Snakes and lizards are found everywhere in the world, except at very high latitudes, on cold mountaintops, and in the Arctic and Antarctic. At high latitudes and elevations, temperature becomes a limiting factor for animals that rely on external heat sources (ectotherms). Nevertheless, many lizards and snakes have evolved adaptations, such as viviparity (bearing live young), that facilitate living in cold environments.
Throughout the world, most lizard assemblages contain mixtures of iguanians, gekkotans, and autarchoglossans. More diverse squamate faunas tend to have proportionately greater numbers of species of autarchoglossans, whereas less diverse faunas have relatively more iguanians. In assemblages with a substantial number of autarchoglossans, most of the iguanian and gekkotan fauna is arboreal, saxicolous (lives among rocks), nocturnal, or active in the shade. In contrast, where squamate assemblages lack or have few autarchoglossans, such as in North American deserts and high-elevation habitats in South America, iguanians occupy many microhabitats held by autar-choglossans in mixed assemblages elsewhere. Iguanians and gekkotans probably have been displaced by autarchoglossans throughout their evolutionary history, explaining much of their current ecological and geographical distribution. At the same time, the set of traits that provides autarchoglossans with a competitive advantage throughout the world may constrain their ability to persist in the environments and microhabitats dominated by iguanians and gekkotans.
Gekkotan and autarchoglossan lizards are more species rich in the Old World (30% and 51-52%, respectively) than in the New World (16-19% and 31-33%, respectively). Igua-nians display the opposite pattern, with considerably fewer species in the Old World (18-19%) than in the New World (49-51%). In the New World, Amazonia and Venezuela have high percentages of autarchoglossans (mostly teiids and gymnophthalmids) and a low percentage of iguanians. Igua-nians outnumber scleroglossans in the Caribbean and Central America, and they dominate in Argentina, where there is little scleroglossan diversity, probably because warm seasons are too short to maintain rich autarchoglossan faunas. In the Old World, autarchoglossans are somewhat impoverished in Madagascar, where iguanians and gekkotans are relatively diverse. In South Africa cordylids have reverted to sit-and-wait foraging, possibly owing to a lack of other diurnal sit-and-wait ambush foragers (e.g., agamids) and competition with other actively foraging lizards (scincids, gerrhosaurids, and varanids). Regional trends are even more pronounced at a local level when lizards from particular study sites are considered. Iguania constitute 74% of the saurofauna at twelve New World desert study sites in the Great Basin, Mojave, and Sonoran deserts, but only 8% and 18% at Old World desert study sites in Africa and Australia, respectively.
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