Effects of submergence on cardiac cycle and rate in aquatic and terrestrial birds

Abstract

Electrocardiograms were recorded for various aquatic and terrestrial birds with heads submerged and then returned to air. Instantaneous cardiac rates were calculated and P-R and Q-T intervals were measured at selected times. Bradycardia developed in all birds during submergence, but was most pronounced and longest sustained in aquatic birds. Upon emergence, aquatic birds and pigeons developed instantly a compensatory tachycardia; cardiac rate in the others gradually returned to the presubmergence level. In the first 15 seconds of submersion, P-R and Q-T intervals remained constant in all birds except pheasants, but in ducks undergoing prolonged submergence the Q-T interval increased significantly. This Q-T interval increase lagged approximately 60 seconds behind the onset of bradycardia. Diving ducks were like dabblers, supporting further the hypothesis that differences in their underwater activity cannot be explained by vascular differences. These observations on minimally restrained wild birds are consistent with the hypothesis of a differential vascular control of oxygen during diving. For underwater activity, aquatic birds utilize mechanisms appearing to some extent in all birds.