The southern alligator lizard (Elgaria multicarinata) exhibits a courtship behavior during which the male firmly grips the female’s head in his jaws for many hours at a time. This extreme behavior counters the conventional wisdom that reptile muscles are fast to fatigue and incapable of powering high endurance behaviors. We conducted in situ experiments in which the jaw-adductor muscles of lizards were stimulated directly while bite force was measured simultaneously with a force transducer. Fatigue tests were performed by supramaximally and bilaterally stimulating the muscles for several minutes with a series of tetanic trains. Our results show that a substantial sustained force gradually develops during the first few minutes of the fatigue test. This sustained force persists after peak tetanic forces have declined to a fraction of their initial magnitude. The observed sustained force during in situ fatigue tests is consistent with the natural courtship behavior of these lizards and likely reflects physiological specialization. The results of molecular analysis reveal that the jaw-adductor muscles contain masticatory myosin and tonic myosin fibers. We propose that the presence of tonic fibers may explain the unusual sustained force properties during mate-holding behavior. The characterization of muscle properties that facilitate extreme performance in specialized systems may help reveal general mechanisms, especially when done in light of convergently evolved systems exhibiting similar performance characteristics.