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Scorpions strengthen their claws and stingers with metal, effectively turning these weapons into the equivalent of a steel-capped boot.

The use of metals to strengthen vulnerable body parts – such as teeth in vertebrates like Komodo dragons (Varanus komodoensis) – is already known, and the areas of a scorpion that contain metals are visible as stains to the naked eye.

at the University of Queensland, Australia, and his colleagues examined the claws and stingers of 18 species of scorpion from around the world to determine the extent and composition of their metal reinforcements.

The team used two different X-ray techniques and electron microscopy to examine the scorpions, enabling them to map the presence of three main metals – iron, zinc and manganese. They also found traces of a range of other elements, including copper, nickel, silicon, chlorine, titanium and bromine.

The metals are largely found within the tips of the stingers and along the cutting edge of the claws, as well as in their mouth and teeth and in their tarsal claws, making their weapons “like a steel-toe-capped boot”, Campbell says. The rest of the animal’s exoskeleton is still hard, but much softer in comparison.

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Scorpions all fluoresce light green or blue under ultraviolet light. But metal-enriched parts of the body don’t glow when exposed to UV, the team found.

It isn’t yet known how the scorpions obtain the metals that they incorporate into their exoskeletons, though their prey is the most likely source.

The team also found that different scorpion species had more metal in different parts of their bodies, and this is related to their behaviour. “What we identified was that when zinc was high in the claws, it would be low in the stinger, and vice versa,” says Campbell. “Because scorpions use their weapons so differently, it is possible that metal enrichment has adapted to provide beneficial biomechanical properties in the weapons where it is most needed by the scorpion.”

Metal enrichment in animal tissues appears to be more common than once thought, says at King’s College London. “A growing number of studies are pointing this out in vertebrate teeth as well,” he says. “The next logical step after discovering these features is to try to understand how they have evolved across major lineages, and this study is a pioneering one for that reason.”