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The effect of oxygen and carbon dioxide on the diving behaviour and cardiac physiology of lesser scaup ducks (aythya affinis) Borg, Kim A.

Abstract

The present study was designed to determine the relative influence of oxygen (O₂) and carbon dioxide (CO₂) on the cardiovascular and behavioral adjustments during voluntary diving in lesser scaup ducks. Of particular interest was how these animals allocate time during the dive cycle (travel time, foraging time and surface time) in response to various levels of inspired O₂ and CO₂ during short and long horizontal dives that are energetically more costly than vertical dives. Heart rate was monitored to determine the influence of peripheral chemoreceptors on cardiovascular control during voluntary dives as well as during dives when the animals were temporarily trapped underwater. Compared with normoxia, hyperoxia (50% O₂) exposure significantly increased dive duration in long dives, whereas severe hypoxia (9%) exposure significantly decreased dive time in both short and long dives. Hypercapnia (5%) had no effect on dive duration, but significantly increased surface interval duration. Mean dive heart rate during short dives (211.8 ± 6.9 beats min⁻¹) was significantly higher than long dives (179.6 ± 5.0 beats min⁻¹) although heart rate during all dives remained above resting levels. Heart rate during diving was unaffected in response to hyperoxia and hypercapnia during both short and long dives, but gradually declined in long dives after severe hypoxia exposure. A faster developing bradycardia to 90 beats min⁻¹ at the end of short dives and to 70 beats min⁻¹ at the end of long dives was evoked when the animals were trapped underwater. There were no significant differences in the response between trap dives after normoxia or hyperoxia. A model of oxygen store utilization during a voluntary dive quantified the critical Pa[sub O₂] that necessitates cardiovascular adjustments in order to meet the metabolic needs of the tissues. During severe hypoxia, the critical Pa[sub O₂] of 21 mmHg was reached 19 seconds into theoretical dives which corresponded with initiation of bradycardia in our severe hypoxia trials. The results suggest that O₂ and CO₂ levels in lesser scaup ducks are managed through changes in diving behavior without any major cardiovascular adjustments occurring throughout the dive. When the animals are pushed to the extremes of their endurance and Pa[sub O₂] levels drop to threshold levels, a chemoreceptor induced bradycardia is evoked to conserve the remaining oxygen in the body.

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