Ontogeny and caudal autotomy fracture planes in a large scincid lizard, Egernia kingii

Many lizard species use caudal autotomy, the ability to self-amputate a portion of the tail, as an effective but costly survival strategy. However, as a lizard grows, its increased size may reduce predation risk allowing for less costly strategies (e.g., biting and clawing) to be used as the primary defence. The King's skink (Egernia kingii) is a large scincid up to approximately 244 mm snout to vent length (SVL) in size when adult. Adults rely less on caudal autotomy than do juveniles due to their size and strength increase during maturation. It has been hypothesised that lower behavioural reliance on autotomy in adults is reflected in loss or restriction of caudal vertebrae fracture planes through ossification as caudal intra-vertebral fracture planes in some species ossify during ontogenetic growth. To test this, we used micro-CT to image the tails of a growth series of seven individuals of E. kingii. We show that fracture planes are not lost or restricted ontogenetically within E. kingii, with adults retaining between 39-44 autotomisable vertebrae following 5-6 non-autotomisable vertebrae. Even though mature E. kingii rely less on caudal autotomy than do juveniles, this research shows that they retain the maximum ability to autotomise their tails, providing a last resort option to avoid threats. The potential costs associated with retaining caudal autotomy are most likely mitigated through neurological control of autotomy and E. kingii's longevity.

At What Cost? Trade-Offs and Influences on Energetic Investment in Tail Regeneration in Lizards Following Autotomy

Caudal autotomy, the ability to shed a portion of the tail, is a widespread defence strategy among lizards. Following caudal autotomy, and during regeneration, lizards face both short- and long-term costs associated with the physical loss of the tail and the energy required for regeneration. As such, the speed at which the individual regenerates its tail (regeneration rate) should reflect the fitness priorities of the individual. However, multiple factors influence the regeneration rate in lizards, making inter-specific comparisons difficult and hindering broader scale investigations. We review regeneration rates for lizards and tuatara from the published literature, discuss how species' fitness priorities and regeneration rates are influenced by specific, life history and environmental factors, and provide recommendations for future research. Regeneration rates varied extensively (0-4.3 mm/day) across the 56 species from 14 family groups. Species-specific factors, influencing regeneration rates, varied based on the type of fracture plane, age, sex, reproductive season, and longevity. Environmental factors including temperature, photoperiod, nutrition, and stress also affected regeneration rates, as did the method of autotomy induction, and the position of the tail also influenced regeneration rates for lizards. Additionally, regeneration could alter an individual's behaviour, growth, and reproductive output, but this varied depending on the species.

The Review of the Autotomy of Agamid Lizards with Considerations about the Types of Autotomy and Regeneration

We present a review of the data on the intervertebral autotomy and regeneration of agamid lizards based on an analysis of information obtained over a 35-year period after the publication of thorough reviews (Arnold, 1984, 1988 and Bellairs, Bryant, 1985). It is supplemented by our own studies of 869 specimens of agamid lizards (Sauria, Agamidae) stored in the herpetological collections of the Zoological Institute of the Russian Academy of Sciences (St. Petersburg, Russia) and the Zoological Museum of the Moscow State University (Moscow, Russia), represented by 31 species of 16 genera. The manifestations of the ability for autotomy and regeneration in phylogenetic lineages within the family—Leiolepidinae, Amphibolurinae, Agaminae, Draconinae—are considered. A comparative morphological analysis of the structure of the caudal vertebrae was carried out using the Computer Microtomography Methods (micro-CT) in the following ecomorphological types of agama: (1) with developed abilities to caudal autotomy and regeneration, (2) with the ability to caudal autotomy but without regeneration and (3) without the ability to autotomy. The phenomenon of intervertebral autotomy (urotomy) in snakes is considered too. Possible ways of evolution of the ability to caudal autotomy as a defense strategy against predators are discussed in the phylogenetic context.

