For a period of 6 months, a group of eight tufted ducks (‘control’ ducks) were kept on a shallow outdoor pond and performed short dives to obtain food (maximum depth, 0.65 m; observed mean duration, 10.9 ± 0.54s). At the same time, a group of seven tufted ducks (‘dive-trained’ ducks) were kept on an adjacent deeper and partly covered pond, and performed ‘extended’ dives under the surface mesh in order to feed (maximum distance to food, 10m; observed mean dive distance, 6.0 ± 0.25 m; observed mean duration, 24.8 ± 0.58 s). At the end of this time, the calculated total usable oxygen store was approximately the same in control and dive-trained ducks (44 and 42mlO2STPDkg−1, respectively), although the relative quantities of usable oxygen in each of the three main storage sites (respiratory system, blood and skeletal muscle) differed between groups.

The end-expiratory lung/air sac volume was found to be significantly smaller (P<0.01) in the dive-trained ducks (165mlBTPskg−1) than in the control ducks (232mlBTPs−1). The dive-trained ducks, however, had a significantly greater (P<001) blood volume (141mlkg−1) than the control ducks (lO7mlkg−1), although the blood oxygen capacity and several haematological indices (measured haemoglobin content, red blood cell count and haematocrit, and calculated mean corpuscular haemoglobin and mean corpuscular haemoglobin concentration) were statistically the same in both groups. Mean corpuscular volume was significantly greater (P<0.05) in the dive-trained ducks. The myoglobin content of the myocardium was the same in both groups. The pectoralis muscle and the locomotory leg muscles, however, contained significantly higher concentrations of myoglobin in the dive-trained ducks than in the control ducks (pectoralis, P<0.05; lateral gastrocnemius and semitendinosus, P<0.01)<001).

It is suggested that the anatomical adaptations which occur in response to chronic increases in diving activity may increase the aerobic diving capacity of the tufted duck by effecting a decrease in buoyancy (reduced end-expiratory lung/air sac volume) and an increase in blood oxygen storage capacity (hypervolaemia). Locomotory muscle function may be maintained in the face of decreasing oxygen delivery during extended dives by means of increased myoglobin content after dive-training.

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