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Cardiac physiology and diving behaviour of double-crested cormorants (phalacrocorax auritus) Enstipp, Manfred Roland

Abstract

Heart rate and dive behaviour were monitored in adult double-crested cormorants during shallow and deep diving and after exposure to different breathing gas mixtures to investigate the role of intravascular chemoreceptors in cardiac and behavioural control during voluntary diving. A data logger was used to record heart rate and dive behaviour of cormorants diving within a shallow (1 m) and deep (13 m) dive tank. Pre-dive heart rate in both shallow and deep diving birds was about three times the resting heart rate, falling abruptly upon submersion to around 200 - 250 beats-min⁻¹. During shallow diving most birds showed a secondary drop in heart rate after 5 - 10 s into the dive to around the resting level. In contrast, during deep diving heart rate stabilised at the initial bradycardie level or decreased further only very slightly. Mean dive heart rate (± S.D.) was significantly lower during shallow diving (163.2 ± 14.0 beats-min"1) compared to deep diving (216.4 ± 7.7 beats-min"1), but in both cases was significantly above the resting value (137.9 ± 17.5 beats-min⁻¹). Exposure to a hyperoxic gas mixture before shallow diving significantly increased mean dive heart rate, while exposure to a hypoxic gas mixture in both the shallow and deep dive tank significantly reduced mean dive heart rate. In contrast, hypercapnic gas before diving had no significant effect on dive heart rate. These results suggest that the cardiac response to voluntary diving in double-crested cormorants is strongly influenced by changes in arterial oxygen tension (Pao₂) throughout the dive. Dive duration was unaffected by alterations in inspired gases, but surface interval duration decreased after hyperoxia and increased after hypoxia. The most efficient dive pattern (highest dive/pause ratio) was observed after hyperoxic exposure. To investigate the ontogeny of the cardiac response to voluntary diving, heart rate was also recorded from naive double-crested cormorant chicks. The cardiac response to first ever and subsequent voluntary submergence was similar to the response observed in adult cormorants. Heart rate was also monitored in a separate group of chicks in which the first exposure to water was during whole body forced submergence. Again, the observed response was similar to the adult forced submergence response, although the cardiac response of chicks to forced submergence was more extreme than in voluntary submergence. These results do not support the hypothesis that cormorants 'learn' the appropriate cardiac response to voluntary diving via habituation or conditioning of the ' classical dive response'.

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