Concept 30.2: Animals Get Food in Three Major Ways

If we look at animals in the broadest way possible, we realize that they get their food in three major ways. Some target easily visible, individual food items. Some collect tiny—often invisibly tiny—food particles, which they must gather in great numbers to obtain a sufficient amount. Finally, some depend on symbioses with microbes to obtain critical food needs.

These three methods are not mutually exclusive. Many animals employ two methods. For example, as we note below, there is a growing realization that we humans are probably far more dependent on microbes than previously thought.

Some animals feed by targeting easily visible, individual food items

An osprey (fish hawk; Pandion haliaetus) catching a fish provides an iconic example of feeding by targeting easily visible, individual food items (FIGURE 30.4). The osprey spots an individual fish from the air while flying. Then it plunges to the water to seize the fish with its talons in a targeted way. This type of feeding also occurs when a lion kills a zebra with a bite to its neck, or when a crab grasps a clam with its claws and crushes the clam’s shell to get to the meat inside.

Figure 30.4: An Osprey Eats by Targeting Individual Food Items

Animals that eat leaves and grasses also exemplify this type of feeding. As a deer plucks leaves from a tree, it targets each leaf. Many grazing animals are remarkably selective, targeting some types of plants but avoiding others. Bees target individual flowers to collect their nectar and pollen. Squid catch fish one at a time.

Suspension feeders collect tiny food particles in great numbers

The water in a lake or ocean often contains great numbers of suspended organic particles, many of them too tiny to be seen with the naked eye. For example, photosynthesis in bodies of water is typically carried out by single-celled algae suspended in the water’s sunlit, upper layers. Each milliliter of this water can contain hundreds or thousands of individual algal cells, each microscopically small. Tiny animals that eat the algae are often also abundant. Dead algal cells, dead microscopic animals, and the feces of tiny animals are additional types of common, suspended organic particles. Here we use the term “particles” to refer to small organic objects that may be plant or animal, living or dead.

Clams, many fish, many aquatic insects, and numerous other aquatic animals have evolved ways of collecting tiny particulate foods. Such animals are called suspension feeders in recognition of the fact that they feed on suspended particles. These animals were originally called filter feeders because people assumed they use sievelike or filterlike body parts to collect particles. However, we realize today that many of these animals do not sieve or filter the water, and specialists now prefer the more general term “suspension feeders.”

Collecting tiny particles is not the only challenge suspension feeders face. To obtain sufficent amounts of food, these animals also must be able to collect the particles in huge numbers. A single clam eats millions of individual algal cells each day. Suspension feeders usually do not target individual food particles, but instead collect huge numbers indiscriminately.

A common definition of suspension feeding is that it is a process by which an animal collects individual food particles that are only 1/100th the size of the animal or smaller. By this definition, many whales are classed as suspension feeders. About half the living species of whales have no teeth. Instead, they have closely spaced plates (made of a material like our fingernails) that allow them to collect small organisms from the water (FIGURE 30.5A). Some of the biggest whales, measuring 20–30 meters long, feed principally on crustaceans that are each about the size of a rice grain. These whales are suspension feeders by the definition we are using. The two biggest fish alive today, the whale shark (Rhincodon typus) and basking shark (Cetorhinus maximus), are also suspension feeders, as are the huge devil (manta) rays (Manta).

Figure 30.5: Two Suspension Feeders (A) A toothless grey whale showing the array of plates in its mouth. The plates, made of a fingernail-like material, are called baleen, and the toothless whales are often called baleen whales. (B) Mussels (a type of mollusk) feed by pumping environmental water over and through their gills, which remove tiny food particles from the water.

Most suspension feeders, however, are less than 30 centimeters long and feed on particles that are microscopic or nearly microscopic in size. These small suspension feeders include sardines, anchovies, menhadens, and many other commercially important fish. They also include scallops, mussels, clams, and many other aquatic invertebrates (FIGURE 30.5B).

Many animals live symbiotically with microbes of nutritional importance

A symbiosis is an intimate, long-term association between two different types of organisms. Many animals have evolved symbioses with microbes, such as bacteria or algae, in which the microbes synthesize nutritionally important molecules for the animals. Because the microbes live inside the animals, the animals do not eat them. However, molecules transported directly from the microbes to the animals serve the same functions as foods, and thus we will call such molecules foods.

