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Autotomy (from the Greek auto = "self-" and tomy = "severing") or self amputation is the act whereby an animal severs one of its own appendages, usually as a self-defence mechanism designed to elude a predator's grasp. Geckos, skinks and other lizards that are captured by the tail will shed part of the tail structure and thus be able to flee. The detached tail will continue to wriggle, creating a deceptive sense of continued struggle and attracting the predator's attention away from the fleeing prey animal. The animal can partially regenerate its tail over a period of weeks. The new section will contain cartilage rather than bone and the skin will have different colouration, typically darker and with little or no pattern.
Autotomy in lizards is enabled by special zones of weakness at regular intervals in the vertebrae below the vent. Essentially, the lizard contracts a muscle to fracture the vertebra itself rather than break the tail between two vertebrae. Sphincter muscles in the tail then contract around the caudal artery to minimise bleeding.
Other animals, such as crabs, brittle stars, lobsters and spiders, can also lose and regenerate appendages when necessary for survival. Autotomy occurs in some kinds of octopus for survival and for reproduction: the specialized reproductive arm (the hectocotylus) detaches from the male during mating and remains within the female's mantle cavity.
The evertion of the guts of sea cucumbers when stressed is also a form of autotomy.
See also
"Autotomy"