Functional implications of pedipalp chela shapes in scorpions

Arie van der Meijden and co-workers have previously published a paper on claw size and pincer force in scorpions. A follow-up paper is now in press on the functional implications of pedipalp chela shapes in scorpions.

Abstract:
Scorpions depend on their pedipalps for prey capture, defense, mating and sensing their environment. Some species additionally use their pedipalps for burrowing or climbing. Because the pincers or chelae at the end of the pedipalps vary widely in shape, they have been used as part of a suite of characters to delimit ecomorphotypes. We here evaluate the influence of the different chela cuticular shapes on their performance under natural loading conditions. Chelae of 20 species, representing seven families and spanning most of the range of chela morphologies, were assigned to clusters based on chela shape parameters using hierarchical cluster analysis. Several clusters were identified corresponding approximately to described scorpion ecomorphotypes. Finite element models of the chela cuticulae were constructed from CT scans and loaded with estimated pinch forces based on in vivo force measurements. Chela shape clusters differed significantly in mean Von Mises stress and strain energy. Normalized FEA showed that chela shape significantly influenced Von Mises stress and strain energy in the chela cuticula, with Von Mises stress varying up to an order of magnitude and strain energy up to two orders of magnitude. More elongate, high-aspect ratio chela forms showed significantly higher mean stress compared with more robust low-aspect ratio forms. This suggests that elongate chelae are at a higher risk of failure when operating near the maximum pinch force. Phylogenetic independent contrasts (PIC) were calculated based on a partly resolved phylogram with branch lengths based on an alignment of the 12S, 16S and CO1 mitochondrial genes. PIC showed that cuticular stress and strain in the chela were correlated with several shape parameters, such as aspect ratio, movable finger length, and chela height, independently of phylogenetic history. Our results indicate that slender chela morphologies may be less suitable for high-force functions such as burrowing and defense. Further implications of these findings for the ecology and evolution of the different chela morphologies are discussed.

Reference:
van der Meijden A, Kleinteich T, Coelho P. Packing a pinch: functional implications of chela shapes in scorpions using finite element analysis. J Anat. 2012; Early View paper. [Subscription required for fulltext]

Thanks to Dr. van der Meijden for sending me this paper!

"Scorpion Strongmen" - Who's got the strongest grip?

The size of scorpion claws (pincers) vary a lot. Some species has long and slender pedipalps (like Leiurus), while others have broad and powerful claws (like Pandinus). But who's got the strongest pincer force?

A. van der Meijden and co-workers have published a comparison of claw size and pincer force in seven species. Scorpions with the highest claws had the strongest grip. The authors suggest that slender claws may be an adaption to speed (catching quick and elusive prey) rather than maximum force. Large claws may be an adaption to defense against predators, but can also have other functions (like burrowing).

Abstract:
A key feature of the ancient body plan of scorpions is the pincer or chela. These multifunctional structures vary considerably in size and shape between different scorpion species. Here we provide the first comparative data on the pinching performance of the chelae of seven species of scorpions exemplifying the extremes of the shape range from slender to robust; Leiurus quinquestriatus, Androctonus amoreuxi, Androctonus australis, Hadogenes sp., Pandinus imperator, Scorpio maurus and Pandinus cavimanus (in the order of decreasing chela height to width ratio). Size-corrected chela height correlates highly with maximum pinch force. Independent contrasts suggest that the correlation of chela width, height and fixed finger length with maximum pinch force is independent of phylogeny, suggesting an adaptive component to the evolution of chela shape and performance.

Reference:
van der Meijden A, Herrel A, Summers A. Comparison of chela size and pincer force in scorpions; getting a frist grip. J Zool. 2009:1-7. DOI:10.1111/j.1469-7998.2009.00628.x [This is an early view article - Subscription required for fulltext]

Thanks to Dr. van der Meijden for sending me the paper!