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Part One explores lizard lifestyles, answering such questions as why lizards are active when they are, why they behave as they do, how they avoid predators, why they eat what they eat, and how they reproduce and socialize. In Part Two the authors take us on a fascinating tour of the world's manifold lizard species, beginning with iguanians, an evolutionary group that includes some of the most bizarre lizards, the true chameleons of Africa and Madagascar. We also meet the glass lizard, able to break its tail into many highly motile pieces to distract a predator from its body; lizards that can run across water; and limbless lizards, such as snakes. Part Three gives an unprecedented global view of evolutionary trends that have shaped present-day lizard communities and considers the impact of humans on their future.
In this chapter we introduce the highly active teiids and lacertids, the diminutive gymnophthalmids, and the secretive, long-lived xantusiids. Teiids, lacertids, and gymnophthalmids are in constant motion, maintaining a distance between themselves and other creatures-including humans-and frequently looking back to keep tabs on what could, after all, be a potential predator. Any sudden move by an observer, and the lizard darts off; although it immediately initiates foraging behavior again, its vigilance never ceases.
These are the lizards that dominate terrestrial habitats in the New World and much of the Old World (excluding Australia). Most of these lizards live in open habitats -even Amazonian forest teiids, such as Dracaena and Crocodilurus, favor large swamps or edges of rivers with sun exposure. Tiny gymnophthalmids appear at first glance to be exceptions because many are found in undisturbed rain forest. However, when scale is considered, most live in the open: a patch of leaf litter in western Amazonia is as much an open habitat to Prionodactylus eigenmanni as the bank of a small stream flowing through the same forest is to Neusticurus ecpleopus.
Teiids, gymnophthalmids, and lacertids (all of which form the clade Lacertiformes) are tied together evolutionarily because they share a common ancestor. Teiids are most closely related to gymnophthalmids, and both are presently restricted to the New World. Lacertids, the sister taxon to the teiid-gymnophthalmid clade, are an Old World group. Ecological counterparts in their respective habitats, lacertiforms share a number of identifying features: they have elongate, streamlined bodies compared with many iguanians; alert and often fast moving, they forage actively, primarily on the surface of the ground (with notable exceptions), discriminating between prey and nonprey using chemical cues; and many, but not all, are active at high body temperatures.
Xantusiids, or night lizards, differ considerably from teiids, gymnophthalmids, and lacertids, and their phylogenetic position is less certain. They may be the sister taxon to Lacertiformes, but they could be more closely allied to the Annulata, the group containing, among other things, wormlike amphisbaenians (see chapter 9). Another possibility is that they are related to Gekkota. We include xantusiids here recognizing that future studies may place them elsewhere within the evolutionary tree of lizard families. Xantusiids live in enclosed spaces. Some occupy crevices in rocks (Xantusia henshawi), others (X. vigilis) live under decaying remains of Joshua trees or clumps of large beargrass, Nolina bigelovi. Xantusiid eyes are capped over like those of most geckos; they have elliptical pupils and are active in dark places. Their body temperatures while active are considerably lower than those of Lacertiformes.
The teiids we know today are a New World family, ecological counterparts of Old World lacertids. Historically, teiids had a wider distribution, at least in the Northern Hemisphere. Their northern distribution is restricted to deserts, the southern half of the Great Plains, and the coastal plain of the southeastern United States. A now extinct group, polyglyphanodontines, was diverse in the late Cretaceous of North America, and they also occurred in the Upper Cretaceous of Mongolia. The fossil record indicates that New and Old World polyglyphanodontines had diverged considerably from each other by mid to late Cretaceous. A faunal exchange during early Cretaceous sent some from east to west; others, however, went from west to east. The fossil record does not clearly indicate whether teiids originated in the Old or New World, though ancient teiids apparently colonized much of the New World. By the end of the Cretaceous, all Northern Hemisphere teiids had gone extinct (Nydam 2000). Modern teiids, therefore, diversified from ancestors remaining in subtropical and tropical areas of the New World, where they achieved the remarkable evolutionary success we see today. A single genus, Cnemidophorus, reinvaded and diversified in the northern half of the New World.
Several characteristics distinguish teiids from other lacertiforms. Teiid head scales are separate from skull bones; those of lacertids are fused to the skull. Teiid teeth have solid bases and are "glued" to jaw bones with cementum; those of lacertids are hollow, and cementum is absent. Indeed, cementum is such a prominent teiid character that it can be used to distinguish fossil teiid jawbones from those of all other fossil lizards. Additional characteristics include generally small, granular scales on the dorsal surface, with large, rectangular scales forming distinct transverse rows ventrally. All teiids have fully formed legs and a fairly distinctive overall morphology consisting of streamlined bodies, long tails, relatively pointed snout, eyelids, and long hind limbs. They are active foragers and lay eggs.
