Animals vary in their ability to tolerate asphyxia. Among aquatic species, some are well adapted to asphyxia associated with the apnea of their diving behavior. The related mechanisms and their regulation are not unique to aquatic animals, rather they are extensions of similar reactions noted in terrestrial species. Our understanding of asphyxia has grown in large part from research on aquatic mammals and birds and by comparing the responses of these natural breath-holding specialists with those of other animals. Studies in nature and in the laboratory have both contributed to this knowledge. The divers have been shown to rely ultimately on oxygen conservation and enhanced anaerobic reserves, producing a strategic retreat into a hypometabolic state.