Power and Speed of Swimming Dolphins

Abstract

The belief that dolphins have an extraordinary ability to generate propulsive power or can modify the water flow over their bodies to the low-resistance laminar flow is pervasive in the perception of the animals by the public and by some of the biology communities. I analyzed measured swimming speeds for dolphins of the Stenella-Delphinus morphology. A conservative hydrodynamics model equivalent to 119% of the drag of a rigid fusiform body (the added 19% compensates for the drag induced by propulsion) and a metabolic rate of 13.4 times the projected resting metabolic rate are CQIJsistent with measured maximum steady-state speeds. Measured routine swimming speeds are also consistent with this same hydrodynamics model and with metabolic rates for routine activity of other swimming and terrestrial homeotherms. Energy expenditure for swimming dolphins is entirely within expected ranges and no extraordinary mechanisms are necessary to explain observations. The streamlined shape and smooth skin of dolphins indicate that swimming energetics played a major role in the evolution of the group. Misconceptions about power requirements and speeds of dolphin swimming have been due primarily to errors in estimating swimming speeds (Lang and Norris, 1966) in conjunction with observations of non-sustainable burst speeds. These misconceptions are exemplified by an apparent paradox presented by Gray (1936): any object which moves through the water as fast as dolphins are reported to swim must have the high-drag turbulent form of £low about them; dolphins do not have the power to swim at the reported speeds if the flow over their bodies is turbulent; they can swim at the reported speeds if the flow is of the low-drag laminar form but there is no known mechanism to change turbulent flow to laminar. Because dolphins have a mysterious and romantic image, this paradox pervades the public awareness of dolphins and also is found among biologists. Most analyses of dolphin swimming have used generalized models (Blake, 1983a) which may not apply to specific cases or have related the hydrodynamics power requirements and metabolic power production at maximum burst speeds (see Gray, 1968 and Lang, 1974 for review). Measurement or computation of these parameters at maximum burst speeds is difficult and imprecise because the transient nature of bursts permits use of specialized mechanisms. Accelerating swimming motions differ from sustained swimming motions (Weihs and Webb, 1983), and high-power, short-duration bursts can be supported by anaerobic metabolism in contrast to the aerobic metabolism of sustained effort (Bartholomew, 1977). Gray (1968) recognized that duration is an important factor but data for metabolically steady swimming speeds were lacking. However, now it is possible to describe dolphin swimming using current understandings of hydrodynamics, metabolism, and measured, metabolically steady swimming speeds. I have divided this presentation into three parts: measurements of swimming speed, hydro-dynamics, and metabolic power. Using conservative estimates and assumptions, I generate estimates of power requirements and power production for dolphins of the Stenella-Delphinus morphology, a group homomorphic in the necessary hydrodynamic aspects of general shape and size (see Nowak and Paradiso, 1983), while swimming at routine and maximum steady-state speeds. Comparing the hydrodynamics model and the metabolic model at two separate swimming speeds (instead of just one) reduces the probabilities that the models are erroneous but also coincident. MEASURED SWIMMING SPEEDS Maximum Steady-State Speeds Accurate measurements of maximum, sustainable, steady-state speeds of swimming dolphins are rare. However, Au and Perryman (1982) report measured sustained swimming speeds of