Role of coloration in antipredator strategies of Pristidactylus achalensis (Squamata: Leiosauridae) related to sex and stages of predation

The interaction between behavioural and phenotypic traits, such as coloration, plays a specific role at different stages of the predatory sequence. Cryptic coloration involves a match to the background that reduces the risk of detection, and it is usually optimized by immobility behaviour. In lizards, sexual dichromatism and within-individual changes of colour can influence the level of their crypsis and thus influence antipredator behaviour and the decision to flee. Here, we examined variation in coloration and antipredator behaviour between the sexes of the Achala copper lizard (Pristidactylus achalensis). We measured sexual dichromatism and crypsis in accordance with avian vision. We also exposed lizards to a raptor dummy to assess the colour change and behaviour at the detection and approach stages of the predatory event. Finally, we tested escape behaviour in the natural habitat. We found that males were more conspicuous than females. Additionally, individuals showed distinct behaviours at different stages of the predation simulation, but there were no differences between the sexes. However, in their natural habitat males initiated flight at greater distances than females, which might be related to their more conspicuous coloration. In summary, this study shows how differences in crypsis between the sexes affect escape behaviour during the approach stage of predation.

Tail autotomy works as a pre-capture defense by deflecting attacks

Caudal autotomy is a dramatic antipredator adaptation where prey shed their tail in order to escape capture by a predator. The mechanism underlying the effectiveness of caudal autotomy as a pre-capture defense has not been thoroughly investigated. We tested two nonexclusive hypotheses, that caudal autotomy works by providing the predator with a "consolation prize" that makes it break off the hunt to consume the shed tail, and the deflection hypothesis, where the autotomy event directs predator attacks to the autotomized tail enabling prey escape. Our experiment utilized domestic dogs Canis familiaris as model predator engaged to chase a snake-like stimulus with a detachable tail. The tail was manipulated to vary in length (long versus short) and conspicuousness (green versus blue), with the prediction that dog attacks on the tail should increase with length under the consolation-prize hypothesis and conspicuous color under the deflection hypothesis. The tail was attacked on 35% of trials, supporting the potential for pre-capture autotomy to offer antipredator benefits. Dogs were attracted to the tail when it was conspicuously colored, but not when it was longer. This supports the idea that deflection of predator attacks through visual effects is the prime antipredator mechanism underlying the effectiveness of caudal autotomy as opposed to provision of a consolation prize meal.

When one tail isn't enough: abnormal caudal regeneration in lepidosaurs and its potential ecological impacts

Abnormal caudal regeneration, the production of additional tails through regeneration events, occurs in lepidosaurs as a result of incomplete autotomy or sufficient caudal wound. Despite being widely known to occur, documented events generally are limited to opportunistic single observations - hindering the understanding of the ecological importance of caudal regeneration. Here we compiled and reviewed a robust global database of both peer-reviewed and non-peer reviewed records of abnormal regeneration events in lepidosaurs published over the last 400 years. Using this database, we qualitatively and quantitatively assessed the occurrence and characteristics of abnormal tail regeneration among individuals, among species, and among populations. We identified 425 observations from 366 records pertaining to 175 species of lepidosaurs across 22 families from 63 different countries. At an individual level, regenerations ranged from bifurcations to hexafurcations; from normal regeneration from the original tail to multiple regenerations arising from a single point; and from growth from the distal third to the proximal third of the tail. Species showing abnormal regenerations included those with intra-vertebral, inter-vertebral or no autotomy planes, indicating that abnormal regenerations evidently occur across lepidosaurs regardless of whether the species demonstrates caudal autotomy or not. Within populations, abnormal regenerations were estimated at a mean ± SD of 2.75 ± 3.41% (range 0.1-16.7%). There is a significant lack of experimental studies to understand the potential ecological impacts of regeneration on the fitness and life history of individuals and populations. We hypothesised that abnormal regeneration may affect lepidosaurs via influencing kinematics of locomotion, restrictions in escape mechanisms, anti-predation tactics, and intra- and inter-specific signalling. Behaviourally testing these hypotheses would be an important future research direction.

