Autotomy
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Basic Information

Autotomy is the behavior and biological process by which some animals break off their own limbs, such as when an iguana drops its tail to distract a predator. In some species this is an intentional action that can be done entirely autonomously, while in other species the limbs are just grown in such a way that they break off easily if much external force is applied.

Many species are able to regrow the lost body parts, but in some species (or individual situations) the regrown parts may be visually different or significantly inferior. For example, some species of lizard are born with a bony tail, but if they shed it they can only regrow a shorter version with a core of cartilage instead of bone. For many species, dropping a limb is a final recourse reserved for the most dire of situations. Doing so may have high costs in terms of blood loss, a weakening of the immune system, or even social costs (reduced chance of mating, being marked as weak, etc) associated with now looking different from others of their kind. In some cases, the act of autotomy may actually cause the death of the creature in some or even all situations. It still might be a useful evolutionary feature if it has even a chance of sparing them, or if it protects the community instead of the individual.

Examples:
Lizard: As mentioned, several species of reptile can engage in Caudal Autotomy, the breaking off of their own tails, and some of these species can regrow their tail after doing so. In some species, the tail is a more vibrant color than the rest of the body, which may make predators more likely to strike the "disposable" limb instead of a more vital bit of anatomy. In some species, the severed tail continues to twitch and thrash for some time after removal. This may distract the predator, fooling them into struggling with the removed bits instead of chasing the main body of the now-escaping prey. In some situations, it may even cause a predator to assume that they had mis-judged the target entirely, and retreat themselves because they interpret the flopping tail to be a new-arrived snake that is a dangerous rival rather than something they can eat.

Amphibean: Several species of amphibean, such as some breeds of salamander, engage in Caudal Autotomy as well, and are usually quite skilled and speedy in terms of regrowing the dropped tail. Some species of salamander can regenerate not just their tails, but legs, nerves, eyes, and even portions of their brain if it gets damaged.

Crustacean: There are several species of crab that can drop legs or claws, either during a struggle or in response to environmental stimuli.

Echinoderm: Some types of Starfish can self-amputate limbs defensively, and in some the lost limb can actually grow back an entire body, effectively being a form of asexual reproduction. Many species of Sea Cucumber eviscerate themselves, ejecting their own stomach or other organs to frighten or distract predators. Though that sounds horrible, they can usually regrow the missing organs within a few days.

Insect: Some species of Bee or Wasp have a barbed stinger that breaks off via autotomic process, if the victim's skin is thick enough for the pointy bits to get lodged within them. Noteworthy for gaming, this means that if your honeybee is a giant specimen, or the victim was under the effects of a shrink ray, the bee might be able to sting repeatedly instead of the traditional one time. Ouch. Also, in some species, the male reproductive organs break off during sex, and become a plug that prevents the female from mating with other males later.

Mammal: The only examples in mammals are a few species of mouse that can shed patches of skin to escape the jaws of a predator. If they escape, they are able to regrow skin, sweat glands, hair and all. These African spiny mice also feature guard hair that is stiff and pointy similar to a hedgehog, so not only do they shed the skin, but they also leave the predators with a prickly painful mess to deal with.

Mollusc: Several varieties of slug and snail can detach parts of their tail. Many varieties of seaslug or nudibranch can self-amputate a portion of their tail, cerata, or mantle skirt. Many species of octopus sever the male's reproductive organs for entirely different (non-defensive) reasons: so the female can take it with them.

Spider: Some orb-weavers detach legs when those legs have been stung by bee or wasp attacks, which has the effect of preventing the venom from entering the rest of their body.

Sources

Game and Story Use

  • Reptilian Humanoids, Dragons, or other lizard creatures might have a detachable tail, and this could lead to some very memorable moments in gaming.
    • They might drop it when fleeing, or the self-amputation might be an uncontrolled response that happens during a surprise round or minor morale failure that might not end the fight.
      • Maybe this species of reptillian rarely flees anymore, but they've maintained their ancestral habit of shedding their tails in combat.
      • PCs are almost certainly going to ignore a dropped tail, unless you make it dangerous in some way, such as if it has an envenomed stinger. You might allow the creature to make some sort of deception check or treat the tail like an illusion spell that fools the PCs into targeting it for a turn or two.
    • Such a creature that's already missing a tail, or had one grow back stunted or discolored, would be an interesting way to visually distinguish between characters or monsters. "Do you attack the one with the stubby white tail, or the one with the long blue tail?"
      • It could also mean several possible conflicting things about the creature:
        • it may indicate that they are a coward who drops their tail at the first sign of danger,
        • or that they are a tough bruiser who gets into fights all the time (and only loses their tail in some percentage of those struggles),
        • or that they were in a fight just a few days ago and aren't yet at full health.
    • Some species of lizard cannot regrow bones in their tails, and so may have a replacement tail made of cartilage instead.
      • This could be the chink in their armor. That said they obviously evolved to usually survive losing a tail, so that implies a limit to how much damage you can do to that body part. You could model this by having a lower Armor Class on the tail, but making hits there either do half damage or have a damage cap (something like, "No more than 15 damage total can affect the tail. Any damage beyond this limit is ignored as the attack destroys the tail, making it no longer a viable target/weakness.")
        • If the players know these limits, it might be a way of making an encounter that's tailored to the mix of power-levels for the part. The big fighter will have the best odds of landing a fatal body blow, but the other PCs still have a good chance of hitting the tail and whittling it's health down by a third.
  • Autotomized parts that remain dangerous are definitely an interesting notion.
    • Stingers that continue to inject further doses of venom over time. You have to dig it out quickly, before the lethal dose is reached!
      • Something like a cone shell's dart could quite easily autotomize after hitting, allowing the creature to deploy another striker (although presumably only if under verifiable attack from multiple opponents, otherwise it would simply hit the same opponent repeatedly and waste "ammunition").
    • Claws, pincers, pedipalps or mandibles that disconnect after striking back at a foe, and maybe lock-on and continue to squeeze the grappled foe, while the main body runs away. Again, damage-over-time for the target until you make a strength check to break free of the severed parts that have you in a choke-hold.
    • Aggressive creatures that vomit up their own inverted stomach to digest prey outside their body, and then later slurp up the pool of nutrients left behind. If the point is to make them extra gross and weird, they may even be able to reabsorb their discarded stomach in some way.
  • Creatures that live in a hive, colony or full-on community might well have autotomizing behavior that kills the individual, but collectively protects the hive, swarm, nest or town.
    • I guess you could argue the acidic blood of the xenomorph in Aliens is sort of along this line. It is of minimal help to the individual monster when they are wounded, but greatly complicates efforts by intelligent prey to burn out the infestation in retaliation, or to turtle-up effectively in a fortified position.
  • Similar powers may be possessed by undead, who may be able to separate their zombie heads, crawling hand, etc.
  • Looking for a way to spice up your Deep Ones, or other Cthulhu Mythos entities that have grown a little stale and lost their shock value? Have them literally tear themselves apart in their attempts to hunt the PCs. It will definitely make them seem more alien and inhuman, that's for sure!
  • Lots of "fun" overlap is potentially possible with body horror, alien hand syndrome, doppelgaenger, starfish aliens, etc, if, like the thing in The Thing, a monster can shed body-parts and have them become autonomous agents of their own.
  • Combined with regeneration or some other inherent healing factor this becomes an option for all sorts of things - the standard fRPG troll for example, can well afford to sacrifice a limb or two so that it can escape with its life.
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