Ingestion in animals

Ingestion is the process of taking food into the body to satisfy nutritional and energy needs. Although basic nutritional requirements are remarkably similar for all animals, mechanisms of ingestion are exceedingly diverse. This diversity stems from the varied nature of available food sources and the resulting behavioral adaptations and specific body forms required to procure adequate nutrition. Because it is convenient to classify feeding strategies by the type of food consumed, organisms are often described as herbivores, carnivores, omnivores, and saprovores. Although very descriptive, these terms alone are not sufficient to fully describe the feeding adaptations used by animals, especially when considering invertebrates, which make up more than 97 percent of all animal species. Accordingly, this section will expand on these ideas by further categorizing ingestion by the type, size, and consistency of food, while also describing behavioral adaptations that lead to ingestion.

Small Particle Ingestion

Numerous animals live exclusively on a diet of very small particles that include bacteria, algae, plankton, and detritus. Although most small-particle consumers are relatively small animals, they range from the tiniest animal-like organisms (protozoans) to the largest (whales). Many mechanisms have evolved to permit the ingestion of small food particles. One common method is endocytosis. During this process, the outer membrane of a cell surrounds a food particle and engulfs it within the cytoplasm, forming a food vacuole (a cellular organelle used for digestion). Protozoans best illustrate this mode of ingestion. The amoeba, for example, uses pseudopodia (extensions of its cell membrane) to surround and ingest prey. Paramecia uses cilia (tiny hairlike processes on cell membranes that beat in a coordinated manner) to guide food particles into an oral region by endocytosis before ingestion.

Multicellular animals may also ingest small food particles by endocytosis. Sponges use flagella (motile, whiplike structures resembling long cilia) to aid in the gathering of small particles of food. The body cavity of sponges is lined with flagellated cells called choanocytes, or collar cells. The beating action of their flagella creates currents that move water through the body cavity, where food particles are removed and incorporated by the choanocytes.

Filter feeding is another strategy commonly used to obtain small bits of food. Cilia, mucus sheets, flagella, tentacles, and nets are often used as filtering devices. Rotifers (small aquatic invertebrates), for example, use a special double-banded ciliary system to transport water and filter out suspended food particles, which are conducted to the mouth by a third set of cilia. In general, sessile (immobile) organisms (such as sponges, rotifers, and some oysters) are called filter feeders. They get their name from the filter-like organ they use to strain water, sand, and other marine debris from the plankton, algae, and other microscopic organisms they eat.

Numerous free-moving animals also use filtering devices to obtain food. Sea cucumbers (animals related to starfish) live on the bottom of the seabed. By extending and periodically retracting sticky tentacles, they capture and ingest small food items. Clams, mussels, and some snails produce a sticky mucus that covers ciliated cells. The mucus traps fine suspended food particles, which are transported to the mouth by ciliary action. Mussels, for example, continuously pass water between their mucus-covered gill filaments for respiration, simultaneously trapping food particles. Similarly, herring and mackerel (fast-swimming fish) possess special structures called gill rakers, which act as sieves to catch plankton from water that continually passes over their gills for the process of respiration. Basking sharks and whale sharks also feed on plankton that is strained from water that enters their mouth and flows over their gills. Baleen whales use a filter consisting of a curtain of parallel filaments (called baleen) attached to the upper jaw to feed. These whales engulf a mouthful of water containing krill (small, shrimplike plankton) and use their tongue to force the water out, trapping krill in the hairlike edges of the baleen. The mouthful of krill is then swallowed. Flamingos feed in a similar manner. They use specially adapted beaks lined with filaments to strain plankton from the muddy bottoms of their aquatic habitat. Some fast-water caddis fly larvae spin tiny silk nets that filter small food items. The nets are then periodically gleaned. Finally, some spiders may be considered filter feeders because they use their webs to “filter” flying insects from their environment. Filter feeders are important for the environment, and an abundance of filter feeder species indicates a healthy ecosystem. For example, oysters and mussels that ingest their food use filter feeding to remove plankton, bacteria, and toxins from the environment. In the Chesapeake Bay, where filter feeder species sharply declined in the 2010s and early 2020s, the water experienced excessive nitrogen and phosphorous concentrations, severely impacting the bay's wildlife.

Large Item Ingestion

In contrast with small-particle consumers, numerous animals ingest large food items. Some of these organisms are Saprovores, such as earthworms. These worms consume large masses of soil and dead leaves, from which they obtain usable organic matter. Some planarians (flatworms) extend a long extensible tube (called a pharynx) from their mouth to ingest decaying material. Other examples of Saprovores include millipedes, wood-eating beetles, some sea cucumbers, many roundworms, and a few snails.

An extraordinary number of animals have adapted to eat plants. Eating plants requires special structures to free plant material for ingestion. Although invertebrates lack true teeth, they have other structures to obtain plants or plant parts. Snails have a unique structure in their mouth called a radula, which acts as a miniature rasping file that scrapes plant material from surfaces and rasps through vegetation. The freed plant material is then ingested. Sea urchins scrape algae by using a highly developed oral apparatus composed of five large pointed plates. Termites use strong jaws made of chitin, the hard structural component of their external skeleton, to cut tiny chunks of wood for ingestion.

