The effects of accelerational swimming and body size on mechanical energy costs of diving were examined by comparing canvasback (Aythya valisineria Wilson), redhead (A. americana Eyton) and lesser scaup (A. affinis Eyton) ducks (mean body mass 1.275, 1.013 and 0.817kg, respectively) in steady versus unsteady models of locomotion. Steady models assume constant speed, whereas unsteady models incorporate acceleration and deceleration with each propulsive stroke.
The surface areas of the ducks increased linearly with body mass (r2=0.96), with all species falling on the same curve.
Body drag D of frozen ducks towed under water increased with body mass M and speed U according to the relationship
D = −0.946+0.826M+0.614U+0.825U2, r2=0.95.
In kinematic analyses, regression curves of percentage of total stroke distance versus percentage of stroke duration did not vary within individuals, sometimes varied between individuals and always varied between species. In diving ducks, analyses of a single sequence per individual and several individuals per species will usually provide an accurate kinematic description for the species.
The degree of acceleration and deceleration during a stroke increased with decreasing body size among species.
The power phase lasted for 66–70% of stroke duration, and the ducks accelerated for the first 84% of this phase. During the power phase, work against drag was 10–12%, work against buoyancy was 36–38%, and inertial work in acceleration was 49–54° of the total mechanical work done.
Mechanical energy costs of descent, as estimated by the unsteady model, were 47–75% higher than estimates from the steady model. Accelerational stroking had greater effects on descent costs as body size decreased.
Present address: Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071, USA.