Caudal autotomy is a defensive adaptation observed in many lizards and some salamanders (there are some scattered reports of this behaviour being observed in certain snake species). The vertebrae at the joint between the tail and the pelvis are weakened and easily broken, so when the animal is threatened by the predator or seized by the tail, the tail breaks off. The actual loss of the tail occurs as the animal tightly contracts the muscles at the posterior of the pelvis, breaking the vertebrae and causing the tail to fall off. The fact that the tail is lost through muscular contraction rather than an external force (ie., being torn off by the predator) has the added benefit of reducing blood loss, as the muscle contraction closes the major arteries and veins running from the trunk of the body to the tail.

Caudal autotomy is a particularly effective antipredator behaviour since the predator is often left with the wriggling tail as a meal while the animal makes its escape. The dropped tail may continue to move and writhe for up to ten minutes after separation, making it a particularly interesting prey item to the predator.

Losing one's tail is not an insignificant experience for the animal in question. While the adaptation may allow the animal to escape, the risk of infection is significant since the wound left can be of substantial size. Most species will regenerate the tail in relatively short order, but these regenerated tails often have a deformed appearance. This regeneration is costly, however. While growing back its tail, the animal ceases growth and, during the reproductive season, gamete production.

As a final note, some species of herpetile do not drop just their tails, but also their digits or limbs.

God help me, but I can't stop giggling over the idea of a three-legged, twelve-toed tailless lizard telling his buddy: "You know, this voluntary leprosy really is something, isn't it!"