RESUMO
Lesser scaup ducks were trained to dive for short and long durations following exposure to various gas concentrations to determine the influence of oxygen (O2) and carbon dioxide (CO2) on diving behavior and heart rate. Compared with normoxia, hyperoxia (50% O2) significantly increased the duration of long dives, whereas severe hypoxia (9% O2) significantly decreased the duration of both short and long dives. Hypercapnia (5% CO2) had no effect on dive duration. Surface intervals were not significantly altered by the oxygen treatments, but significantly increased following CO2 exposure. Heart rate during diving was unaffected by hyperoxia and hypercapnia, but gradually declined in long dives after severe hypoxia. Thus, our results suggest that during the majority of dives, O2 and CO2 levels in lesser scaup ducks are managed through changes in diving behavior without any major cardiovascular adjustments, but below a threshold PaO2, a bradycardia is evoked to conserve the remaining oxygen for hypoxia sensitive tissues. A model of oxygen store utilization during voluntary diving was developed to estimate the critical PaO2 below which bradycardia is initiated (approximately 26 mmHg) and predicted that this critical PaO2 would be reached 19s into a dive after exposure to severe hypoxia, which corresponded exactly with the time of initiation of bradycardia in the severe hypoxia trials.