birds. Roadrunners catch and eat large insects, lizards, and small snakes. Many species of fish prey on other fish. Some species of fish that live in the deep ocean, where other animal life is scarce, have jaws so huge they can swallow another fish substantially larger than themselves. All of these animals obtain energy and essential nutrients by eating other animals.
Mammals in the order Carnivora are highly specialized to prey on other animals. They have specialized digestive systems that are inefficient at digesting plants. This order includes dogs, cats, bears, weasels, and seals. Within this group, the cats, family Felidae, have evolved to prey almost exclusively on other mammals. Lions have evolved specialized behaviors, teeth, and digestive systems that allow them to pursue and kill much larger mammalian herbivores. They are able to consume huge amounts of meat by gorging on the kill, which they slowly digest. This specialized behavior is necessary because lions may go several days or longer between kills. Cheetahs are even more specialized. They have evolved speed at the expense of strength and are now so specialized that their diet consists almost exclusively of the small gazelles that graze the African plains. Grazers and filter feeders eat vast quantities of food with low nutritional value. Carnivores consume smaller amounts of food with higher nutritional content, but they have to work harder to get it.
see also Foraging Strategies.
Clark, E. "Gentle Monsters of the Deep: Whale Sharks." National Geographic 182 (1992):6.
Curtis, Helena, and N. Sue Barnes. Biology, 5th ed. New York: Worth Publishing, 1989.
Miller, G. Tyler Jr. Living in the Environment, 6th ed. Belmont, CA: Wadsworth Publishing Company, 1990.
Purves, William K., and Gordon H. Orians. Life: The Science of Biology. Sunderland, MA: Sinauer Associates Inc., 1987.