This study investigates the functional anatomy of the flipper of the jackass penguin (Speniscus demersus). The flippers of birds (n = 15) were dissected and described. Several adaptations from the typical avian wing were noted and reasons were proposed for these. The conclusion is that the osteological and myological adaptations of the flipper are designed to enable the penguin to progress very effectively through water, while the vascular adaptations provide a highly efficient mechanism for thermoregulation.