Precise monitoring of porpoising behaviour of Adelie penguins determined using acceleration data loggers

A new method using acceleration data loggers enabled us to measure the porp oising behaviour of Adelie penguins (Pygoscelis adeliae), defined as a cont inuous rapid swimming with rhythmic serial leaps. Previous hydrodynamic mod els suggested that leaping would be energetically cheaper when an animal sw ims continuously at depths of less than three maximum body diameters below the water surface. In the present study, free-ranging Adelie penguins leapt at a mean speed of 2.8 m s(-1) above the predicted threshold speed (0.18-1 .88 m s(-1)). Wild penguins reduced drag by swimming deeper (0.91 m) and di d not swim continuously within the high-drag layer while submerged. This in dicates that previous calculations may be incomplete. Moreover, leaps repre sented an average of only 3.8% of the total distance travelled during the p orpoising cycle, which would make energy savings marginal. Among the six pe nguins used in our study, two did not porpoise and three porpoised for less than 7 min, also indicating that this behaviour was not important during t ravel to and from foraging sites, as has been previously suggested, Birds m ainly porpoised at the start and end of a trip. One explanation of porpoisi ng might be an escape behaviour from predators.