Autotomy

Abstract: Evolutionary modification has produced a spectrum of animal defence traits to escape predation, including the ability to autotomize body parts to elude capture. After autotomy, the missing part is either replaced through regeneration (for example, in urodeles, lizards, arthropods and crustaceans) or permanently lost (such as in mammals). Although most autotomy involves the loss of appendages (legs, chelipeds, antennae or tails, for example), skin autotomy can occur in certain taxa of scincid and gekkonid lizards. Here we report the first demonstration of skin autotomy in Mammalia (African spiny mice, Acomys). Mechanical testing showed a propensity for skin to tear under very low tension and the absence of a fracture plane. After skin loss, rapid wound contraction was followed by hair follicle regeneration in dorsal skin wounds. Notably, we found that regenerative capacity in Acomys was extended to ear holes, where the mice exhibited complete regeneration of hair follicles, sebaceous glands, dermis and cartilage. Salamanders capable of limb regeneration form a blastema (a mass of lineage-restricted progenitor cells) after limb loss, and our findings suggest that ear tissue regeneration in Acomys may proceed through the assembly of a similar structure. This study underscores the importance of investigating regenerative phenomena outside of conventional model organisms, and suggests that mammals may retain a higher capacity for regeneration than was previously believed. As re-emergent interest in regenerative medicine seeks to isolate molecular pathways controlling tissue regeneration in mammals, Acomys may prove useful in identifying mechanisms to promote regeneration in lieu of fibrosis and scarring.

Abstract: Many organisms have the ability to shed an appendage (autotomy) to escape a predator or fouled molting event. Despite its immediate advantage on survivorship, autotomy can have important consequences for locomotion, foraging, survivorship, and/ or reproduction. Thus, regeneration is a way that animals alleviate some of the costs associated with losing an appendage. Like autotomy, however, appendage regeneration can have important consequences for a variety of aspects of fitness; in a wide range of amphibians, reptiles, fishes, and arthropods, the allocation of resources to regenerate a lost appendage negatively affects somatic or reproductive growth. Previous research into the costs associated with regeneration has provided a strong framework to explore how trade-offs associated with regeneration may have influenced its evolution. However, all research to date describing the costs and benefits associated with autotomy and regeneration have compared individuals autotomizing and regenerating an appendage with individuals that have never lost an appendage. I suggest that for studies of the evolutionary significance of regeneration, an alternative comparison is between individuals experiencing autotomy without regeneration and individuals experiencing autotomy with regeneration. Future work in this direction promises new insights into the evolution of regenerative tendencies, as well as how regeneration may be influencing animal form and function. Key words: autotomy, costs, evolution, regeneration, trade-offs. [Behav Ecol]

Abstract: Autotomy is defined herein as the shedding of a body part, where (1) the loss of the body part is defensive (autotomy helps prevent the whole animal from being compromised and is in response to external stimuli); (2) shearing occurs by an intrinsic mechanism along a breakage plane (there has been selection for certain body parts to be pulled off easily); and (3) the loss is controlled - the animal moves away from the trapped limb, the loss is under some form of central control (neural or hormonal), or the body part is detached quickly. Autotomy (under this defensive definition) has evolved independently for a diverse array of body parts in many taxa; we have summarised available information for over 200 invertebrate species. The advantages of autotomy include escape from entrapment, an effective form of attack, expulsion of an infected body part or in limiting wounding. We discuss how the incidence of autotomy may therefore be correlated with various traits such as limb function, sex differences, other defence mechanisms, habitat disturbance, and sociality. There are also costs associated with autotomy. Short-term costs include loss of a specialised appendage or organ, reduced speed and stability, or even death. Long-term costs include compromised foraging and feeding (often leading to reduced growth), altered anti-predator, competitive or reproductive behaviour, and even defective development. Regenerating lost appendages may also incur significant costs for the individual. We examine the costs and benefits of autotomy, and discuss the evolutionary selective pressures that contribute to the prevalence and effectiveness of autotomy in invertebrates.