Interactions of body shape, body size and stroke‐acceleration patterns in costs of underwater swimming by birds

Abstract

Summary For birds, mammals and turtles, costs of swimming by foot propulsion are usually much higher than for propulsion by wings or foreflippers. The propulsive efficiency with which limbs impart thrust to the water is greater for lift‐based wings than for drag‐based feet, but different acceleration patterns during oscillatory strokes may also alter total drag on the body fuselage (head and trunk). Because wing propulsion allows thrust on both upstroke and downstroke, whereas foot propulsion in many species (perhaps excepting grebes) has little or no thrust on the upstroke, foot propulsion requires higher speeds during a smaller fraction of the stroke to maintain the same mean speed. Because drag increases non‐linearly with increasing speed, higher instantaneous speeds in drag‐based foot propulsion may cause greater total drag on the body fuselage. Tow‐tank measurements have shown that foot‐propelled birds that swim with long necks extended have lower fuselage drag at high speeds than do wing‐propelled birds that swim with necks retracted. This difference might reduce the higher costs of drag‐based foot propulsion, but such effects must be evaluated in the context of drag at a range of speeds throughout oscillatory strokes. In quasi‐steady models of horizontal swimming underwater, stroke‐acceleration curves for both foot and wing propulsion were applied to a range of bird shapes and sizes. Higher fuselage drag during foot propulsion increased mechanical costs of transport (MCOT, J kg−1 m−1) by 26–40% in various species. Thus, a large fraction of the different costs of wing and foot propulsion might be explained in terms of drag on the body fuselage, independently of the propulsive efficiency of stroking limbs. When drag curves for different body shapes were combined with different oscillatory stroking patterns, swimming with a long neck extended did not reduce the higher total drag associated with drag‐based foot propulsion. Thus, although size and shape can affect drag measured at different constant speeds, effects of drag on locomotor costs depend much more on stroke‐acceleration patterns of different swimming modes.