Autotomy literally translates as “self-cutting” and refers to the ability of certain animals to actively remove (shed) a part of the body as part of a defensive response.

The strictest definition of autotomy has three criteria:

1. Removing the body part has the potential to improve an animal’s survival: Autotomy is a defensive behavior

2. The body part is removed along a specially adapted breakage plane: Autotomy happens in a specific location

3. Removal is mediated by a central system: Autotomy is controlled by nerves or hormones

Tinbergen’s 4 questions take an integrated approach to behavior by looking at behavior on multiple levels, integrating both proximate (immediate) and ultimate time scales and studying both "snapshots" (instantaneous) and "stories" (taking place over a period of time) of behavior. Tinbergen's four questions can be broken down into mechanism (causation), adaptive value (long-term function), ontogeny (development), and phylogeny (evolution). By examining these four areas, one can gain a better understanding the behavior as a whole.

In all cases, autotomy is a mechanism that is used for self-preservation. Under the pressure of a predator, an organism can “drop” a limb, allowing the animal to escape from a predator’s grasp and providing a distraction. However, if the animal manages to get away, it is left in a vulnerable state as it regrows the lost body part. Since there are such high costs to autotomy, the behavior exists in species for which it is a highly effective escape strategy and could make a difference between the life and death of the organism. (Read more about Adaptive value.)

Among species with an autotomous ancestor, autotomy is lost in those species that have a more effective and less costly escape mechanism. Autotomy is found in a wide range of animals, from lizards and salamanders to crabs, insects, and sea slugs. It appears to have evolved repeatedly, especially in invertebrates. (Read more about Phylogeny.)

The mechanisms of autotomy are different in different animals, but there are a few basic “rules” of autotomy mechanisms that all autotomous animals share. First, autotomy occurs along a specialized breakage plane, sometimes known as a fracture plane. Second, autotomy is controlled by a central system - usually either the nervous system or the endocrine system. (Read more about Mechanism.)

Not all autotomous animals are born with the ability; in some species, the ability to autotomize develops as the individual grows. The mechanisms of autotomy also develop with maturation. In other species, though, we see the opposite pattern: individuals lose the ability to autotomize as they mature. (Read more about Ontogeny.)

Check out this cool video! Hank Green did a lesson on his YouTube channel, SciShow, about Regeneration and Autotomy.