Metal-reinforced scorpions evolved to kill
Scorpions are optimized hunters, whose skills have been honed over millions of years of evolution. An armored exoskeleton, powerful pincers, a venomous stinger: almost everything in their anatomy helps them either hunt insects, small mammals and reptiles, or defend themselves against snakes and birds. But for years, entomologists have been aware of a potential secret weapon in arthropod biology: metal reinforcements.
Researchers have already detected traces of metals in the exoskeletons of at least some of the 3,000 known species of scorpions. At the same time, experts were unsure of the distribution and concentration of these metals.
“We knew that metals strengthened the weapons in the arsenals of certain species, [but] we don’t know if all scorpion weapons contain metal,” Sam Campbell, an environmental scientist at the University of Queensland in Australia, said in a statement.
The answer might come from the way they rely on their stingers and claws. Some species of scorpions wield their venomous barbs more than their claws, while others deploy the opposite strategy. Campbell and his colleagues hypothesized that the distribution of trace metals might correspond to whether or not a species prefers its stingers or pincers.
While pursuing a Smithsonian fellowship at the National Museum of Natural History in Washington, DC, the team used microanalytical methods such as high-resolution electron microscopy and x-ray analysis to examine specimens of 18 distinct scorpion species. Their results published in the Journal of the Royal Society Interface The claws and darts found contain concentrations of metal.
“The National Museum of Natural History’s large scorpion collection has allowed us to analyze metal enrichment in a wide range of scorpion species more than ever before using these techniques,” said Edward Vincenzi, a researcher at the Museum Conservation Institute and co-author of the study.
The results revealed a pair of distinct metallic layers in the scorpions. Stingers reliably contained large amounts of zinc in their needle-like tips, followed by a layer of manganese. The distribution is also similar in the pincers. In the moving part known as the tarsus, Campbell’s team identified zinc or a combination of zinc and iron along the cutting edge of the claw.
However, the usefulness of each metal does not exactly match the researchers’ hypothesis. Although they predicted that stronger, more crushing claws would contain more zinc, they found higher zinc levels in thinner, longer claws, typically used with stingers.
“This indicates a role for zinc beyond hardness, perhaps playing a larger role in durability,” Campbell said. “After all, long claws must grab the prey and prevent it from escaping before being injected with venom. This is an interesting finding because it suggests an evolutionary relationship between the way a weapon is used and the specific properties of the metal that reinforces it.”
The team’s findings have major ramifications for understanding the broader world of arthropods and insects. Scorpions are far from the only creatures to incorporate traces of metals into their anatomy. By establishing a clear foundation for future analyses, researchers can study how these evolutionary adaptations may arise in bees, wasps, spiders and other animals.
“The microscale methods we used allowed us to identify individual transition metals with an extremely high level of detail, showing us how nature skillfully designed these metals into the scorpion’s weapons,” Vincenzi added.
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