Lizards are reptiles of the order Squamata, which they share with
the snakes (Ophidians). They are usually four-legged, with external
ear openings and movable eyelids. Species range in adult length
from a few centimeters (some Caribbean geckos) to nearly three meters
Some lizard species called "glass snakes" or "glass
lizards" have no functional legs, though there are some vestigial
skeletal leg structures. They are distinguished from true snakes
by the presence of eyelids and ears. The tail of glass lizards,
like many other lizards, will break off as a defense mechanism,
Many lizards can change color in response to their environments
or in times of stress. The most familiar example is the chameleon,
but more subtle color changes occur in other lizard species as well
(most notably the anole, also known as the "house chameleon"
Lizards typically feed on insects or rodents. A few species are
omnivorous or herbivorous; a familiar example of the latter is the
iguana, which is unable to properly digest animal protein. Until
very recently, it was thought that only two lizard species were
venomous: the Mexican beaded lizard and the closely-related Gila
monster, both of which live in northern Mexico and the southwest
United States. However recent research at the University of Melbourne,
Australia and Pennsylvania State University has revealed that in
fact many lizards in the iguanians and monitor (lizard) families
have venom-producing glands. None of these poses much danger to
humans, as their poison is introduced slowly by chewing, rather
than injected as with poisonous snakes. Nine toxins previously thought
to only occur in snakes have been discovered, and a number of previously
unseen chemicals as well.
These revelations are prompting calls for a complete overhaul of
the classification system for lizard species to form a venom clade.
"These papers threaten to radically change our concepts of
lizard and snake evolution, and particularly of venom evolution,"
says Harry Greene, a herpetologist at Cornell University in New
Most other lizard species are harmless to humans (most species
native to North America, for example, are incapable even of drawing
blood with their bites). Only the very largest lizard species pose
threat of death; the Komodo dragon, for example, has been known
to attack and kill humans and their livestock. The Gila Monster
and Beaded Lizard are venemous however, and though not deadly, can
inflict extremely painful and powerful bites. The chief impact of
lizards on humans is positive; they are significant predators of
pest species; numerous species are prominent in the pet trade; some
are eaten as food (for example, iguanas in Central America); and
lizard symbology plays important, though rarely predominant roles
in some cultures (e.g. Tarrotarro in Australian mythology).
Most lizards lay eggs, though a few species are capable of live
birth. Many are also capable of regeneration of lost limbs or tails.
Lizards in the Scincomorpha family, which include skinks (such
as the blue-tailed skink), often have shiny, iridescent scales that
appear moist. However, like all other lizards, they are dry-skinned
and generally prefer to avoid water. All lizards are able to swim
if needed, however, and a few (such as the Nile monitor) are quite
comfortable in aquatic environments.
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