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.

Leave it all behind: a taxonomic perspective of autotomy in invertebrates.

Ecology and evolution unfold in spatially structured communities, where dispersal links dynamics across scales. Because dispersal is multicausal, identifying general drivers remains challenging. In a coordinated distributed experiment spanning organisms from protozoa to vertebrates, we tested whether two fundamental determinants of local dynamics, top-down and bottom-up control, generally explain active dispersal. We show that both factors consistently increased emigration rates and use metacommunity modelling to highlight consequences on local and regional dynamics.

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Bottom-up and top-down control of dispersal across major organismal groups

This is the first study to examine the direct metabolic costs of autotomy, the voluntary shedding of an appendage as an escape mechanism, in invertebrates. We investigated the effects of limb autotomy upon endurance and metabolic cost of locomotion in the field cricket Gryllus bimaculatus. Compared with control (intact) crickets, animals that had autotomised a single hindlimb were slower, stopped more often, moved a shorter distance and expended more energy doing so. Both the cost of locomotion (COT) and minimal cost of locomotion (MCOT) were significantly higher for autotomised animals. We compare these data with locomotion energetics of 36 other invertebrate species, and discuss the results in terms of the biomechanics of walking in crickets.

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Just drop it and run: the effect of limb autotomy on running distance and locomotion energetics of field crickets (Gryllus bimaculatus).

When attacked, crickets may shed or 'autotomize' an entrapped limb in order to escape a would-be predator. We examined the relationship between limb autotomy, running speed and susceptibility to future predation in house crickets (Acheta domestica). Hind limb autotomy resulted in a significant reduction in escape speed and ability to jump during the escape run, and greater predation by both lizards (striped skink Mabuya striata punctatissima) and mice (pouched mouse Saccostomus campestris). Although limb autotomy may enable a house cricket to escape a predatory encounter, autotomy of even one hind limb results in immediate costs to escape speed in crickets and makes the animal more vulnerable to subsequent predator encounters.

Increased susceptibility to predation for autotomized house crickets (Acheta domestica)

Current optimal escape theory focuses on economic distance-based models that predict that animals will flee at greater distances when risk of capture is greater. Although these models have been tested extensively on vertebrate prey animals using large approaching stimuli (e.g., humans), it has never been tested on an invertebrate generalist predator with a stimulus that is in the size range of potential prey. I presented adult jumping spiders, Phidippus princeps, with a small black model to test flight decisions when physically handicapped, under different levels of threat, and on surfaces that potentially hindered escape. Predator approach speed, running surface texture, and leg autotomy had no effect on flight decisions. I also measured running distance and speed under different levels of hunger and energy state to test how these variables affect the decision to flee or to turn and defend oneself against a predator. When prodded, larger spiders fled shorter distances before switching to a defensive posture, hungry spiders fled longer distances than sated spiders, and rested spiders ran faster than tired spiders. There is likely a trade-off between body size and energy stores when deciding to flee from a threat or turning to defend oneself. These findings 1) reflect differences in how predators and prey assess risk in their environment and the distances at which they treat an approaching object as threatening and 2) suggest that future studies should focus on how animals optimize escape decisions in ways other than traditional distance-based economic models (e.g., relying absolutely on crypsis). Copyright 2009, Oxford University Press.

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When predators become prey: flight decisions in jumping spiders

Leg autotomy in spiders is a relatively common occurrence, usually resulting from agonistic interactions with predators or conspecifics. While autotomy has immediate benefits, due to enhanced survival probability, it also potentially decreases future fitness. One possible cost of losing a leg is a reduction in burst running speed, which may affect prey capture and predator avoidance behaviors in wandering spiders. We examined sprint speed in intact and autotomized males and females of the wolf spider Pirata sedentarius from two stream banks in New York in an effort to determine if the sexes differed in either sprint speed or the potential cost of leg loss. Autotomy was fairly common in the field, with 18–22% of spiders missing at least one leg at our two sites. Males and females did not differ in frequency of leg loss. Females were larger than males (both in terms of mass and structural size) and ran significantly faster both before and after autotomy. Sprint speed was uncorrelated with body size...

Pasha Apontes

Papers

Between-sex Variation in Running Speed and a Potential Cost of Leg Autotomy in the Wolf Spider Pirata sedentarius