Teiids occur throughout the southern and western continental United States, across Mexico and Central America, on many Caribbean islands, and into much of South America (excluding the high Andes and extreme southern parts of South America). Nine genera are recognized: Ameiva, Callopistes, Cnemidophorus, Crocodilurus, Dicrodon, Dracaena, Kentropyx, Teius, and Tupinambis (Presch 1974). One genus, Cnemidophorus, with 56 named species (Wright 1993), has undergone an extensive adaptive radiation. Ameiva and Kentropyx are well represented also, but Tupinambis, Callopistes, Teius, Dracaena, and Dicrodon contain only a few species. Crocodilurus is monotypic, that is, represented by a single species, C. lacertinus. Teiids are divided into two subfamilies, Teiinae (Ameiva, Cnemidophorus, Dicrodon, Kentropyx, and Teius) and Tupinambinae (the remaining four genera). Teiids vary in body size, ranging from the small whiptail Cnemidophorus inornatus (55 mm SVL) to large tegus, Tupinambis (500 mm SVL), and caiman lizards, Dracaena (approx. 300-450 mm SVL).
The 56-plus species of Cnemidophorus (= carrying leg armor), commonly known as racerunners or whiptails, range from southern Idaho through Central America and the Caribbean all the way to Argentina. They reach their largest body sizes in northern South America and on some Caribbean islands, with size generally diminishing as latitude increases (although exceptions exist: C. tigris, for example, is smaller in southern parts of its geographic range). Several species of Cnemidophorus remain unnamed, including one found in Manaus, Brazil. Many "species" of whiptails are unisexual; in these, males do not exist and females reproduce by parthenogenesis. As soon as a female reaches sexual maturity, she begins producing daughters that are genetically identical to herself (see chapter 6). Parthenogenesis occurs in some other teiids (e.g., Kentropyx) as well, and in some other squamate families.
Herpetologists have had difficulty identifying whiptails and working out their relationships. As Charles Lowe points out in his insightful introduction to Biology of Whiptail Lizards (Genus Cnemidophorus) (Wright and Vitt 1993), the famous herpetologist Edward Drinker Cope considered this genus the most difficult in all of herpetology. Yet Cope had seen only the tip of the iceberg: parthenogenetic Cnemidophorus were not discovered until 1958 (not published until 1962), adding a new element of confusion to the genus.
Discovery of parthenogenesis in East Asian Lacerta by Ilya Darevsky (1958) sent the American herpetologists Richard Zweifel and Charles Lowe deep into their collections searching for the answer to a question implicit in an observation first made by Sherman Minton (1958); why were there no males in Cnemidophorus tesselatus? Indeed, not only did C. tesselatus prove to be parthenogenetic, but a swarm of parthenogenetic Cnemidophorus were found to occur all across the southwestern United States and northern Mexico, all very difficult to sort out. Zweifel and Lowe (1966) discovered three characters-number of scales around midbody, number of scales between paravertebral stripes, and number of scales on toes-that were sufficient to distinguish one species of Cnemidophorus from another. Of course, in those days we didn't have the fancy molecular techniques that now dominate systematic herpetology, but at least Cope's most difficult genus could finally be sorted out into species.
Among teiids, whiptails are most widespread geographically and appear tied to open habitats to a much greater extent than Ameiva, Kentropyx, or most larger-bodied genera. These lizards are most frequently encountered on beaches and desert fiats, in tropical dry forest, and along edges of relatively closed habitats such as forests. They often use roads and trails to get to open patches within forest in tropical regions. As a group, they are also among the most terrestrial of teiids, rarely entering water or climbing into vegetation.
In addition, whiptails-which appear quite nervous while foraging, often darting off at the slightest provocation -are among the most active of Lacertiformes. Their high activity levels are supported in part by their high body temperatures. Cnemidophorus deppii, for example, which live on open beaches in western Central America, average nearly 40°C while active.
In temperate North and South America, whiptails are active during summer, with activity falling off rapidly as fall approaches. In North America, adults disappear underground in August (though juveniles remain active until September or October), and reproduction is highly seasonal, occurring in spring and early summer (as, for example, in Cnemidophorus inornatus and C. neomexicanus; Christiansen 1971). In tropical environments where annual temperatures are relatively constant, activity and reproduction can occur nearly year round (as in C. ocellifer in northeastern Brazil; Vitt 1983).
