Does the unusual phenomenon of sustained force circumvent the speed-endurance trade off in the jaw muscle of the southern alligator lizard (Elgaria multicarinata)?

The jaw muscles of the southern alligator lizard, Elgaria multicarinata, are used in prolonged mate-holding behavior, and also to catch fast prey. In both males and females, these muscles exhibit an unusual type of high endurance known as sustained force in which contractile force does not return to baseline between subsequent contractions. This phenomenon is assumed to facilitate the prolonged mate-holding observed in this species. Skeletal muscle is often subject to a speed-endurance trade off. Here we determine the isometric twitch, tetanic, and isotonic force-velocity properties of the jaw muscles at ∼24°C as metrics of contractile speed and compare these properties to a more typical thigh locomotory muscle, to determine whether endurance by sustained force allows for circumvention of the speed-endurance trade-off. The specialized jaw muscle is generally slower than the more typical thigh muscle; time to peak twitch force, twitch 90% relaxation time (p<0.01), and tetanic 90% and 50% relaxation times (p<0.001) are significantly longer, and force-velocity properties are significantly slower (p<0.001), in the jaw than the thigh muscle. However, there seem to be greater effects on relaxation rates and shortening velocity that on force rise times; there was no effect of muscle on time to peak, or 50% of tetanic force. Hence, the jaw muscle of the southern alligator lizard does not seem to circumvent the speed-endurance trade-off. However, the maintenance of force rise times despite slow relaxation, potentially enabled by the presence of hybrid fibers, may allow this muscle to meet the functional demand of prey capture.