Researchers had previously identified the tongue retractor muscle (m. hyoglossus) as a “supercontracting” muscle – one that can bypass limitations on its ability to stretch and shorten. In the USD lab of Christopher Anderson, Ph.D., associate professor of biology, two additional muscles involved in feeding (m. sternohyoideus sperficialis and m. sternohyoideus profundus) were found to function in the same way, but each with its own mechanism.

An article on their research, “Feats of supercontractile strength: Functional convergence of supercontracting muscle properties among hyoid musculature in chameleons,” appears in the March 26 issue of the journal Proceedings of the Royal Society B: Biological Sciences.

For chameleons to project their tongues up to 2.5 times their body length and with incredible speed, their muscles have evolved specializations, Anderson explained.

“We found that two muscles associated with the hyoid – or tongue skeleton – of chameleons are able to exert force over a much broader range of muscle lengths than typical vertebrate skeletal muscle,” he said. “This helps to explain how chameleons are able to achieve such remarkable tongue projection distances and still pull their prey back into their mouths.”

In the lab, researchers measured the ability of the force produced by the two hyoid muscles compared to supercontracting tongue retractor muscle and a typical skeletal muscle. They also examined the muscle fibers using transmission electron microscopy and examined images of embryonic muscle development associated with the hyoid to see how these muscles developed.

Both hyoid muscles were able to overcome typical muscle limitations, but only m. sternohyoideus superficialis did so as a supercontracting muscle: through the structural modifications of the basic contractile unit of muscle fibers called sarcomeres. The other muscle (m. sternohyoideus profundus) used a different strategy to overcome muscle stretch limitations by capitalizing on uneven length change across the muscle, allowing for different muscle fibers to be increasing their force while others may be decreasing theirs as the muscle changes length.

“The discovery of a second supercontracting muscle and an additional muscle with a different mechanism for overcoming contraction limits shows that chameleons possess a more diverse set of muscle adaptations than previously known,” said Nikole Schneider, a doctoral student in The Anderson Lab and one of the study’s authors. “Identifying common developmental origins for these muscles helps link their function to evolutionary processes, offering insights into how chameleons, and potentially other animals, adapt their muscle structure for specialized tasks.”

For Anderson, who is also the chair of the Chameleon Specialist Group of the International Union for Conservation of Nature, this research expands our understanding of one of nature’s most captivating and unusual species.

“Despite centuries of curiosity about how chameleons are able to project their tongues from their mouth with such explosive performance, the fact that we are still able to shed light on how chameleons are able to perform some of their most extreme and characteristic behaviors is extremely rewarding,” he said.

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