The shift in feeding behavior from catching prey with the tongue to jaw prehension had numerous ramifications, ultimately leading to the scleroglossan suite of innovations: enhanced chemosensory ability, active foraging, high active body temperatures, selection of high-payoff food, an enhanced ability to find hidden and sedentary prey, and an improved capacity to capture agile prey. As sit-and-wait ambush foragers, iguanians find mobile prey visually. High numbers of ants, insect larvae, grasshoppers, spiders, beetles, and other hymenopterans in their diets suggest that they sample somewhat randomly among arthropods in their immediate microenvironments. Dietary specialization on ants has occurred several times among iguanians, and, in a few cases, entire clades were generated. For example, all species of horned lizards in the North American iguanian genus Phrynosoma are ant specialists, suggesting that ant specialization evolved early in the evolutionary history of this clade and was carried through to all present day descendants.
Many insect larvae, eusocial termites, and other nonmobile arthropods escaped detection by iguanians but could not evade scleroglossans. Access to these resources allowed explosive diversification within the Scleroglossa. In contrast to iguanians, scleroglossans are active foragers (with a few exceptions) with keen chemosensory systems that can add nonmobile prey to their diets. Use of chemical cues by scleroglossans to discriminate prey also facilitates avoidance of noxious prey items. Dietary differences between Iguania and Scleroglossa are subtle, but some abundant prey (ants, other hymenopterans, and beetles) eaten by iguanians are underrepresented in scleroglossan diets. These prey often contain noxious chemicals (particularly alkaloids) and may be discriminated against based on chemical signals detectable by scleroglossans but not by most igua-nians. Because alkaloids are metabolic toxins, avoidance of them may have opened up new metabolic opportunities for scleroglossans, allowing for higher activity levels as well as prolonged activity at high body temperatures.
Just as iguanians took lingual feeding to its logical end point, autarchoglossans took jaw prehension and chemore-ception to their logical extremes in varanid lizards and snakes. Having evolved superior chemosensory abilities, autar-choglossans became fierce competitors and awesome predators of their more primitive relatives, iguanians and gekkotans. Many gekkotans escaped from autarchoglossans and other diurnal predators by becoming nocturnal. Switching to a nighttime existence, geckos found an unexploited virtual cornucopia of nocturnal insects, such as crickets, moths, and spiders. To avoid autarchoglossans, iguanians became arboreal, shifted to shady microhabitats, or moved up into colder habitats at higher elevations. Some became herbivorous and evolved large body size (iguanines and leiolepidines).
Herbivory evolved several times within Iguania, producing the subfamilies Iguaninae and Liolaeminae within Iguanidae and the subfamily Leiolepidinae within Agamidae. Most of these herbivores are larger in body size compared with their carnivorous relatives. Herbivory either released these iguanians from body size constraints associated with reliance on arthropod prey or drove the evolution of large body size, perhaps as an antipredator tactic—these are the largest iguanians. These herbivorous lizards shifted their foraging behavior, becoming grazers, and enhanced their chemosensory abilities, using the tongue-vomeronasal system to detect chemical signals. Numerous other correlates of herbivory developed, including an enlarged fermentation chamber in the gut and use of microorganisms for digestion of cellulose.
Other iguanians diversified, maintaining rudimentary vomerolfaction, relatively small size, crypsis, sit-and-wait foraging, and relatively low activity levels while subsisting on a wide variety of arthropods. Most insectivorous iguanians eat some ants, and ant specialization has occurred several times. Herbivory also has evolved several times within Scleroglossa, with similar results. Avoidance of plants containing noxious chemicals could have been a driving force behind evolution of chemosensory food discrimination in these lizards. Remaining scleroglossans were dominated by carnivorous, actively foraging clades, although numerous evolutionary reversals took place. Cordylids and some snakes, for example, armed with the scleroglossan arsenal of innovations, reverted to ambush foraging. Evolutionary reversals in diet and foraging modes occurred in cordylids and xenosaurids, along with the associated loss of ability to discriminate prey chemically. Such reversion back to sit-and-wait ambush foraging demonstrates the attractiveness of low-energy requirements and camouflage offered by the iguanian lifestyle.
