Prevalence and Mechanisms of Sustained Force Production in the Musculature of the Southern Alligator Lizard ( <i>Elgaria multicarinata</i> )

The southern alligator lizard (Elgaria multicarinata) exhibits prolonged mate-holding behavior. This behavior maybe be underpinned by a phenomenon known as sustained force, observed in the jaw muscles of this species, in which the muscle fails to relax between subsequent contractions. However, it is not clear if sustained force is unique to the jaw muscles used in mate-holding, or what the mechanisms responsible for this phenomenon are. In situ and in vitro muscle preparations were used to determine peak force, sustained force, and relaxation rate in the jaw-adductor complex, the iliotibialis 2 (IT2), and the gastrocnemius muscles of E. multicarinata during repeated contractions to determine if sustained force is unique to the jaw. There was a significant effect of muscle on sustained force with only the jaw muscles exhibiting sustained force. To determine if sustained force was due to a slowing of relaxation or the engagement of a passive element such as the molecular spring titin, the inter-contraction interval was increased in the jaw-adductor complex to allow for greater relaxation time. There was a significant effect of inter-contraction interval on sustained force with sustained force being lost and the muscle fully relaxing and force returning to baseline between contractions with a longer inter-contraction interval. The uniqueness of sustained force to the jaw muscle provides support for it being an adaptation for mate-holding that may observed in multiple vertebrate groups. The loss of sustained force with increased inter-contraction interval implicates changes to relaxation process in the mechanism of the development of sustained force. This suggests that this is a fundamentally different mechanism from the catch mechanism used for prolonged maintenance of force in some invertebrates.