All skinks presumably derived from a single common ancestor (i.e., they are monophyletic). They have large, symmetric, shieldlike scales on the head. Most skinks have smooth, glossy, circular scales, but a few have sharp or keeled scales. Bony plates, known as osteoderms, underlie skink scales. One distinctive feature of skinks is the bony secondary palate in the roof of the mouth, which separates the respiratory and digestive passages. (These are confluent in most other lizards.) Other lizards pant when they are thermally stressed (which cools the roof of the mouth, into which large blood sinuses can dissipate heat), but skinks do not pant, perhaps because the secondary palate impedes heat exchange.
Skinks have repeatedly evolved reduced appendages; numerous different evolutionary groups have produced completely limbless forms (Acontias, Anomalopus, Barkudia, Brachy-meles, Coeranoscincus, Feylinia, Lerista, Melanoseps, Ophiomorus, Ophioscincus, Scelotes, Scolecoseps, Sepsophis, Typhlacontias, and Typhlosaurus). These 15 genera are found in Africa, Asia, and Australia. A single North American genus, Neoseps, approaches limblessness, with both front and hind limbs reduced to tiny appendages. The same is true of many skinks in Southeast Asia (e.g., species of Larutia and Riopa). Legless skinks, as well as those with reduced limbs, are typically burrowers and usually have small eyes and consolidated head shields. All degrees of limb reduction can occur within a single genus.
In most lizards, including some skinks, inguinal fat bodies protrude into the abdominal cavity from the pelvic area. (These store valuable energy reserves used in reproduction.) Members of the large Sphenomorphus group of skinks, however, have lost these fat bodies and rely on their tails to store fat. Tail loss thus can be costly. In many skinks, the tails of juveniles are markedly brighter than the tails of adults. Red, blue, and yellow tails are thought to lure the attention of predators away from the body.
Skinks have a wide variety of eye types. As in other lizards, only the lower eyelid moves; it is lowered to open the eye and raised to close it. The ancestral condition is a freely movable, scaly, opaque eyelid. Several other derived states exist among skinks: some species have a freely movable eyelid with a clear, disclike central scale, or window, through which the lizard can see even when the eye is closed. In other species of skinks the eyelid is fused immovably in the raised position, with an expanded clear area through which they can see. Still other skinks have a clear eyelid fused all around, forming a spectacle similar to those of geckos and snakes. Larger skinks tend to display the ancestral condition, with movable, opaque eyelids, but most smaller skink species have more derived eyelid conditions. Permanently capped eyes in small skinks limit evaporative water loss and protect the eye.
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