Why Do Animals Use Autotomy? Key Mechanisms and Benefits
The other term for Autotomy is self-amputation, which refers to the ability of certain animals to release a portion of their body that has been grasped by an external agent. A notable example is found in lizards, which break off their tails when they are captured by a predator. The phenomenon has also been observed in worms, salamanders, and spiders. Sometimes the cast-off part is regenerated.
Furthermore, sting autotomy, or the self-amputation of the stinger and its glands, occurs in many social Hymenoptera as a form of colony defence, particularly against vertebrates, and may be due to the size and shape of the sting barbs. Chemical cues released by the detached venom apparatus could help other attacking individuals locate the predator.
Geckos, skinks, lizards, salamanders, and tuatara, for example, can shed their tails when they are caught by a predator. They would be able to flee from their predator and regrow the tail later if they do so. The detached tail would wiggle and is considered to be a protective mechanism in terms of diverting the predator's attention away from the fleeing animal. After successfully evading their predators, some of them, such as certain species of geckos, return to eat their detached tail. Certain species of male octopuses shed their arms, i.e. the hectocotylus, for mating as well as for reproduction purposes.
Tail Autotomy
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As a defensive strategy, most iguanid lizards undergo tail autotomy or the loss of the tail. When a predator grabs the tail, it snaps off and wiggles violently for several minutes. A cartilaginous fracture plane runs through the vertebral body and neural arch of each caudal vertebra. To protect the hemipenes and fat deposits, fracture planes are absent in the cranial part of the tail. In adults, the fracture planes in iguanas are basically replaced by bone as the lizards grow, so the tail breaks off less easily.
When compared to the original tail, the lost tail is supported by a cartilaginous rod and covered with smaller darker scales in an irregular pattern. Tail autotomy is uncommon in agamid lizards, monitors, and true chameleons, and these species do not regenerate tails lost due to trauma or amputation.
If the tail does not break off cleanly during autotomy, the regenerated tail can be deformed, resulting in a forked tail or a blunt knob.
Autotomy in Starfish
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Linckia multifora and Linckia guildingii are two species of starfish found in Hawaii that exhibit autotomy, or the shedding of one or more arms on a regular basis. The arms can also be referred to as "comets'' because they can move independently and each one can develop into a new individual. Despite being disconnected from the nervous system and the water vascular system, they continue to exhibit normal behaviour patterns.
A study conducted in Hawaii discovered that arm detachment was not a sudden event. The majority of fractures occurred about 1 inch (2.5 cm) from the disc and began with a small crack on the lower surface of the arm. This progressed laterally and upwards towards the dorsal surface. The tube feet on the arm as well as those on the body then pulled the two parts of the animal apart until they parted. It is possible that the process will take about an hour to complete. The damaged tissue healed in about ten days, and the animal grew a new arm over several months. Breaks occurred in a variety of locations on the arm, with Crozier noticing a specific breaking zone in Coscinasterias tenuispina. The immediate cause of autotomy is not always obvious.
Lizard Tail Autotomy
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Let's talk about lizard autotomy! A few lizards from various families have thick tails covered in large, hard, spiny scales. A tail that swings vigorously from side to side is an effective defence against snakes, especially if the lizard's head and body are in a burrow or wedged between rocks.
In another way, the tails of some lizard species are useful in defence. When captured, some lizards voluntarily shed their tails, which wiggle violently, temporarily confusing the predator and allowing the lizard to escape. Each vertebra in the tails of tail-shedding lizards has a fracture plane that can be split voluntarily by twitching the tail muscle.
Simultaneous nerve stimulation in the severed portion causes it to twitch for a few seconds after separation. The tail is usually broken in one place, but there are some lizards, particularly the so-called glass snakes (Ophisaurus), break their tails into several pieces. The stump heals quickly, and a new tail grows; however, the regenerated tail is often shorter and has simpler scales than the original.
Autotomy in Echinoderms
This article lists the structures that are autonomous in each echinoderm class, assesses the extent to which autotomy precedes regeneration in natural populations, reviews current knowledge of the morphology of autotomy planes and mechanisms that affect fracture at autotomy, and comments on autotomy-related issues raised by studies of the cellular events of regeneration.
Each autotomy plane can be thought of as a collection of breakage zones that run through the individual anatomical components of the autonomous structure. Some breakage zones in any one autotomy plane are permanent sites of weakness that are fractured by external forces, while others are potential sites of weakness that lose tensile strength only at the time of autotomy. The latter occurs primarily in mutable collagenous structures, though there are a few examples of endogenous rupturing in muscles.
FAQs on Autotomy Explained in Biology
1. What is autotomy in biology?
Autotomy, also known as self-amputation, is a defence mechanism where an animal voluntarily sheds or discards a part of its body, such as a limb or a tail, to escape from a predator's grasp. The detached part often continues to move, distracting the predator and allowing the animal to flee and survive.
2. What are some common examples of autotomy in the animal kingdom?
Autotomy is observed in various animal groups. Some common examples include:
- Lizards: Many species can detach their tails when caught, which later regenerates.
- Starfish (Echinoderms): They can shed one or more arms if they are trapped or attacked.
- Crabs and Lobsters: These crustaceans can drop a claw or leg to escape a fight or a predator.
- Insects: Some insects, like grasshoppers and stick insects, can shed a leg if it is held.
- Spiders: Certain spiders will detach a leg to get away from a threat.
3. How does the process of autotomy work at a biological level?
Autotomy is a highly controlled process that occurs at specific weak points called 'fracture planes'. In a lizard's tail, for instance, these planes exist within the vertebrae. When threatened, the lizard contracts specific muscles to break the vertebra at one of these planes. Simultaneously, sphincter muscles around the arteries and veins constrict to minimise bleeding and tissue damage, facilitating a clean break.
4. Is it painful for an animal like a lizard to lose its tail?
While animals that perform autotomy have the neurological capacity to feel pain, the process is adapted to be a quick, life-saving reflex rather than a painful injury. The detachment occurs at pre-determined weak points, and the entire event is a rapid, instinctual response to extreme stress. The immediate survival advantage likely overrides any sensation of pain during the event.
5. What is the difference between autotomy and a regular injury?
The key difference lies in control and adaptation. Autotomy is a voluntary, controlled process that happens at a specific, evolved fracture point to minimise harm. An injury, on the other hand, is an uncontrolled, accidental event that can occur anywhere on the body and often results in significant damage and blood loss. Autotomy is a defensive strategy, whereas an injury is simply damage.
6. Are there any disadvantages or costs for an animal after performing autotomy?
Yes, autotomy comes with significant biological costs. The primary disadvantage is the high energy expenditure required to regenerate the lost body part. Additionally, losing a part can lead to:
- Loss of important energy reserves (e.g., fat stored in a lizard's tail).
- Impaired mobility, balance, or ability to climb.
- Reduced social status or attractiveness to mates.
- Temporary vulnerability until the part is regrown.
7. Why can't humans and other mammals perform autotomy?
Mammalian limbs are far more complex than the appendages of animals that use autotomy. Our limbs contain bone marrow for blood cell production, intricate networks of nerves and blood vessels, and are not designed with 'fracture planes'. The loss of a limb would be catastrophic due to uncontrollable bleeding and a high risk of infection. Instead of limb-shedding, mammals have evolved other defence mechanisms and advanced wound healing and immune systems to deal with injuries.
