Feeding Strategies
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.
Grazing
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.
Browsing
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.
Burrowing
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
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, and 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.
Predation
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).
Symbiosis
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.
Scavenging
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.
References
Alcock, John. Animal Behavior, 7th ed. Sunderland, MA: Sinauer Associates, Inc., 2001. Owen, Jennifer. Feeding Strategy. Chicago: University of Chicago Press, 1980.