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Video still showing Hadogenes paucidens in the arena, with three mirrors providing a total of four views. The scorpion is induced to close its left chela by stimulation with a brush hair mounted on a thin carbon fibre rod, which can be seen over the body of the scorpion in the middle views.
Source publication
Here we analyzed the speed of chela closing in nine species of scorpions using high speed camera videos and 3D Digitation process. We also correlated the measured closing speed with biteforce data already available and what we found is a negative correlation between these two parameter. This result suggest us the existence of a functional trade-off...
Context in source publication
Context 1
... arena composed of three mirrors was filmed from above, permitting views of the closing chela from four different angles, three of which were approximately or- thogonal (Fig. 2). The arena size and camera distance were changed to optimize the spatial resolution for each species. Each view was calibrated before each recording session by placing a three-dimensional (3D) printed calibration shape with 12 landmarks in the arena. After calibration, a scorpion was placed in the arena, which elicited a defensive ...
Citations
... Chelate pedipalps are one of the most striking structures displayed by scorpions. These modified prosomal appendages serve disparate functions, including predation (Lamoral 1971;Simone and van der Meijden 2018;Leeming 2019;Cunha et al. 2022), defence (van der Meijden et al. 2013), and burrowing (Harington 1977;Abdel-Nabi et al. 2004). As such, the mechanics (van der Meijden et al. 2012a;Bicknell et al. 2022), structure (Zhao et al. 2016;Zhang et al. 2023), and general biology (Snodgrass 1952) of pedipalps have been discussed at length. ...
... The thick endocuticle in the sagittal orientation illustrates that chelae fingers are reinforced to limit development of cracks within the cuticle (Zhang et al. 2023). These observations illustrate that U. manicatus chelae are well adapted for handling prey (Farley 1999;Simone and van der Meijden 2018;Kellersztein et al. 2019). ...
... Combined with the robust morphology and the array of adaptions within the cuticular microstructure and elemental composition, this high pinch force suggests that U. manicatus likely uses its chelae to attack, capture, and crush prey. This contrasts with forms that display lower force values (<1 N) and rapid chelae closing speeds, using the chelae to hold prey for incapacitation by the stinger (Casper 1985;van der Meijden et al. 2013;Simone and van der Meijden 2018). This observation also aligns with the life mode of U. manicatus as a predator that waits at burrow entrances for prey (Holden 1997) and uses pedipalps, rather than the stinger, to initially subdue prey (Simone and van der Meijden 2018). ...
Pedipalps-chelate 'pincers' as the second pair of prosomal appendages-are a striking feature of scorpions and are employed in varied biological functions. Despite the distinctive morphology and ecological importance of these appendages, their anatomy remains underexplored. To rectify this, we examined the pedipalps of the Australian black rock scorpion, Urodacus manicatus, using a multifaceted approach consisting of microcomputed tomography, scanning electron microscopy, energy dispersive X-ray spectroscopy, and live pinch force measurements. In doing so, we document the following aspects of the pedipalps: (1) the musculature in three dimensions; (2) the cuticular microstructure, focusing on the chelae (tibial and tarsal podomeres); (3) the elemental construction of the chelae teeth; and (4) the chelae pinch force. We recognise 25 muscle groups in U. manicatus pedipalps, substantially more than previously documented in scorpions. The cuticular microstructure-endo-, meso-, and exocuticle-of U. manicatus pedipalps is shown to be similar to other scorpions and that mesocuticle reinforces the chelae for predation and burrowing. Elemental mapping of the chelae teeth highlights enrichment in calcium, chlorine, nickel, phosphorus, potassium, sodium, vanadium, and zinc, with a marked lack of carbon. These elements reinforce the teeth, increasing robustness to better enable prey capture and incapacitation. Finally, the pinch force data demonstrate that U. manicatus can exert high pinch forces (4.1 N), further highlighting the application of chelae in subduing prey, as opposed to holding prey for envenomation. We demonstrate that U. manicatus has an array of adaptions for functioning as a sit-and-wait predator that primarily uses highly reinforced chelae to process prey.
... Early studies of skeletal anatomy considered the mechanical function encoded by G as an essential evolutionary driver of postural variations in ungulates (Gregory 1912b); as determinant of limb function during locomotion (Gray 1944); and as primary explanation for the diversity of mammalian skeletal shape in general (Smith and Savage 1956). In the last three decades alone, G has been the principal functional metric in the study of the ecological speciation in Darwin's finches (Herrel et al. 2009); the evolution of feeding mechanisms in fish (Westneat 1994(Westneat , 2004; the evolution of skull form in bats (Dumont et al. 2014;Santana et al. 2012); the morphological diversification of jaw morphology in vertebrates (Anderson et al. 2011;Deakin et al. 2023) and of chela morphology in scorpions (Simone and van Der Meijden 2018); bite force estimation in extinct species (Sakamoto et al. 2010(Sakamoto et al. , 2019; the early evolution of biting-chewing performance in the hexapoda (Blanke 2019); the ecological and taxonomic diversification of crown mammals (Tseng et al. 2023), and the diet of Mesozoic mammals (Morales-García et al. 2021); and the variability of rates of phenoptypic evolution in cichlids (Burress and Muñoz 2023). There is probably no other biomechanical metric that has been measured as extensively, and that has served as foundation for such far reaching comparative, ecological and evolutionary conclusions. ...
