Tail Instantly Breaks Off

blue and orange lizard on green tree trunk

Biological Strategy

Tokay gecko

Jeanette Lim

Image: Tontan Travel / Flickr / Some rights reserved

Modify Size/Shape/Mass/Volume

Many living systems alter their physical properties, such as size, shape, mass, or volume. These modifications occur in response to the living system’s needs and/or changing environmental conditions. For example, they may do this to move more efficiently, escape predators, recover from damage, or for many other reasons. These modifications require appropriate response rates and levels. Modifying any of these properties requires materials to enable such changes, cues to make the changes, and mechanisms to control them. An example is the porcupine fish, which protects itself from predators by taking sips of water or air to inflate its body and to erect spines embedded in its skin.

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Protect From Animals

Animals–organisms that range from microscopic to larger than a bus–embody a wide variety of harms to living systems, including other animals. They threaten through predation, herbivory, defense, and parasitism, and they compete for resources such as water, nutrients, and space. Any given living organism commonly faces threats from a variety of animals, requiring strategies that effectively defend from each. Trout and other bony fish, for example, escape predators by having scales made of very thin, flake-like pieces of bone covered with slippery mucus. They also have behavioral strategies such as camouflage, fast swimming, and twisting and turning to achieve release from a predator’s grip.

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Expel Solids

Living systems must often discharge solids–such as seeds, eggs, pollen, and mineral salts–for reproduction, protection, or other reasons. Solids can be easily moved when a living system applies force to them. But creating that force requires energy, so living systems must either have efficient strategies worth the energy investment or use an outside force (such as gravity). For example, a tropical mistletoe positions its pollen so that the weight of a bird landing nearby triggers the pollen’s release onto the bird’s forehead. The bird then fertilizes other mistletoes that it visits.

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Reptiles

Class Reptilia (“creeping”): Lizards, crocodiles, turtles, snakes

Reptiles retain some of the key characteristics that first enabled vertebrates to live permanently on land. They have dry skin covered in scales made of keratin that help prevent water loss. (Amphibians still need to stick close to water to keep their skin moist.) Many reptiles reproduce by laying eggs that are watertight and have a yolk, so eggs can develop on land while staying moist and nourished. Reptiles are also ectotherms, meaning they don’t produce their own body heat. This means they burn through energy much more slowly than “warm blooded” creatures of the same size. It also means that they need to keep warm to keep active, which is why you might see them spending a seemingly inordinate amount of time simply basking in the sun.

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The tail of the Tokay gecko instantly breaks off during a predator attack with the help of pre-formed lines of weakness.

Geckos are small lizards that can escape an attacking predator using an unusual strategy—by instantly losing their tails. This process of actively shedding a whole body part is called autotomy. When a predator grasps onto a gecko, releasing the tail can help the gecko wriggle free and escape while the attacker is holding onto a severed tail or distracted by it.

In the Tokay gecko (Gekko gecko), structures in the tail appear to help the shedding process. First, the base of the tail has built-in lines of weakness going across it, similar to perforated lines or score lines that make pieces of paper easier to tear apart. These lines of weakness, or fracture planes, cross the tail’s skin, muscles, bones, and other tissues. Like many animals, the gecko’s muscles form segments spanning the length of the body and tail. Sheets of connective tissue separate neighboring segments. The fracture planes run through the connective tissue between muscle segments and continue through the bony vertebrae that make up the backbone in the tail.

When not under threat, the gecko’s tail is likely held in place by adhesion between the two sides of a fracture plane. This adhesion may be enhanced by the shape and arrangement of the muscle segments. Each segment can be thought of as a sideways “W” that interlocks with neighboring W-shaped segments. Compared to simple flat surfaces, the W-shaped structures have more surface area for adhesion. Micro-sized structures on the tips of individual muscle fibers also appear to play a role in tail adhesion and release. Researchers hypothesize that the shape of muscle fiber tips at the fracture plane can change to reduce adhesion during autotomy, making the tail easier to release. Contracting muscles around the fracture plane are also likely to help break tissues and release the gecko’s tail. Adhesion in this system appears to be a balance between enabling easy tail release when it’s needed, but preventing accidental release when it’s not.

Autotomy occurs in many other animals, including other lizards, as well as amphibians and sea stars. Many of these animals can also regenerate their lost body parts over time.

Image: Sanggaard et al. 2012 / CC BY - Creative Commons Attribution alone

A) Blue stain: connective tissues and scales; red stain: muscles; white: adipose tissues. B) Close up of the fracture plane in the skin. C) Electron microscope image showing cells that might be part of a “cellular zipper” in the fracture plane. © 2012 Sanggaard et al. Unique Structural Features Facilitate Lizard Tail Autotomy. PLoS ONE 7(12): e51803. http://dx.doi.org/10.1371/journal.pone.0051803

Image: Sanggaard et al. 2012 / CC BY - Creative Commons Attribution alone

Scanning electronic microscope image of the W-shaped muscles extending from the gecko tail stump. The tips of individual muscle fibers are visible on most sides of the muscle. © 2012 Sanggaard et al. Unique Structural Features Facilitate Lizard Tail Autotomy. PLoS ONE 7(12): e51803. http://dx.doi.org/10.1371/journal.pone.0051803

Image: Sanggaard et al. 2012 / CC BY - Creative Commons Attribution alone

A and B) Parts of the W-shaped muscles extend from the tail stump. C) The exposed end of the gecko where the tail once attached shows pockets where extensions from the W-shaped muscles fit. © 2012 Sanggaard et al. Unique Structural Features Facilitate Lizard Tail Autotomy. PLoS ONE 7(12): e51803. http://dx.doi.org/10.1371/journal.pone.0051803

Image: Richard Ling / CC BY SA - Creative Commons Attribution + ShareAlike

Last Updated April 20, 2018

References

“…[gecko] caudal autotomy relied on biological adhesion facilitated by surface microstructures. Results based on bio-imaging techniques demonstrated that the tail of Gekko gecko was pre-severed at distinct sites and that its structural integrity depended on the adhesion between these segments.” (Sanggaard et al. 2012:1)

“Our data suggest that caudal autotomy in lizards is a biological friction- and adhesion-based phenomenon. The tail contains “score lines” at distinct horizontal fracture planes where the tail may be released as a response to predation. These scores penetrate all the way through the tissue where the structural integrity is maintained by adhesion forces. The interdigitation arrangement of muscles is likely to facilitate adhesion by generating a larger surface area of interaction. This architecture increases the contact area and most likely decreases the risk of accidental tail loss as opposed to a simple “end-to-end” arrangement. “Mushroom-shaped” microstructures at the muscle termini are observed after autotomy. These structures are likely to facilitate tail release by reducing the adhesion between tail segments.” (Sanggaard et al. 2012:6-7)

Journal article

Unique Structural Features Facilitate Lizard Tail Autotomy

PLoS ONE, 7(12): e51803 | 19/12/2012 | Sanggaard KW; Danielsen CC; Wogensen L; Vinding MS; Rydtoft LM; Mortensen MB; Karring H; Nielsen NC; Wang T; Thøgersen IB; Enghild JJ

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