Feeding and digestion

Many of the prominent and interesting adaptations of reptiles are related to the capture and digestion of food. Most reptiles seize prey as individual items, and feeding strategies can largely determine the shape of the head and characteristics of the skull and jaws.

The reptilian skull varies in relation to feeding requirements. Skulls of turtles and crocodilians are comparatively rigid and compact. Those of lizards and especially snakes exhibit evolutionary reduction of structure and articulation of elements to produce a highly movable skull. The skull of a snake contains eight links that have kinetic joints that allow rotation and an impressive complexity of possible movements. When a snake swallows prey, which is ingested whole, the two sides of the skull alternately "walk" over the prey. Multiple recurved teeth pull the prey into the throat and esophagus. Lizards have mandibles that are joined at the front of the mouth, whereas the mandibles of snakes are attached only by muscles and skin. These elements in snakes can spread apart and move backward and forward independently. The kinetic structure of the skull of snakes allows very large gapes that accommodate prey larger in circumference than the snake's body. Snakes capable of larger gape, such as pit vipers, can swallow larger prey. Prey subdued and swallowed by these snakes can be enormous, sometimes exceeding the body mass of the snake that ate it.

Many snakes seize prey and swallow it as they struggle. However, snakes that consume animals capable of inflicting harm generally subdue them by means of constriction or en-venomation. Toxic secretions that immobilize prey are contained in Duvernoy's gland in the upper jaw of many colubrid snakes. These glands are homologous to the venom glands of viperid and elapid snakes. Envenomation of prey occurs with sharp but unmodified teeth or with specialized, enlarged fangs that have evolved from teeth at the front of the maxilla in viperids and elapids or near the rear of the maxilla in colu-brids. Fangs are grooved or hollow like a hypodermic needle and inject venom released from the venom glands into the struck prey. Toxic venom immobilizes prey and aids in digestion. Both of these functions are probably more important than is a defensive function.

The teeth of squamate reptiles are generally specialized for seizing and holding prey and have less variability in structure than do teeth needed for mastication in mammals. Reptilian teeth are attached to bone and often undergo replacement several times during the lifetime of individual animals. The teeth of crocodilians are attached inside sockets by ligaments in a manner similar to the attachment of mammalian teeth. One of the remarkable adaptations in a few species of snakes is hinged teeth that facilitate swallowing of hard-bodied prey. Teeth are absent in living turtles, being replaced by a tough, keratinized beak.

The reptilian gut is similar to that of many other vertebrates in being elongated, compartmental, and complex. Digestion depends on gut motility, orchestration of multiple hydrolytic enzymes, and appropriate conditions of temperature and pH. Digestion proceeds most rapidly at warmer body temperatures, and some species of reptiles select body temperatures that are higher during digestion than during nonfeeding periods. On the other hand, foods tend to putrefy and are regurgitated if body temperatures are too low. The importance of mastication or reduction of food to smaller particles varies

Rattlesnake Illustration Feeding
Fang erection by the African puff adder (Bitis arietans). The maxillary and prefrontal bones are highlighted. (Illustration by Barbara Duperron)

with species and is generally much less than is required by mammals. Because of low metabolic rates and generally high efficiencies of converting food to assimilated energy, most reptiles do not need food frequently.

Results of studies mainly of snakes indicate that physiological responses of the gut are regulated adaptively in relation to foraging modes and feeding habits. After feeding, snakes that feed infrequently on relatively large prey undergo remarkable increases in metabolic rate, mass of intestinal tissue, and rate of nutrient transport across the gut wall. After digestion and between feedings, the gut atrophies, and digestive functions are down-regulated. These changes are more modest in actively foraging snakes that feed with relatively greater frequency and thus maintain the gut in a more constant state of readiness. The down-regulation responses in snakes that feed less frequently presumably evolved to conserve energy otherwise spent on gut maintenance during extended periods without feeding. Snakes, such as vipers and pythons, that generally capture prey by ambush may feed once a month or less frequently.

Various turtles and lizards are omnivorous, but relatively few lineages have evolved strict herbivory. Herbivorous reptiles exhibit morphological specializations of the gut and depend to varying degrees on fermentation by populations of symbiotic microorganisms that usually reside in enlarged for ward portions of the large intestine. Plant material has a lower energy content than does animal tissue, and the high fiber content of plants can be very resistant to digestion. Digestion of plant material requires more time and is less efficient than is generally true of animal tissue. Among lizards, strict her-bivory is associated with relatively large body size, possibly as an evolutionary consequence of the energetics involved. Fruits and flowers are common foods that are relatively rich in energy and nutrients, but some lizards and land tortoises have diets composed largely of leaves.

Studies of the Galápagos marine iguana (Amblyrhynchus cristatus) have elucidated important couplings between food resources, body size, and survival during periods of food limitation. These lizards are herbivorous reptiles that feed on submerged algae along the rocky shorelines of the Galápagos archipelago of Ecuador. Smaller marine iguanas are prevalent on islands that have lower production of marine algae, apparently because the smaller animals have a higher foraging efficiency than do larger iguanas. Moreover, when food is scarce (e.g., during El Niño events), larger animals suffer higher mortality than do smaller animals. Thus population shifts to smaller body sizes can occur during or after periods of resource limitation. Amazingly, changes in bone metabolism enable marine iguanas to shrink reversibly during times of energy shortages. Thus shrinkage in body size has been shown to coincide with low food availability after El Niño events, whereas body length has been shown to increase during subsequent La Niña conditions, when food is more abundant. Marine iguanas demonstrating greater shrinkage survive longer than do larger iguanas because the efficiency of foraging increases and the total energy expenditure decreases relative to larger animals.

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