Birds use horny beaks to obtain and ingest plant material. Cardinals and grosbeaks, for example, are well adapted to hull, and then consume the nutritional portion of large seeds. The bee-eaters are skilled hunters and use their medium-sized, pointed beaks to catch insects, mainly bees, mid-air. After catching them, the bee-eater stuns the bee and removes its stinger before swallowing it whole. Herbivorous mammals use specialized teeth to obtain and chew plant material. Rodents (such as beavers, porcupines, and mice), rabbits, and hares have chisel-like front teeth (called incisors) that are used to gnaw, slice, or pull off plant material. Other herbivorous mammals (including cows, sheep, deer, moose, elk, and giraffes) lack upper incisors and, therefore, use their lower incisors pressed against the roof of their mouth to pull off leaves. The ingested food is then chewed by grinding with premolars and molars.

In contrast to herbivores, many animals capture other animals and eat them. To be effective, these carnivorous predators must have appropriate behavioral adaptations to find and capture prey, as well as specialized structures to seize and hold their victims. Jellyfish use tentacles that are equipped with stinging cells to grasp and subdue animals, whereas the tentacles of squid and octopuses have suction-cup-like structures to grasp and manipulate prey. The carnivorous giant water bug hunts and captures small fish, which are relatively large prey for an insect. These water bugs use their legs to seize and hold fish and their piercing mouthparts to suck juices from their victims. Fish, amphibians, and reptiles have pointed teeth to seize and hold prey. It also is common for them to swallow their food whole. Snakes, for example, swallow whole items such as birds’ eggs and small mammals. In addition, a snake’s jaws are held together by elastic ligaments, permitting it to spread apart and ingest victims larger than its own head. Some large tropical snakes consume small pigs and deer. Chameleons and frogs swallow their prey whole, but in contrast to snakes, they use a long and sticky tongue that rapidly shoots out to capture insects. Some predatory carnivores (such as lions, tigers, bears, and dogs) have long, pointed, daggerlike teeth called canines to pierce and kill their prey. Carnivores also may have knifelike molars (called carnassials) that are used to slice flesh from bones. Carnivorous birds (such as hawks, eagles, and owls) use long, sharp talons to seize and kill small animals. Their beaks can be used to tear small pieces of food for ingestion. Further, many carnivorous animals possess specialized mechanisms for paralyzing victims. Jellyfish, centipedes, spiders, scorpions, and some snakes possess structures that inject toxins that inhibit the nervous system of their prey. Finally, some electric eels may locate and stun their prey with electrical discharges.

Liquid Ingestion

Many animals live exclusively on a liquid diet. Herbivores such as bees, butterflies, moths, hummingbirds, and some bats derive nutrition by consuming plant nectar. As a consequence of their feeding, these animals help plants reproduce by dispersing pollen. Hummingbirds have specially adapted long, narrow beaks and long tongues to suck nectar from flowers. Nectar-consuming bats also have long, narrow faces and tongues. Aphids, and many other insects that consume plant sap, have highly specialized mouthparts that pierce plants and act as miniature straws to suck sap.

Also, a number of carnivores are adapted to consume a liquid diet. As ghoulish as it might sound, ingesting blood is the most common mechanism of feeding for these animals. Mosquitoes, for example, are equipped with a syringelike mouthpart called a proboscis. Although the male sips nectar, the female mosquito uses her proboscis to pierce the skin and suck blood. As with most bloodsucking animals, mosquitoes secrete an anticoagulant that prevents blood from clotting (and also makes people itch). Some flies use a similar mode of feeding, but the common housefly generally laps up food and sugary solutions. Leeches are also well adapted for bloodsucking: They have a suction-cup-like mouth that clings tenaciously to a host while their jaws make a Y-shaped cut in the skin. Further, leeches have a muscular pharynx (throat) that literally pumps blood from their host. Ticks, which are related to spiders and scorpions, have tiny heads that are designed to burrow into the skin of their host and suck blood. They also are vectors for potentially serious diseases such as Lyme disease and Rocky Mountain spotted fever. Although vampire bats consume blood, they do not suck it. Instead, they lap blood with their tongue as it oozes from a shallow scrape in the skin made by their teeth.

Spiders also are adapted to live on a liquid diet; however, they prey on insects that have a tough external skeleton that is not easily ingested. To feed, they first pierce the insect’s exoskeleton with hollow jaws and pump in strong digestive juices that liquefy the internal contents of their prey. Later, the spider sucks the insect empty. Some young birds, such as pigeons and emperor penguins, feed on a regurgitated milklike secretion (called crop milk) that is produced by their parents’ crop. In addition, all mammals begin their life as fluid feeders, ingesting milk produced by their mothers.

Finally, endoparasites comprise a group of animals that consume a liquid diet by living inside other organisms. Some eat host tissues, while others rely on their host to digest food for them and, as a result, lack a digestive system. Tapeworms, for example, have a specially adapted anterior end with hooks and suckers to maintain a fixed position in their hosts’ gut while they consume predigested food. In contrast, hookworms (a type of roundworm) do have a digestive system, and they use their mouth opening and toothlike structures to draw and ingest blood from the inside of their host.

