Many people confuse geckos and salamanders. At first glance, it's easy to understand why -- both types of animal are usually small and have similar body shapes. Despite these similarities, geckos and salamanders are very different animals that are separated by millions of years of evolution.
Classification and Diversity
Geckos and salamanders are not closely related clades; salamanders are amphibians, while geckos are members of the class Reptilia. While approximately 360 salamander species inhabit the globe, geckos are a much more diverse group, represented by almost 1,500 species.
One of the key innovations of the ancestor of modern reptiles and mammals was the development of the shelled, amniotic egg. Unlike the eggs of most amphibians, which have no shell and must be deposited in the water or some very damp location, the shell of a reptile’s egg prevents it from desiccating. This allowed reptiles to colonize habitats far from water, in places few salamanders could follow. Some salamanders practice external fertilization, while those that engage in internal fertilization do so by juxtaposing their cloacas -- no salamander species possess an intromittent organ. All geckos, by contrast, practice internal fertilization, and the males use paired hemipenes for sperm transfer.
Salamanders have moist, smooth and sometimes slimy skin that is very permeable to water; geckos have skin that is variably smooth, warty or rough, and relatively impermeable to water. Both geckos and salamanders shed their skin at regular intervals, and both often eat the shed skin after completing the process. Both types of animals often seek out damp retreats before the shedding process begins.
Though there are a few exceptions, most geckos and salamanders have the same general body plan, consisting of four well-developed limbs, a well-defined head and a long tail. Most of the members of both groups are relatively small, generally less than a foot in length. However, a few aquatic salamanders reach very large sizes; hellbenders (Cryptobranchus alleganiensis) reach more than 2 feet in length, and the world’s largest salamanders, the Asian giant salamanders (Andrias sp.), may reach or exceed 5 feet and weigh more than 75 pounds. By comparison, the largest gecko species, Rhacodactylus leachianus, does not quite weigh 1 pound and barely exceeds a foot in length.
All geckos and adult salamanders are carnivores -- most species subsist on worms, insects and other invertebrates. Aquatic salamanders typically feed on aquatic invertebrates, but large species hunt fish and other salamanders as well. Some aquatic species -- particularly aquatic larvae -- filter organic debris from the water column. A few gecko species add flower nectar and soft fruit to their diet as well.
Many salamanders never leave the water, while no gecko routinely enters the water. Many gecko species have colonized the trees, and one group of species (genus Ptychozoon) has even evolved gliding ability and wing-like membranes; by contrast, few salamander species venture into the trees. Additionally, some geckos live in arid deserts, unlike salamanders, which must live in damp habitats.
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