A North American species, Cnemidophorus tigris, ranges from southern Idaho through Sonora and Baja California. Because of its wide distribution, it exemplifies geographic variation in ecological traits. In the north, C. tigris are active at both lower body temperatures and lower ambient environmental temperatures than in the south. Their seasonal period of activity is also shorter in the north. Frequencies of broken regenerated tails are higher in the south (Pianka 1970a). Most whiptails eat a variety of insects (particularly termites) and spiders, but some, such as C. lemniscatus, add fruits to their diet, and a few, such as C. murinus and C. arubensis, are herbivorous.
Ameiva, which occur through southern Mexico, Central America, and much of South America, with numerous island species in the Caribbean, are very much like Cnemidophorus in general morphology and behavior. As in Cnemidophorus, Ameiva are terrestrial. Diets are varied but include a diversity of invertebrates, small vertebrates, and fruits (Vitt and Colli 1994; Censky 1996). A. ameiva in South America has been best studied. This alert, fast-moving, large (190 mm SVL) lizard is common in virtually every open habitat within its range, even entering cities to forage alongside dogs in garbage heaps! It is among the most conspicuous of lizards in Venezuelan llanos, cerrado, caatinga, and lowland rain forest habitats. Ameiva ameiva maintain body temperatures of 37°C and higher, regardless of habitat, and can be seen in strikingly large numbers in lowland forest along roads, trails, and river edges where direct sunlight hits the ground (Sartorius et al. 1999).
Like many other teiids, Ameiva ameiva digs a burrow where it remains while inactive. Burrows generally have a single entrance and are left open while the lizard is inside. When on flat ground, burrows are shallow, with the terminal chamber just under the surface. If an intruder digs into one of these burrows, the lizard typically emerges through the roof of the terminal chamber and dashes off. On steep hillsides, burrows often go straight into the bank and may likewise be rather shallow. Although lizards have no escape routes from these shelters, their location-sometimes as much as 4.5 m up the bank-likely discourages many predators.
In Central America, several species of Ameiva occur along beaches and in dry forest of the west coast, including A. festiva and A. quadrilineata. Ameiva festiva also occurs in rain forest of eastern Central America, along the Caribbean. Just north of Río San Juan in Nicaragua, A. festiva is a common forest species near treefalls. In many respects, including coloration and color pattern, it appears nearly identical ecologically to Kentropyx pelviceps of lowland Amazonian forest, foraging and basking in treefalls but also entering forest to forage for brief periods before its body temperature falls (Vitt and Zani 1996d). However, it doesn't climb like Kentropyx.
At least 12 species of Ameiva occur on the Lesser Antilles. Each island has a single species, with one exception: the Anguilla Bank has two, A. corax and A. pleii (Censky and Paulson 1992). Unlike Anolis, which colonized the island arc from the west (Puerto Rico), Ameiva colonized from the mainland to the south. When sea levels were much lower during the Pleistocene, islands were larger and separated by less water. The fact that most islands have but a single species suggests that Ameiva are their own strongest competitors. On the Anguilla Bank, the two species are different in body size, with adult male A. pleii reaching 181 mm SVL and A. corax reaching only 132 mm SVL. Those 50 mm may represent the minimum body size difference allowing coexistence. Of course, juveniles are much more similar in size and probably face intense interspecific competition as a result.
This divergence in body size, both within and among genera, is in fact fairly common among teiids (see below). Unlike iguanian species, which vary considerably in overall morphology and often segregate by microhabitat, teiid species are strikingly similar morphologically (with a few exceptions). Yet because they cover large areas while foraging, and forage at about the same time of day, encounters among individuals and between species occur frequently. Hence, divergence in body size appears to allow coexistence. Examination of body sizes and DNA-based phylogeny for populations of C. tigris and C.
Excerpted from LIZARDS by ERIC R. PIANKA LAURIE J. VITT Copyright © 2003 by Regents of the University of California. Excerpted by permission.
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Part I. Lizard Lifestyles
1. Evolutionary History and Phylogeny
2. Getting Around in a Complex World
3. Lizards as Predators
4. Escaping Predators
5. Social Behavior
6. Reproduction and Life History
7. Reflections of the Real World
Part II. Lizard Diversity
9. From Geckos to Blind Lizards
10. From Racerunners to Night Lizards
12. From Girdled Lizards to Knob-Scaled Lizards
13. Monsters and Dragons of the Lizard World
Part III. Synthesis
14. Historical Perspective
15. Lizards and Humans
Appendix: Taxonomic Summary of Lizard Genera