Strouhal number analysis for the swimming of little penguin (Eudyptula minor): Proof of efficient underwater propulsion

Abstract

Penguins are the utmost specialized wing-propelled divers as their flapping action is accountable for the entire underwater propulsion. However, numerical evidence of how efficient their underwater propulsion is has not been studied to date. Literature sources suggest that propulsive efficiency peaks when the Strouhal number (St) ranges between 0.2 and 0.4. Therefore, in this study, using field data for free-ranging Little penguins, we estimated the range of Strouhal numbers for Little penguins and also investigated how this range varies with the shape and depth of dives to better understand the behavioural adaptations for underwater swimming. Moreover, it was argued by some researchers that the St value is not sufficient to ensure higher propulsive efficiency because St estimates are based on assumptions that might be incorrect; therefore, to evaluate the reliability of the St estimate, we also calculated the Reynolds and Swimming numbers. Those estimated values precisely designate that, as anticipated, penguins employ efficient propulsion mechanisms.