Pitching stability of diving seabirds during underwater locomotion: a comparison among alcids and a penguin

Abstract Background In animals, recoil motion resulting from underwater propulsion can destabilise trajectory and decrease locomotory performance. The posture of diving seabirds fluctuates simultaneously with their appendage beat, especially in the vertical plane, because of the recoil force of the beat. Seabirds that fly and swim face trade-offs between maximising their locomotory performance in one medium relative to the other, and flightless penguins were hypothesised to have higher underwater pitching stability than alcids that fly and swim. To test this hypothesis, we investigated the in situ pitching stability of three species of diving seabirds, including a penguin, Pygoscelis adeliae, and two species of alcids, Cerorhinca monocerata and Uria lomvia. A high-resolution gyroscope data logger was attached to the back of each bird and recorded the angular velocity of the body during the descent phase of dives. Results For all three species, the root mean square (RMS) of the angular velocity, which indicated the level of angular fluctuation, was negatively correlated with the depth. Many factors, such as the dorsoventral acceleration resulting from wing beat, dive angle, speed, and current depth, as well as the maximum depth in each dive, significantly affected the angular velocity RMS. The angular velocity RMS at a given depth (e.g. 5 and 10 m) significantly increased with the maximum depth of the dives, suggesting buoyancy regulation relative to the target depth to reduce the destabilising angular momentum in all three species. During entire descent periods, the angular fluctuation was generally lower in P. adeliae than in the two species of alcids, supporting the hypothesis of a higher pitching stability in penguins. Furthermore, the angular fluctuation of U. lomvia was lower than that of C. monocerata at deeper depths, suggesting higher pitching stability and more efficient underwater locomotion in U. lomvia. Conclusions This study demonstrated a difference in the pitching stability, which is an important component of underwater locomotory efficiency, of a penguin and two alcid species while diving freely in natural conditions. In situ angular fluctuation data obtained by gyroscope would be useful to understand the locomotory strategy of swimming animals.