Re-regeneration to reduce negative effects associated with tail loss in lizards

Many species of lizard use caudal autotomy, the ability to self-amputate a portion of their tail, regenerated over time, as an effective anti-predation mechanism. The importance of this tactic for survival depends on the degree of predation risk. There are, however, negative trade-offs to losing a tail, such as loss of further autotomy opportunities with the regenerated tail vertebrae being replaced by a continuous cartilaginous rod. The common consensus has been that once a tail has been autotomised and regenerated it can only be autotomised proximal to the last vertebral autotomy point, as the cartilage rod lacks autotomy planes. However, anecdotal evidence suggests that although the regenerated portion of the tail is unable to autotomise, it can re-regenerate following a physical shearing event. We assessed re-regeneration in three populations of the King’s skink (Egernia kingii), a large lizard endemic to south-west Western Australia and surrounding islands. We show that re-regeneration is present at an average of 17.2% across the three populations, and re-regenerated tissue can comprise up to 23.3% of an individual’s total tail length. The ability to re-regenerate may minimise the costs to an individual’s fitness associated with tail loss, efficiently restoring ecological functions of the tail.

Phenotypic selection by kelp gulls against pear-shaped shells of the Antarctic limpet Nacella concinna

The Antarctic limpet (Nacella concinna) presents two ecotypes related to different water depths: littoral individuals have a robust and taller shell while sublittoral individuals are thinner and flatter. Among the environmental factors possibly causing this divergence, avian predation upon littoral individuals has been mooted but has received little research attention. The kelp gull (Larus dominicanus) is the principal consumer of littoral limpets in the study area. We used shells from littoral and sublittoral zones, and from gull middens to examine – through linear morphometrics and elliptic Fourier analysis – whether selective predation exists, and to evaluate the mode of selection that could be operating on different traits. We found that limpet individuals with the apex displaced towards the anterior side were more likely to be predated. However, a remarkable result was that gulls select dorsal pear-shaped rather than elliptical shells and that there is a directional selection against pear-shaped limpets. However, in contrast to previous studies, we did not find that the proportions of elliptical or pear-shaped individuals differed between the littoral and sublittoral zones. This discrepancy could be related to variations in predatory intensity in different locations. These results suggest that where gulls have a strong influence, their effect could be a key factor in dorsal differentiation in limpet shells.

The ecology and evolution of autotomy

Autotomy, the self-induced loss of a body part, occurs throughout Animalia. A lizard dropping its tail to escape predation is an iconic example, however, autotomy occurs in a diversity of other organisms. Octopuses can release their arms, crabs can drop their claws, and bugs can amputate their legs. The diversity of organisms that can autotomize body parts has led to a wealth of research and several taxonomically focused reviews. These reviews have played a crucial role in advancing our understanding of autotomy within their respective groups. However, because of their taxonomic focus, these reviews are constrained in their ability to enhance our understanding of autotomy. Here, we aim to synthesize research on the ecology and evolution of autotomy throughout Animalia, building a unified framework on which future studies can expand. We found that the ability to drop an appendage has evolved multiple times throughout Animalia and that once autotomy has evolved, selection appears to act on the removable appendage to increase the efficacy and/or efficiency of autotomy. This could explain why some autotomizable body parts are so elaborate (e.g. brightly coloured). We also show that there are multiple benefits, and variable costs, associated with autotomy. Given this variation, we generate an economic theory of autotomy (modified from the economic theory of escape) which makes predictions about when an individual should resort to autotomy. Finally, we show that the loss of an autotomizable appendage can have numerous consequences on population and community dynamics. By taking this broad taxonomic approach, we identified patterns of autotomy that transcend specific lineages and highlight clear directions for future research.