Feeding on Food Masses

Feeding on Food Masses
Among the most interesting animal adaptations are those that have evolved for procuring and manipulating solid food. Such adaptations and the animals bearing them are largely shaped by what the animal eats. Predators must be able to locate, capture, hold, and swallow prey. Most carnivorous animals simply seize food and swallow it intact, although some employ toxins that paralyze or kill prey at time of capture. Although no true teeth appear among invertebrates, many have beaks or toothlike structures for biting and holding. A familiar example is the carnivorous polychaete Nereis, which possesses a muscular pharynx armed with chitinous jaws that can be everted with great speed to seize prey (Figure 17-3A). Once a capture is made, the pharynx is retracted and the prey swallowed. Fish, amphibians, and reptiles use their teeth principally to grip the prey and prevent its escape until they can swallow it whole. Snakes and some fishes can swallow enormous meals. Gripping of prey, together with absence of limbs, is associated with some striking feeding adaptations in these groups: recurved teeth for seizing and holding prey and distensible jaws and stomachs to accommodate their large and infrequent meals (Figure 34-3). Birds lack teeth, but their bills are often provided with serrated edges or the upper bill is hooked for seizing and tearing prey (see Figure 29-11,).

Many invertebrates are able to reduce food size by shredding devices (such as the shredding mouthparts of many crustaceans) or by tearing devices (such as the beaklike jaws of the cephalopod molluscs). Insects have three pairs of appendages on their heads that serve variously as jaws, chitinous teeth, chisels, tongues, or sucking tubes. Usually the first pair serves as crushing teeth; the second as grasping jaws; and the third, as a probing and tasting tongue.

Figure 34-3 This African egg-eating snake, Dasypeltis, subsists entirely on hard-shelled birds’ eggs, which it swallows whole. Its special adaptations are reduced size and number of teeth, enormously expansible jaw provided with elastic ligaments, and teethlike vertebral spurs that puncture the shell. Shortly after the second photograph was taken, the snake punctured and collapsed the egg, swallowed its contents, and regurgitated the crushed shell.

Figure 34-4 Structure of human molar tooth. The tooth is built of three layers of calcified tissue covering: enamel, which is 98% mineral and the hardest material in the body; dentine, which composes the mass of the tooth and is approximately 75% mineral; and cementum, which forms a thin covering over the dentine in the root of the tooth and is very similar to dense bone in composition. The pulp cavity contains loose connective tissue, blood vessels, nerves, and tooth-building cells.

True mastication, the chewing of food as opposed to tearing or crushing, is found only among mammals. Mammals usually have four different types of teeth, each adapted for specific functions. Incisors are designed for biting, cutting, and stripping; canines are for seizing, piercing and tearing; premolars and molars, at the back of the jaw, are for grinding and crushing (Figure 34-4). This basic pattern is often greatly modified in animals having specialized food habits (Figure 34-5; see also Figure 30-10). Herbivores have suppressed canines but well-developed molars with enamel ridges for grinding. The well-developed, self-sharpening incisors of rodents grow throughout life and must be worn away by gnawing to keep pace with growth. Some teeth have become so highly modified that they are no longer useful for biting or chewing food. An elephant’s tusk (Figure 34-6) is a modified upper incisor used for defense, attack, and rooting, and the male wild boar has modified canines that are used as weapons. Many feeding specializations of mammals are described on.

Figure 34-5 Mammalian dentition. A, Teeth of gray fox, a carnivore, showing the four types of teeth; B, Woodchuck, a rodent, has chisel-like incisors that continue to grow throughout life to replace wear; C, White-tailed deer, a browsing ungulate, with flat molars bearing complex ridges suited for grinding.

Herbivorous, or plant-eating, animals have evolved special devices for crushing and cutting plant material. Some invertebrates have scraping mouthparts, such as the radula of snails (Figure 16-3). Insects such as locusts have grinding and cutting mandibles; herbivorous mammals such as horses and cattle use wide, corrugated molars for grinding. All these mechanisms disrupt the tough cellulose cell wall to accelerate its digestion by intestinal microorganisms, as well as to release cell contents for direct enzymatic breakdown. Thus herbivores are able to digest food that carnivores cannot, and in doing so, convert plant material into protein for consumption by carnivores and omnivores.

Figure 34-6 An African elephant loosening soil from a salt lick with its tusk. Elephants use their powerful modified incisors in many ways in the search for food and water: plowing the ground for roots, prying apart branches to reach the edible cambium, and drilling into dry riverbeds for water.