Studying Ingestion

Much of what has been learned about ingestion has come from simple observation of feeding behavior combined with careful note-taking. In fact, observation and data collection, along with analysis and interpretation, are the most fundamental of all scientific activities. Most people have seen a robin use its beak to pull an earthworm from the ground or a cat capture a mouse. By closely watching the lifestyle and daily activities of animals, biologists have discovered what food items are eaten and how they are ingested.

The naked eye is insufficient for observing very small animals, and for this activity, microscopes aid biologists. These tools have permitted observation of endocytosis of paramecia and bacteria by the amoeba as well as other feeding mechanisms of protozoans. Microscopic, inert latex beads are used to study the direction and power of feeding currents generated by cilia and flagella and the formation of food vacuoles. For example, paramecia will direct beads into their oral region by ciliary action and then engulf them by endocytosis. Dyes are also commonly used to study the direction and action of feeding currents. This method has revealed that flagellated choanocytes of sponges move water in through small pores in the sides of the sponge and out a single larger opening at the top.

To supplement simple observation, animals are often filmed or photographed by researchers or by the movement-sensing equipment scientists set up in the wild to capture an animal’s ingestion habits. These procedures not only provide a permanent record of the event but also permit additional analysis to be conducted at some future time. Further, feeding mechanisms that occur very quickly are difficult to analyze with the naked eye. These events can be recorded by high-speed cinematography and later played back at a slower speed for analysis. This method of study has been used to observe the lightning-fast movement of a chameleon’s tongue and the way bats catch insects with their wing membranes while in flight. Alternatively, very slow feeding events, such as endocytosis or a snake swallowing a rat, can be recorded by time-lapse photography and later viewed at a faster speed for analysis.

Mechanisms of ingestion may also be inferred by carefully analyzing the body design of an animal. For example, birds possessing beaks that have an arrangement of tightly packed vertical filaments, as found in flamingos, feed by filtering. In contrast, birds with long, pointed beaks, such as woodpeckers, probe for food in narrow places. Finally, the contents of the stomach and fecal samples from animals may be analyzed to determine the type and size of the food items that were eaten.

The Necessity of Food

Animals have an absolute requirement for food. Animals must ingest food items because, unlike photosynthetic organisms, they cannot manufacture all the necessary nutrients they require from raw materials. Animals require food both as a fuel source to provide energy for locomotion and metabolism and as building blocks for growth, maintenance, and repair. Obtaining sufficient food is of paramount importance for survival. Therefore, the limited availability of food is selected by animals that have the most successful feeding strategies and body designs for procurement and ingestion of nutrients. Because different sources of nutrition are utilized, the selection pressures for obtaining food may result in vastly different feeding mechanisms among closely related animals. For example, giant water bugs, termites, and aphids are all classified as insects but rely on different diets and, therefore, possess divergent methods of feeding and structures for ingestion. Conversely, selection pressures for obtaining food may result in the development of similar body structures in distantly related species (convergent evolution). Baleen whales and flamingos, for example, are classified in different taxonomic groups (mammals and birds) but have similar feeding methods and, therefore, similar structures for ingestion. Thus, specific feeding behaviors and structures for ingestion are primarily shaped by the nature of the food items being utilized, and the vast diversity of feeding mechanisms reflects a similar diversity in food sources.

Ultimately, the source of energy to create food comes from the Sun. Photosynthetic organisms use light to synthesize energy-rich organic compounds, such as glucose, from energy-poor inorganic compounds, such as carbon dioxide and water. One exception to this scheme occurs in certain regions of the ocean floor near the thermal vents. Far removed from sunlight and organic material derived by photosynthesis, the food chain of oceanic thermal vents is based upon certain bacteria that synthesize organic compounds from inorganic substances emitted by these undersea geysers.

There is an unavoidable loss of usable material and energy between links in a food chain. This loss occurs because much of the energy stored in food is irreversibly lost when organisms use it for growth, repair, and maintenance. Therefore, it is more efficient to have fewer links in a food chain between its base (usually plants or plankton) and its end (typically large carnivores). With that in mind, it is interesting to note that some of the world’s largest fish (whale sharks) and mammals (whales) are filter feeders of tiny organisms. These massive animals, which require a large amount of energy, avoid extra links in their food chain by feeding on plankton instead of other large animals.

Principal Terms

Carnivore: Any organism that eats animals or animal tissues

Detritus: Small bits of dead matter derived from the decay of plants and animals

Herbivore: Any animal that eats plants or plant material

Invertebrate: Any animal that lacks a backbone

Omnivore: Any animal that eats both plants and animals or their tissues

Parasite: Any organism that lives on or in other living organisms and obtains its food from them

Plankton: Microscopic plants and animals that float in water

Predator: Any organism that kills another living organism to eat it

Protozoan: A single-celled animal-like organism

Saprovore: Any organism that consumes dead or decaying plant or animal matter

Taxonomy: A classification scheme for organisms based primarily on structural similarities; taxonomic groups consist of genetically related animals

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