The effect of age and the influence of the relative size of the heart, brain, and blood oxygen store on the responses to submersion in mallard ducklings

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Abstract
Three newly hatched mallard ducklings showed profound bradycardia on head submersion. Heart rate fell from 485 beats∙min−1 predive to 75 beats∙min−1 after 30 s. Resting heart rates in a group of eight ducklings retested at weekly intervals fell from 430 ± 13 beats∙min−1 at one week after hatching to 183 ± 27 beats∙min−1 at 12 weeks. In spite of the wide difference in resting heart rates, the proportional fall in heart rate by 30 s after head submersion was very consistent, to some 20% of the predive rate. The absolute fall in heart rate during early submergence was much greater at 1 week than at 4 weeks. Previous experience with voluntary head submersion during feeding did not affect the heart-rate response.Predive mean arterial blood pressure, which rose from 77.1 mmHg at 1 week to 180.2 mmHg at 12 weeks was maintained constant, or rose slightly, during submersion in all the animals tested. The tolerance to prolonged submersion was markedly poorer in younger ducklings; birds at 1 week only tolerated some 67 s of apnoeic asphyxia before a breakdown of the circulatory adjustments, while at 12 weeks the same birds tolerated submersion for 240 s with no ill effects. This is partially due to the changing allometric relationships during development between the mass of the brain and the heart, major components of the oxygen sink during submergence, and the blood volume, a major component of the self-contained oxygen store.

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