Convergently evolved linear actuators in ballistic tongues.

Rapid movements in animals depend on efficient energy transfer to overcome inertia. Among vertebrates, dense tissue and limited elastic storage are thought to impose profound constraints on power output, making extreme ballistic performance noteworthy. Here, we show that chameleons (Chamaeleonidae) and some lungless salamanders (Plethodontidae) have independently converged on a shared biomechanical solution: a sliding-based linear actuator that launches the tongue via muscular squeezing of a tapered skeletal rod. This design decouples muscle action from skeletal movement, enabling acceleration of 30-590 G (300-5,740 m/s) and projection speed of 2-5.5 m/s across a 30-fold range in body size, representing some of the most efficient energy transfer in vertebrate movements. Integrating theoretical modeling and experimental results, we show that such sliding-based actuation enables rapid and spatially compact energy transfer (within 3-30 ms over 1-35 mm), circumventing vertebrate muscle's force-velocity trade-offs. Further, although presumably driven by similar selection for ecological versatility and thermal robustness, these ballistic tongues were each assembled via a different sequence of innovations. Our findings reveal how biomechanical modularity, rather than exceptional materials, underlies this vertebrate ballistic innovation. These results also provide bio-inspiration for the engineering of rapid actuators built as hybrids of soft and stiff materials.