Lizard

Snakes evolved from within the lizards, so snakes are in a sense a specialised branch of lizards, but the animals we call lizards share several features that most snakes have lost. Typical lizards have four legs, external ear openings, and movable eyelids, and their lower jaw is a single rigid unit. Snakes are legless, have no ear openings or eardrum, cannot blink because a clear scale covers each eye, and have a lower jaw split into two halves joined by stretchy ligament for swallowing large prey. The tricky cases are the legless lizards, such as slow worms and glass lizards, which look like snakes but still have eyelids, ear openings, and often the ability to shed the tail.

Tail loss, or caudal autotomy, is an escape tactic. Many lizards have weak points built into the tail vertebrae and surrounding muscle, and when a predator grabs the tail the lizard contracts muscles that snap it off at one of these planes. The detached tail thrashes for several minutes, drawing the predator's attention while the lizard runs. It does regrow, but the replacement is usually a simple rod of cartilage rather than segmented bone, is often a different colour and texture, and cannot be shed again at the same spot. Losing the tail has real costs, since the tail stores fat, aids balance and climbing, and signals status, so a tailless lizard is at a disadvantage until it regrows.

Yes. The clearest cases are the Gila monster and the beaded lizards of the southwestern United States and Mexico, heavy-bodied desert lizards that chew venom into a bite wound through grooved teeth in the lower jaw; the bite is intensely painful and can be medically serious but is very rarely fatal to a healthy adult. Beyond these, research in the 2000s found that monitor lizards, including the Komodo dragon, and many iguanian lizards produce venom proteins in glands in the jaw, though in most species the effect is mild. This suggests a single early origin of venom in a large branch of the lizard and snake tree, with most lizards simply not developing it further.

Lizards are increasingly used that way. Because they are ectotherms that rely on external heat, their activity, breeding, and even the sex of their offspring in some species are tightly linked to temperature, so they respond quickly and measurably to climate warming, and populations at the warm edge of a species range have been seen to decline or vanish as the animals lose too much of the day to hiding from heat. Lizards are also abundant, easy to survey, and tied to particular habitats, so changes in which species are present and how common they are can track habitat degradation, invasive predators, and pesticide effects on their insect prey.