DIVING ENERGETICS IN LESSER SCALE (AYTHA-AFFINIS EYTON)

Mechanical and aerobic energy costs of diving were measured simultaneo usly by closed-circuit respirometry in six lesser scaup Aythya affinis Eyton (body mass=591+/-30 g) during bouts of voluntary feeding dives. Durations of dives (t(d)=13.5+/-1.4 s) and surface intervals (t(i)=16 .3+/-2.2 s) were within the normal range for ducks diving to 1.5 m dep th. Mechanical power output (3.69+/-0.24 W kg(-1)) and aerobic power i nput (29.32+/-2.47 W kg(-1)) were both higher than previous estimates. Buoyancy was found to be the dominant factor determining dive costs, contributing 62 % of the mechanical cost of descent and 87 % of the co st of staying at the bottom while feeding. Drag forces, including the contribution from the forward-moving hindlimbs during the recovery str oke of the leg-beat cycle, contributed 27 % and 13 % of the mechanical costs of descent and feeding, respectively. Inertial forces created b y net acceleration during descent contributed approximately 11 % durin g descent but not at all during the feeding phase. Buoyant force at th e start of voluntary dives (6.2+/-0.35 N kg(-1)) was significantly gre ater than that measured in restrained ducks (4.9+/-0.2 N kg(-1)). Loss of air from the plumage layer and compression due to hydrostatic pres sure decreased buoyancy by 32 %. Mechanical work and power output were 1.9 and 2.4 times greater during descent than during the feeding phas e. Therefore, energetic costs are strongly affected by dive-phase dura tions. Estimates by unsteady and steady biomechanical models differ si gnificantly during descent but not during the feeding phase.