Quadrupedal terrestrial locomotion in emperor penguins (Aptenodytes forsteri)

Birds generally employ bipedal terrestrial locomotion; their wings flap synchronously to provide aerial propulsion. Penguins (Spheniscidae) are non-flyers that use their modified wings (flippers) to 'fly' underwater, again using synchronous action. The family is distributed from the equator to the Antarctic; the earliest penguins lived at temperate latitudes. On land, penguins usually walk bipedally and slowly (waddling) because their legs are short and their bodies long and fusiform. Their flippers are held out sideways and show little movement; they may function like balance poles. A few penguin species breed at high latitude and make substantial terrestrial migrations over snow and ice; they intermittently use an alternative terrestrial locomotory mode (tobogganing) in which they lie on their ventral surfaces and use the feet alternately to propel them more quickly. The emperor penguin Aptenodytes forsteri makes two-way trips (<250 km) to breed, encountering both ice and several types of snow. The topography varies from flat areas to steep slopes and ridges. In this study we analysed 13 publicly available video records and demonstrate that the flippers are also used for propulsion (particularly for climbing in deep snow). Gait analysis reveals that all four limbs are employed in a quadrupedal, ipsilaterally coordinated manner: the classic mode of tetrapod terrestrial locomotion. During climbing, tetrapod limb action is supplemented by use of the beak as an anchor. During quadrupedal locomotion over snow the flippers move snow on their power strokes, thereby demonstrating exchange of forces.