Movement is integral to animal life, and most animal movement is actuated by the same engine: skeletal muscle. Muscle input is typically mediated by skeletal elements, resulting in musculoskeletal systems that are "geared": at any instant, the muscle force and velocity are related to the output force and velocity only via a proportionality constant G, the "mechanical advantage". The functional analysis of such "simple machines" has traditionally centred around this instantaneous interpretation, such that a small vs large G is thought to reflect a fast vs forceful system, respectively. But evidence is mounting that a complete analysis ought to also consider the mechanical energy output of a complete contraction. Here, we approach this task systematically, and use the theory of physiological similarity to study how gearing affects the flow of mechanical energy in a minimalist model of a musculoskeletal system. Gearing influences the flow of mechanical energy in two key ways: it can curtail muscle work output, because it determines the ratio between the characteristic muscle work and kinetic energy capacity; and it defines how each unit of muscle work is partitioned into different system energies, i.e. into kinetic vs. "parasitic" energy such as heat. As a consequence of both effects, delivering maximum work in minimum time and with maximum transmission efficiency generally requires a mechanical advantage of intermediate magnitude. This optimality condition can be expressed in terms of two dimensionless numbers, which reflect the key geometric, physiological, and physical properties of the interrogated musculoskeletal system, and the environment in which the contraction takes place. Illustrative application to exemplar musculoskeletal systems predicts plausible mechanical advantages in disparate biomechanical scenarios; yields a speculative explanation for why gearing is typically used to attenuate the instantaneous force output (G_opt < 1); and predicts how G needs to vary systematically with animal size to optimise the delivery of mechanical energy, in superficial agreement with empirical observations. A many-to-one-mapping from musculoskeletal geometry to mechanical performance is identified, such that differences in G alone do not provide a reliable indicator for specialisation for force vs speed- neither instantaneously, nor in terms of mechanical energy output. The energy framework presented here can be used to estimate an optimal mechanical advantage across variable muscle physiology, anatomy, mechanical environment and animal size, and so facilitates investigation of the extent to which selection has made efficient use of gearing as degree of freedom in musculoskeletal "design".
... Trade-offs between prey capture device length and kinematics are also seen in some other arachnids. For instance, in scorpion palpal chelae or claws, there is a negative correlation between closing speed, which is typically higher in species with longer chelae, and closing force, which is higher in species with shorter chelae (Simone and van Der Meijden, 2017). Species with longer chelae have an advantage in opening gape (Simone and van Der Meijden, 2017). ...
... For instance, in scorpion palpal chelae or claws, there is a negative correlation between closing speed, which is typically higher in species with longer chelae, and closing force, which is higher in species with shorter chelae (Simone and van Der Meijden, 2017). Species with longer chelae have an advantage in opening gape (Simone and van Der Meijden, 2017). In this context, such a functional trade-off could hypothetically exist in Amblypygi; taxa with long pedipalps might optimise for larger absolute reach and prey capture accuracy, whilst reducing other important prey capture factors, such as manoeuvrability or time to secure prey. ...
The link between form and function is key to understanding the evolution of unique and/or extreme morphologies. Amblypygids, or whip spiders, are arachnids that often have highly elongated spined pedipalps. These limbs are used to strike at, and secure, prey before processing by the chelicerae. Amblypygi pedipalps are multifunctional, however, being used in courtship and contest, and vary greatly in form between species. Increased pedipalp length may improve performance during prey capture, but length could also be influenced by factors including territorial contest and sexual selection. Here for the first time, we use high-speed videography and manual tracking to investigate kinematic differences in prey capture between amblypygid species. Across six morphologically diverse species, spanning four genera and two families, we create a total dataset of 86 trials (9-20 per species). Prey capture kinematics varied considerably between species, with differences being expressed in pedipalp joint angle ranges. In particular, maximum reach ratio did not remain constant with total pedipalp length, as geometric scaling would predict, but decreased with longer pedipalps. This suggests that taxa with the most elongated pedipalps do not deploy their potential length advantage to proportionally increase reach. Therefore, a simple mechanical explanation of increased reach does not sufficiently explain pedipalp elongation. We propose other factors to help explain this phenomenon, such as social interactions or sexual selection, which would produce an evolutionary trade-off in pedipalp length between prey capture performance and other behavioural and/or anatomical pressures.
