Autotomy

Vertebrates

Reptiles and amphibians

Some lizards, salamanders, snakes, and tuatara when caught by the tail will shed part of it in attempting to escape. In many species the detached tail will continue to wriggle, creating a deceptive sense of continued struggle, and distracting the predator's attention from the fleeing prey animal. In addition, many species of lizards, such as Plestiodon fasciatus, Cordylosaurus subtessellatus, Holaspis guentheri, Phelsuma barbouri, and Ameiva wetmorei, have elaborately colored blue tails which have been shown to divert predatory attacks toward the tail and away from the body and head. Depending upon the species, the animal may be able to partially regenerate its tail, typically over a period of weeks or months. Though functional, the new tail section often is shorter and will contain cartilage rather than regenerated vertebrae of bone, and in color and texture the skin of the regenerated organ generally differs distinctly from its original appearance. However, some salamanders can regenerate a morphologically complete and identical tail. Some reptiles, such as the western fence lizard, develop split or branched tails after autotomy.

Mechanism

The technical term for this ability to drop the tail is 'caudal autotomy'. In most lizards that sacrifice the tail in this manner, breakage occurs only when the tail is grasped with sufficient force, but some animals, such as some species of geckos, can perform true autotomy, throwing off the tail when sufficiently stressed, such as when attacked by ants.

Caudal autotomy in lizards takes two forms. In the first form, called intervertebral autotomy, the tail breaks between the vertebrae. The second form of caudal autotomy is intravertebral autotomy, in which there are zones of weakness, fracture planes across each vertebra in the mid-part of the tail. In this second type of autotomy the lizard contracts a muscle to fracture a vertebra, rather than break the tail between two vertebrae. Sphincter muscles in the tail then contract around the caudal artery to minimize bleeding. Another adaptation associated with intravertebral autotomy is that skin flaps fold over the wound at the site of autotomy to readily seal the wound, which can minimize infection at the autotomy site. Caudal autotomy is prevalent among lizards; it has been recorded in 13 of approximately 20 families.

Effectiveness and costs

Caudal autotomy is present as an anti-predator tactic but is also present in species that have high rates of intraspecific competition and aggression. The Agama agama lizard fights by using its tail as a whip against other conspecifics. It can autotomize its tail but this is met with a social cost - tail loss decreases social standing and mating ability. For example, Uta stansburiana suffers reduced social status following caudal autotomy, while Iberolacerta monticola experiences reduced mating success. Among Coleonyx brevis, smaller eggs or no eggs at all are produced after the tail is lost. However, the regenerated tail in Agama agama takes on a new club-like shape providing the male with a better fighting weapon, such that autotomy and regeneration work together to increase the lizard's ability to survive and reproduce. There are also examples in which salamanders will attack the tails of conspecifics in order to establish social dominance and decrease the fitness of competitors.

Despite this mechanism's effectiveness, it is costly, and is employed only after other defenses have failed. One cost is to the immune system: tail loss results in a weakened immune system which allows for mites and other harmful organisms to have a larger negative impact on individuals and reduce their health and lifespan. Since the tail plays a significant role in locomotion and energy storage of fat deposits, it is too valuable to be dropped haphazardly. Many species have evolved specific behaviors after autotomy, such as decreased activity, to compensate for negative consequences such as depleted energy resources. Some such lizards, in which the tail is a major storage organ for accumulating reserves, will return to a dropped tail after the threat has passed, and will eat it to recover part of the sacrificed supplies. Conversely, some species have been observed to attack rivals and grab their tails, which they eat after their opponents flee.

There are also adaptations that help mitigate the cost of autotomy, as seen in the highly toxic salamander Bolitoglossa rostrata, in which the individual will delay autotomy until the predator moves its jaws up the tail or holds on for a long time, allowing the salamander to retain its tail when toxicity alone can ward off predators. Regeneration is one of the highest priorities after autotomy, in order to optimize locomotor performance and recoup reproductive fitness. While regenerating their tails, caudal autotomy is restored at an energetic cost that often hinders body growth or intraspecies interactions.

Autotomy in the fossil record

Fossils of reptiles possessing the ability to autotomize that are not within the lizard family have been found that date back to the Late Carboniferous and Early Permian, belonging to the groups Recumbirostra and Captorhinidae. Two squamate species from the Jurassic period, Eichstaettisaurus schroederi and Ardeosaurus digitatellus, were identified as having intervertebral autotomy planes, and these species were placed in the squamate taxonomy as being an ancestor of current existing geckos.

Mammals

At least two species of African spiny mice, Acomys kempi and Acomys percivali, are capable of autotomic release of skin, e.g. upon being captured by a predator. They are the first mammals known to do so. They can completely regenerate the autotomically released or otherwise damaged skin tissue — regrowing hair follicles, skin, sweat glands, fur and cartilage with little or no scarring. These and other species of rodent are also known to exhibit a so-called "false caudal autotomy," whereby the skin on the tail slides off with minimal force, leaving only the bare vertebral structure. Examples of species possessing this ability are cotton rats (Sigmodon hispidus), eastern chipmunks (Tamias striatus), and degu (Octodon degus).

Fish

The giant oarfish shows evidence of self-amputation of the body posterior to the vent. This amputation can either be just involving the caudal fin and a small number of vertebrae, or it may be the entire posterior part of the body. As the organs are concentrated in the front portion of the body, these amputations do not damage any vital organs. These amputations are noted to occur several times throughout the lifetime of the fish (serial autotomy), and all fish over 1.5 m (4.9 ft) long have bodies shortened by this. It is unclear why these amputations occur, as oarfish have no documented natural predators, so it is unlikely to be a predation response. Despite a common misconception that oarfish are preyed on by sharks, no shark attacks on oarfish have been documented. There is one recorded instance of a pod of pilot whales attacking an oarfish, but they did not eat it.