Like their ancestors, snakes rely heavily on chemosensory cues to locate prey. Not all snakes are active foragers, however; boas, pythons, and vipers have reverted to the iguanian sit-and-wait mode of ambush foraging but armed with a keen chemosensory ability. Two snake subfamilies have evolved infrared receptors ("pits") wired to the optical receptor region of their brains, allowing them virtually to "see" endothermic prey in the dark. Similarly to other snakes, these predatory snakes exploit their sophisticated vomeronasal chemosensory system to locate scent trails and find ideal sites for ambush attacks.
Many snakes are larger than most lizards, and many are dietary specialists. Most eat various vertebrate prey, including fishes, amphibians, lizards, birds, mammals, and even other snakes. Like most lizards, a few snakes consume arthropods, including ants, termites, spiders, centipedes, and scorpions. Some snakes have specialized in other invertebrates, such as earthworms, slugs, and snails. Snake skull morphologic characteristics and dentitions have evolved along many different pathways, each presumably adapting its bearer to efficient exploitation of its own particular prey. Diets of various species of snakes are restricted to amphibian and reptilian eggs, avian eggs, snails, frogs, toads, lizards, other snakes, birds, and mammals. Many snakes will not eat anything outside their own particular prey category. As examples, hog-
nosed snakes (Heterodon) eat only toads, mussuranas (Clelia) eat mostly other snakes, and several snake species (Liophid-ium, Scaphiodontophis, and Sibynophis) feed almost exclusively on scincid lizards.
Skinks have bony plates called osteoderms embedded within their scales, which overlap in the manner of shingles on a roof, providing a sort of armor. Most have smooth scales and are difficult to grasp and hold on to, especially when they are squirming. Nevertheless, some species of snakes have specialized in skinks as prey items. Several of these skink specialists have evolved hinged teeth that fold back when they encounter an osteoderm but ratchet upright between scales, offering a firm purchase. One clade of gekkotan lizards, Pygo-podidae, has converged on the limbless snake body plan. Py-gopodids are known as flap-footed lizards because they have no forelimbs and greatly reduced hind limbs. Two species in one genus of pygopodids, Lialis, feed largely on skinks and have independently evolved hinged teeth.
Many snakes kill their prey by constriction, which requires short vertebrae; heavy, supple bodies; and slow movements. Very fast snakes, such as cobras and racers, have elongated vertebrae with musculature extending considerable distances between vertebrae; such snakes are slender and not as supple and seldom can constrict their prey. Another potent solution
to prey capture, used by about 20% of snakes (the inspiration for the hypodermic needle), is envenomation, which has evolved repeatedly among snakes. Some snake venoms are actually powerful protein enzymes, which begin digesting a prey item even before the snake swallows it. Injecting venom into a large and potentially dangerous prey and then releasing it to run away and die elsewhere protects a snake from being injured by its prey. Using their keen vomeronasal sensory systems, snakes can follow the trail left by the departing enven-omated prey with considerable accuracy to find the dead and partially digested food item. Snakes, monitor lizards, and large teiids use their hydrostatic, long, forked tongues as edge detectors to follow scent trails.
Rear-fanged snakes (opistoglyphs) are thought to have a primitive condition—their fangs are too far back in their mouths for efficient delivery of venom. Such snakes have to chew to inject venom. The family Elapidae, which includes coral snakes and cobras, has permanently erect short fangs (proteroglyph) in the front of the mouth; they also must chew to inject venom. Vipers and pitvipers have by far the most efficient means of injecting venom deep into their prey. They have long, hollow front fangs attached to the maxillary bone, which hinges backward when a snake closes its mouth but swings forward as the mouth is opened. Fangs in such soleno-glyph snakes swing through an arc of 90° from the resting position to the fully erect stabbing position. With use, fangs fall out and are ingested while embedded in prey items, but they are replaced quickly. (A venomous solenoglyph snake has a set of replacement fangs in the roof of its mouth.)
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