On the cardioinhibitory role of water immersion per se in ducklings (Anas platyrhynchos) during enforced submersion

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Abstract
Experiments were done to clarify the contribution of water immersion per se to the cardiac chronotropic response to enforced submergence in ducklings. Four protocols were used in which one or more of the factors claimed to affect diving heart rate were controlled. Heart rate fell rapidly in the first few seconds of submergence in all protocols. This initial fall was unaffected by changes in head position or by breathing 100% oxygen for 5 min before diving. Also, sudden withdrawal of input from lung receptors or cessation of activity in central respiratory neurones did not affect onset of diving bradycardia. We conclude that about one-third of diving bradycardia in ducklings is caused by water immersion per se.