... Maximum pinch force data were only known from the micro-CT scanned specimens of Androctonus bicolor and Pandinoides cavimanus (Simone & Van der Meijden, 2018). As we did not measure in-vivo pinch forces of the micro-CT scanned individuals of other species, pinch forces were estimated for those specimens using data available from other individuals of the same species. ...
... The data were log 10 transformed to linearise variables with different dimensionality and used to produced species-specific linear models, and associated regression coefficients (Table S2). The mechanical advantage of the moveable finger was calculated from the micro-CT scans following Simone & Van der Meijden (2018). However, as in-vivo pinch forces were measured approximately two thirds of the finger length from the joint and not the distal-most point of moveable fingers, the mechanical advantage was corrected by shortening the length of the out-lever by one third. ...
... The results of the scorpion FEAs show similar groupings as in Van der Meijden, Kleinteich & Coelho (2012). The differences in magnitudes of VM stresses (Fig. 3) indicate that some species (e.g., Pandinoides cavimanus, Ophistacanthus maculatus, Hottentotta gentili) could exhibit a higher 'safety factor' (i.e., a measure of how much stronger a morphology needs to be compared to the input forces; Hicks & Wang, 2021) than others (e.g., Caraboctonus keyserlingi, Hadogenes paucidens, Parabuthus transvaalicus), although the pattern does not seem to correspond to defensive behaviour (Van der Meijden et al., 2013) or relative pinch force (Simone & Van der Meijden, 2018). Considering the modelled scorpions in the context of possible ecomorphs, there is limited overlap between groups identified here and recently proposed ecomorphologies (Coelho et al., 2022). ...
Eurypterids (sea scorpions) are extinct aquatic chelicerates. Within this group, members of Pterygotidae represent some of the largest known marine arthropods. Representatives of this family all have hypertrophied, anteriorly-directed chelicerae and are commonly considered Silurian and Devonian apex predators. Despite a long history of research interest in these appendages, pterygotids have been subject to limited biomechanical investigation. Here, we present finite element analysis (FEA) models of four different pterygotid chelicerae-those of Acutiramus bohemicus, Erettopterus bilobus, Jaekelopterus rhenaniae, and Pterygotus anglicus-informed through muscle data and finite element models (FEMs) of chelae from 16 extant scorpion taxa. We find that Er. bilobus and Pt. anglicus have comparable stress patterns to modern scorpions, suggesting a generalised diet that probably included other eurypterids and, in the Devonian species, armoured fishes, as indicated by co-occurring fauna. Acutiramus bohemicus is markedly different, with the stress being concentrated in the proximal free ramus and the serrated denticles. This indicates a morphology better suited for targeting softer prey. Jaekelopterus rhenaniae exhibits much lower stress across the entire model. This, combined with an extremely large body size, suggests that the species likely fed on larger and harder prey, including heavily armoured fishes. The range of cheliceral morphologies and stress patterns within Pterygotidae demonstrate that members of this family had variable diets, with only the most derived species likely to feed on armoured prey, such as placoderms. Indeed, increased sizes of these forms throughout the mid-Palaeozoic may represent an 'arms race' between eurypterids and armoured fishes, with Devonian pterygotids adapting to the rapid diversification of placoderms.
... In the particular case of Chactas sp., males have slenderer chelae than females, which might explain the higher pinch force of the latter. This agrees with previous studies which suggest that scorpions with robust chelae are stronger than species with slender chelae [4,5,35]. Since we did not correct for overall body size, the between-species comparisons are no indication for relative pinch performance. ...
Background:
Scorpions can use their pincers and/or stingers to subdue and immobilize their prey. A scorpion can thus choose between strategies involving force or venom, or both, depending on what is required to subdue its prey. Scorpions vary greatly in the size and strength of their pincers, and in the efficacy of their venom. Whether this variability is driven by their defensive or prey incapacitation functionis unknown. In this study, we test if scorpion species with different pincer morphologies and venom efficacies use these weapons differently during prey subjugation. To that end, we observed Opisthacanthus elatus and Chactas sp. with large pincers and Centruroides edwardsii and Tityus sp. with slender pincers.
Methods:
The scorpion pinch force was measured, and behavioral experiments were performed with hard and soft prey (Blaptica dubia and Acheta domesticus). Stinger use, sting frequency and immobilization time were measured.
Results:
We found that scorpions with large pincers such as O. elatus produce more force and use the stinger less, mostly subjugating prey by crushing them with the pincers. In C. edwardsii and Tityus sp. we found they use their slender and relatively weak pincers for holding the prey, but seem to predominantly use the stinger to subjugate them. On the other hand, Chactas sp. uses both strategies although it has a high pinch force.
Conclusions:
Our results show that scorpionspecies with massive pincers and high pinch force as O. elatus use the stinger less for prey subjugation than scorpionspecies with slenderpincers.
