Habituation of the cardiac response to involuntary diving in diving and dabbling ducks.

1. Bradycardia in response to forced submergence was habituated in dabbling (Anas platyrhynchos, Linnaeus) and diving (Aythya americana, Eyton) ducks by repetitively submerging the animals, each day for several days, for periods of 40 and 20 s, respectively. The onset of pronounced bradycardia was delayed with each successive trial, until little or no bradycardia occurred during submergence. Diving bradycardia is driven by chemoreceptors in the dabbler and caused by stimulation of narial receptors in the diver. 2. Mean arterial blood pressure in dives was unchanged from pre-dive levels in both naive and trained dabbling ducks. PaO2, PaCO2 and pHa at the end of a dive were similar before and after habituation training. 3. Bradycardia occurred in dives by habituated dabbling ducks if the animal breathed 15% O2 before submergence. The ventilatory responses to breathing high and low levels of oxygen were unaffected by habituation training. 4. The changes in blood gases during dives by naive and habituated dabbling ducks were the same: therefore, in the absence of a demonstrated decrement in receptor chemosensitivity or efferent potency, the locus of habituation must reside in the central nervous system.