Tendon springs often influence locomotion by amplifying the speed and power of limb joint rotation. However, less is known about elastic recoil action in feeding systems, particularly for small aquatic animals. Here, we ask if elastic recoil amplifies the speed of gape closing during aquatic food processing in the Axolotl (Ambystoma mexicanum). We measure activation of the adductor mandibulae externus via electromyography and strain of the jaw adductor muscle-tendon unit (MTU), and gape kinematics via fluoromicrometry. The muscle is pre-activated coincident with gape opening, which causes MTU stretch. Activation lasts significantly shorter for fish than cricket processing, and muscle shortening during MTU lengthening yields significantly greater elastic strain for cricket processing. The speed of MTU shortening, which dictates the speed of gape closing is 2.5-4.4 times greater than the speed of the initial shortening of the muscle fascicles for fish and cricket gape cycles, respectively. These data demonstrate a clear role for elastic recoil, which may be unexpected for a MTU in a feeding system of a small, aquatic animal. Amplification of jaw-closing speed resulting from elastic recoil likely confers ecological advantages in reducing prey escape risks during food processing in a dense and viscous fluid environment.
Keywords: evolution; feeding biomechanics; fluid-based movement; functional morphology; speed amplification.