The function of the heart provides a window into the physiology of vertebrates and a means of evaluating their response to the stresses imposed by diverse activities. I developed a miniaturized instrument to record electrocardiograms (ECGs), using open-source Arduino technology, that a snake can wear as a backpack. The backpack can also be used when the snake is in a metabolic chamber, to record the metabolic rate and ECG simultaneously. I first used the ECG backpack to record the physiology of snakes under different temperature conditions, to verify that the system worked as expected. the backpack was then used on different species of snakes to test their responses to purified cardiotonic steroids similar to the toxins naturally found in toads, to understand the effects of those toxins on snakes with different diets, including some that are predators on toxic prey. Many snakes are capable of consuming prey that have evolved toxic defenses, such as toads. Genetic resistance to toad toxins is surprisingly widespread among snakes, but toad-eating is not. That suggests that there may be additional adaptations, beyond genetic resistance, that allow only some species to feed heavily on toads. I found that the effect of toad toxins on snakes depends on the species and sex of the snake, as well as the type of toxin, revealing greater even more complexity than had been expected. Females had notably less dramatic responses to toad toxins, and sexual dimorphism was most marked in species that possess genetic resistance to the toxins but are not known to eat toads in the wild. Nonresistant species did not exhibit sexual dimorphism in their cardiac response. Toad specialists had a smaller response to the toad toxins than did other species, followed by dietary generalists, then resistant nontoad-eating species, and then nonresistant species. Although heart rate and metabolic rate are usually strongly positively correlated, they were not strongly correlated during the experiments with toad toxins or the temperature experiments.