Autotomy and Regeneration
Hulali Alford & Rose Driscoll
Reed College Biology 342 Fall 2015
How does the behavior benefit the animal, in terms of lifetime reproductive fitness (LRF)? Is there a tradeoff in the costs and benefits of the behavior?
Autotomy is, obviously, adaptive in that it allows the animal to survive an encounter with a predator. Autotomy can also be adaptive in scenarios of conspecific aggression or entrapment. However, autotomy can also carry costs in the form of a decrease in motor or feeding ability, the loss of resources stored in the autotomized body part, and/or the energetic cost of re-growing the body part (if applicable). Thus, autotomy exists in species where the benefits outweigh the costs and where there are few effective alternative strategies.
In lizards, caudal (tail) autotomy can have two main benefits: autotomy allows the lizard to escape from a predator that holds it by the tail, and the detached tail serves as a distraction for the predator, giving the animal more time to get away. The former, escape, is most beneficial when there is high predation (Fox et al 1994) and when the lizard is fast enough to effectively elude the predator. In the case of using a detached tail as a distraction, animals may enhance the benefit from autotomy by drawing the predator’s attention to the tail before it is detached behaviorally (by waving it, for example) and/or physiologically (by having a tail that is visually salient); also, the detached tail may move independently in ways that resemble the pre-autotomy movement. Distraction techniques are most beneficial when the tail is acceptable as a food item to the predator, as this improves the likelihood that the predator will not risk chasing and losing the lizard and will simply eat the tail instead. This strategy frequently coincides with shedding of the entire tail, rather than just the smallest possible section (see Mechanism) (Arnold 1984).
1. Autotomy of chelipeds (large front limbs) in porcelain crabs is adaptive because it allows the animal to escape from a predator. Porcelain crabs have relatively large chelipeds (twice as long as the crab’s body) which are used to divert the predator’s attention. Since autotomy occurs with very low stimulation threshold, for these species the benefits are predicted to well outweigh the costs, perhaps because captured individuals have little chance of escape via alternate means. Also, the costs of autotomy with respect to food acquisition are low in this species as the chelipeds are not used in feeding (Wasson et al 2002).
Figure 1. Autotomy in porcelain crabs: when grasped by the chelliped (large front claw) by a much larger predator crab, the porcelain crab detaches its claw and is able to escape. (From Wasson et al, 2002.)
2. Fox et al (1994) found that species under high predation autotomize more readily (i.e. under relatively less pressure, controlling for tail thickness) than related species experiencing low predation. This supports an adaptive role for autotomy in predator escape.
Figure 2. Species under high predation, such as S. gadovae (f) and S. spinosus (e), tend to lose their tails much more readily (i.e., have shallower lines) than their low-predation relatives – S. aenaeus (a) and S. grammicus / S. torquatus (b, d) respectively. Observe the different scales of the graphs when making comparisons: for example, the line for S. gadovae (f) appears much steeper than that for S. aenaeus (a), but while S. gadovae (f) has a slope of ~2/6 ≈ ?, S. aenaeus (a) has a slope of ~5/20 ≈ ¼. Four points (at the lower right corner) were excluded from the regression for S. gadovae (f) as these individuals had previously experienced autotomy and evidence suggests that second autotomies tend to occur much more easily than first autotomies in this species. (From Fox et al, 1994.)
3. P. marmoratus geckos with tails escape predators (such as the marsupial A. swainsonii) more frequently than geckos without tails due to the former’s ability to perform autotomy. Predators are distracted by the autotomized tail and chase that instead of the gecko itself (Daniels 1986).
4. Across invertebrates, the benefits of autotomy can include: escape from predators, competitive conspecifics, and traps such as commercial nets; distraction of a predator; the removal of a diseased or injured appendage; and even attack (Fleming et al 2007), as in the case of the squid O. deletron which practices attack autotomy by grasping an attacker with one tentacle before autotomizing the limb and swimming away. The detached limb keeps holding on to the predator and flashes, allowing the squid to also benefit from the predator distraction aspect of autotomy (Bush 2012).
5. The squid O. deletron minimizes the costs of autotomy through the practice of “economy of autotomy”: arms are autotomized just slightly above where the arm is held (or is holding), minimizing the amount of tissue lost (Bush 2012).
Figure 3. Observed number of arm injuries resulting from autotomy in a sample of O. deletron. Smaller missing sections of the arm were observed more frequently, suggesting that these squid autotomize the minimum amount of arm necessary.
6. Autotomy is an adaptive defense mechanism for the spider Kukulcania hibernalis against some but not all predators. Different predators use different attack techniques; while scorpions grasp a spider with their pedipalps and then paralyze the spider with the tail stinger, centipedes carry their venom in their claws and thus do not allow the spider time to effectively escape through autotomy. With both types of predator, venom inhibits the autotomy reflex (Formanowicz 1990).
Table 1. Use of autotomy by Kukulcania hibernalis spiders against scorpion and centipede predators. Autotomy is an effective defensive mechanism against scorpions, but not against centipedes. (From Formanowicz, 1990.)