Constricting snakes must balance the energetic cost of constriction with the potential danger in releasing their prey too early. Therefore, it would be advantageous for these snakes to possess a mechanism to determine the minimum pressure and duration required to ensure that a prey item has been subdued and is no longer capable of inflicting harm. We hypothesized that Boas (Boa constrictor) modulate their constriction based on endogenous cues from their prey such as a heartbeat. In previous work we demonstrated that Boas respond to a simulated heartbeat in a deceased rat model by constricting with greater pressure and duration than when constricting rats without a simulated heartbeat. We extended this work in the current study by testing how Boas respond to a more realistic model; a rat whose cardiovascular system fails during the constriction event. We presented snakes with rats with a simulated heartbeat that failed halfway into the constriction. Analysis of these data demonstrated that constriction events with a simulated heart which fails ten minutes into the constriction are of intermediate duration and total pressure when compared to constriction tests with no simulated heart and a continuously beating simulated heart. We have also conducted experiments to test the snake's response while constricting live, anesthetized rats. This system allows us to observe the snake's response to an actual rat heartbeat while simultaneously monitoring cardiovascular function in the rat during the constriction. Analyses of these data support our hypothesis that cardiac arrest, rather than suffocation, is the proximate cause of death in rats during constriction.