... From consideration of lever mechanics, longer fingers (i.e. a longer out-lever) provide a lower mechanical advantage, therefore less force is transmitted from chela muscles to the tip of the movable finger. We recently found a negative correlation between pinch force and chela closing speed [267], which means that species with a stronger grip are also slower (Figure 4). Faster chelae may be a suitable weapon to hunt fast prey, but lever mechanics limits the maximum pinch force, and thus restricts the bearer to soft rather than hard-bodied prey. ...
Scorpions possess two systems of weapons: the pincers (chelae) and the stinger (telson). These are placed on anatomically and developmentally well separated parts of the body, that is, the oral appendages and at the end of the body axis. The otherwise conserved body plan of scorpions varies most in the shape and relative dimensions of these two weapon systems, both across species and in some cases between the sexes. We review the literature on the ecological function of these two weapon systems in each of three contexts of usage: (i) predation, (ii) defense and (iii) sexual contests. In the latter context, we will also discuss their usage in mating. We first provide a comparative background for each of these contexts of usage by giving examples of other weapon systems from across the animal kingdom. Then, we discuss the pertinent aspects of the anatomy of the weapon systems, particularly those aspects relevant to their functioning in their ecological roles. The literature on the functioning and ecological role of both the chelae and the telson is discussed in detail, again organized by context of usage. Particular emphasis is given on the differences in morphology or usage between species or higher taxonomic groups, or between genders, as such cases are most insightful to understand the roles of each of the two distinct weapon systems of the scorpions and their evolutionary interactions. We aimed to synthesize the literature while minimizing conjecture, but also to point out gaps in the literature and potential future research opportunities.
... In other words, crabs with one specific claw morphology cannot be quick and strong at the same time. The same is the case with the lever system of scorpion chela, where a functional trade-off between speed and force has been identified across species (Simone and van der Meijden 2017). Likewise, a trade-off between bite force and jaw velocity exists in Darwin's finches, where higher bite forces have been associated with slow beak movements (Herrel et al. 2009). ...
Biting performance is important for feeding, territory defense, and mating in many animals. While maximal bite force is a well-studied trait, other aspects of biting and their variation depending on behavior are rarely considered. Here, we took an innovative approach, where (1) we quantified a novel trait, bite duration in lizards; (2) we examined variation across setups that simulate feeding and antagonistic behavior; and (3) we used F-matrix statistics to investigate how different functional components are optimized for ecological and social demands with respect to individual morphology. Our results did not show differences between the sexes in bite duration, but bite performance varied across experimental setups in males, suggesting a higher functional flexibility in this sex. The investigation of form-function associations revealed that trade-offs, facilitations, and one-to-one relationships are simultaneously involved in the morphological optimization of bite force and duration. Put together, our integrated analysis of two different components of bite performance-force and duration-demonstrates their importance for males in both ecological and social tasks. Our findings also suggest the existence of trade-offs in the morphological optimization of functional components, possibly due to physiological constraints on muscle composition, insertion, and orientation.
Morphological differences between the sexes are a common feature in many groups of animals and can have important ecological implications for courtship, mating, access to prey and, in some cases, intersex niche partitioning. In this study, we evaluated the role of sexual dimorphism in the performance of the two structures that mediate the ability to access prey, the pinchers or chelae and the venomous stinger, in two species of scorpions with contrasting morphologies: Chactas sp., which has marked sexual dimorphism in the chelae, and Centruroides sp., which does not have such marked dimorphism in the chelae. We evaluated aspects such as chela pinch force, toxicity to prey (LD50) and the volume of venom in males and females of each species. We found significant differences between males and females of Chactas sp. in the chela pinch force, volume of venom and LD50. In contrast, for Centruroides sp., no differences between males and females were found in any of these traits. We discuss several potential selective regimes that could account for the pattern observed.
Morphological differences between the sexes are a common feature in many groups of animals and can have important ecological implications for courtship, mating, access to prey and, in some cases, intersex niche partitioning. In this study, we evaluated the role of sexual dimorphism in the performance of the two structures that mediate the ability to access prey, the pinchers or chelae and the venomous stinger, in two species of scorpions with contrasting morphologies: Chactas sp., which has marked sexual dimorphism in the chelae, and Centruroides sp., which does not have such marked dimorphism in the chelae. We evaluated aspects such as chela pinch force, toxicity to prey (LD 50) and the volume of venom in males and females of each species. We found significant differences between males and females of Chactas sp. in the chela pinch force, volume of venom and LD 50. In contrast, for Centruroides sp., no differences between males and females were found in any of these traits. We discuss several potential selective regimes that could account for the pattern observed. ADDITIONAL KEYWORDS: pinch force-LD 50-scorpions-sexual dimorphism-venom.