All reef-building corals contain algae (FIGURE 30.6). The cells of these algae live inside the cells of the corals. A reef-building coral is an animal–algal symbiosis composed of a cnidarian animal (resembling a hydra or anemone) and a specific species of alga in the genus Symbiodinium (see Concept 20.4). The algal cells contain photosynthetic pigments that are responsible for some of a coral’s coloration. By carrying out photosynthesis, the algal cells produce food molecules such as glucose. Some of these food molecules then diffuse out of the algal cells into the animal cells and become food for the heterotrophic animal cells. This symbiosis is of extreme importance nutritionally. Corals can lose their algal cells when stressed (a process called bleaching). If that happens, the corals deteriorate and will die unless they obtain new algal cells.

Figure 30.6: Reef-Building Corals Live Symbiotically with Algal Cells Corals, unlike most animals, are photosynthetic and require sunlight. (A) Coral reefs consist of a diverse array of corals. Each coral is a large colony of hydra-like polyps, which build the coral’s skeleton. (B) Algal cells (often called zooxanthellae) live inside the cells of a polyp’s gastrodermal cell layer. The algae carry out photosynthesis, synthesizing glucose and other nutritionally important molecules for the polyp. Polyps also feed heterotrophically by using stinging cells on their tentacles to capture tiny animals that swim by.

Animals have also evolved nutritionally important symbioses with many types of microbes that are heterotrophic rather than photosynthetic. The ruminant mammals—including cows, sheep, moose, antelopes, deer, and giraffes—are the most famous examples. All ruminants are herbivores and eat foods such as grasses and leaves. They have evolved a complex stomach that consists of four chambers through which food passes sequentially (FIGURE 30.7). The first of these chambers, the rumen, is huge, occupying more than 10 percent of the animal’s body. It is filled with a microbial community consisting of a complex and changeable mix of bacteria, protozoa, yeasts, and fungi living in a nonacidic, saliva-like fluid.

Figure 30.7: A Ruminant’s Stomach Bison, like all ruminants, have a specialized stomach with four compartments that enables them to obtain energy and nutrients from coarse, otherwise indigestible plant material through fermentation. The fermentation is carried out by a mixed microbial community of bacteria, protists, yeasts, and fungi that is housed primary in the rumen, which is not acidified.

When a ruminant eats, its food (after chewing) goes directly to the rumen, which acts like a fermentation vat. Here the resident microbes attack the food, breaking it down and transforming it in ways the animals cannot. The processes carried out by the microbes are called fermentations because they take place without O₂ (the rumen is anoxic). Three functions of the rumen microbes are particularly important:

• They break down cellulose and hemicelluloses (cellulose-like compounds; see Figure 2.10). Vertebrate animals by themselves cannot break these compounds down and thus cannot obtain nutritional value from cellulose, one of the most abundant organic materials on land (see Concept 30.3). By breaking down cellulose, the microbes turn it into small molecules (short-chain fatty acids, such as acetic acid) that the ruminant can absorb into its blood and use for chemical energy.
• The microbes make several types of B vitamins and amino acids that are essential nutrients for the ruminants.
• The microbes enable ruminants to recycle nitrogen, one of the chemical elements that is often in short supply. The microbes take up waste nitrogen from the ruminant’s metabolism and convert it to microbial protein. The animal then digests this protein, obtaining amino acids from which it can synthesize animal proteins. Animals without this recycling process simply excrete their waste nitrogen, and then need to replace it by finding nitrogen-rich foods.

One of the breaking stories in the study of animal nutrition is the recent realization that heterotrophic microbes probably play important nutritional roles in most animals, not just in animals such as ruminants that have evolved large fermentation vats. The microbes of greatest interest are those that live in the lumen—the central cavity—of the intestines and other parts of the gut and collectively are called the gut microbiome.

Humans have a large gut microbiome, consisting mostly of bacteria, which we acquire during birth and after we are born. People differ in the composition of their gut microbiome, and increasing evidence indicates that the gut microbiome has important effects on individual digestion, nutrition, and metabolism.

Figure 30.8: Digestive Systems Are Diverse, but They All Carry Out Digestion and Absorption In each group of animals, specific regions of the digestive tract vary from species to species, in part because different species are adapted to different diets.

Now that we have surveyed the ways in which animals get foods, let’s turn to the ways they process their foods after ingestion. Processing is as important as the chemical composition of food in determining the nutrients that animals obtain.