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First published online September 5, 2008
Journal of Experimental Biology 211, 3009-3019 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.018895

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Consequences of buoyancy to the maneuvering capabilities of a foot-propelled aquatic predator, the great cormorant (Phalcrocorax carbo sinensis)

Gal Ribak^1,*, Daniel Weihs² and Zeev Arad¹

¹ Department of Biology, Technion, Haifa 32000, Israel
² Faculty of Aerospace engineering, Technion, Haifa 32000, Israel

View larger version (12K): [in this window] [in a new window]   Fig. 1. A diagram of the different levels of the obstacle course used to establish the maneuvering capabilities of cormorants. The numbers on the right are the horizontal distances between adjacent barriers (thick vertical lines). The vertical barrier in the center of all levels is B0.5 (see text), which was kept at a fixed position inside the channel. The two neighboring barriers that were moved closer between trials are B0 and B1, respectively. The red line represents the trajectory of the birds through the course.

View larger version (5K): [in this window] [in a new window]   Fig. 2. A representation of the body of cormorants illustrating the points on the body that were digitized in the side-view video (see text for details). Axes depicted on the body with a broken line represent the morphological horizontal and vertical axes. The solid line represents the lateral axis and is shown here to illustrate the terminology used to describe rotations and moments in the vertical plane (pitch).

View larger version (7K): [in this window] [in a new window]   Fig. 3. Mean ± s.d. (N=8) of the observed turning radius for each maneuvering level. The data are for the center of the maneuver when the birds are passing above B0.5 (see Materials and methods for further details).

View larger version (4K): [in this window] [in a new window]   Fig. 4. Mean ± s.d. swimming speeds of the great cormorant (N=8) inside the obstacle course. Data are plotted as a function of the mean turning radius in each difficulty level (see Fig. 3). Squares denote the present study on vertical maneuvers. The triangle denotes data from Ribak et al. (Ribak et al., 2005) for the same birds during unobstructed swimming in a straight line (turning radius=).

View larger version (38K): [in this window] [in a new window]   Fig. 5. Mean pitch angles of the body (B), tail (T) and neck (N) as a function of position of the bird (N=8) inside the obstacle course. (A) Figure of the bird illustrating the different angles. Angles below the horizontal are considered negative. In the graphs each symbol represents the different level of difficulty of the obstacle course. The number in the legend denotes the horizontal distances (cm) between vertical barriers (Fig. 1). The horizontal axis was normalized by dividing it by the distance between the two extreme barriers and shifting the horizontal position of the first barrier to zero. As a result the vertical barriers overlap in all of the maneuvering levels (vertical black lines). Error bars are ± s.d.

View larger version (24K): [in this window] [in a new window]   Fig. 6. Mean pitch angle of the tail (A) and neck (B) relative the long axis of the body of the birds (N=8) as a function of the position of the birds inside the obstacle course. Different symbols denote the different difficulty levels of the obstacle course. The legend describes these levels as the horizontal distance between adjacent barriers (cm) (Fig. 1). Vertical arrows denote the position of the barriers. The horizontal axis was normalized by dividing it by the distance between the two extreme barriers and shifting the horizontal position of the first barrier to zero. Error bars are ± s.d.

View larger version (21K): [in this window] [in a new window]   Fig. 7. Occurrence of foot propulsion as a function of position inside the obstacle course. Each chart represents a maneuvering difficulty level labeled according to the horizontal distance between the adjacent barriers (cm) (Fig. 1). Black bars denote the adjusted horizontal positions within the obstacle course where active paddling (stroke) was observed in the video. Note that the birds are less likely to paddle while entering the obstacle course (horizontal axis=0) and more likely to paddle above the second barrier (horizontal axis=0.5). Vertical arrows denote the positions of the barriers (0, 0.5 and 1.0) on the adjusted horizontal axis.

View larger version (22K): [in this window] [in a new window]   Fig. 8. The turning moments generated by the tail (solid black circles) and neck (open circles) as a function of the birds' position inside the obstacle course. The data are means ± s.d. from eight birds passing through the obstacle course at the highest difficulty level (0.36 m between the barriers, Fig. 1). Note the order of magnitude difference in the scale of the vertical axes. Red symbols are the angular acceleration of the body, which correlates with the moment of the tail. Vertical arrows denote the position of the vertical barriers. The horizontal axis is the same as in Figs 5, 6, 7 to allow equivalent evaluation.

View larger version (14K): [in this window] [in a new window]   Fig. 9. The forces produced normal to the swimming direction as a function of the position inside the obstacle course. The forces are produced by the body, tail and neck moving at an angle of incidence to the flow. The columns are means of the forces from eight birds passing through the obstacle course at the highest difficulty level (0.36 m between barriers, see Fig. 1). The red line is the sum of the three forces. Positive values are directed dorsally, and negative values are directed ventrally. Vertical arrows denote the position of the barriers. The horizontal axis is the same as in Figs 5, 6, 7, 8 to allow equivalent evaluation.

View larger version (11K): [in this window] [in a new window]   Fig. 10. Normal forces during vertical maneuvers of great cormorants as a function of the position inside the obstacle course. Each black line is the net normal force generated by a single bird (out of N=8 birds). The net force is calculated by summing the forces normal to the instantaneous swimming direction from the motion of the body, tail, neck and foot propulsion. The forces are calculated for the birds passing through the obstacle course at the highest difficulty level (0.36 m between barriers, see Fig. 1). Filled and open columns denote centrifugal force and the component of buoyancy that is normal to the swimming direction, respectively. These forces are estimated based a mean body mass of 2 kg and the average instantaneous swimming speed, swimming direction and turning radius (see text). All forces are positive when they are directed above the bird's trajectory and negative when they are directed below it. The vertical arrows point to the position of the barriers on the adjusted horizontal axis. The horizontal axis is the same as in Figs 5, 6, 7, 8, 9 to allow equivalent evaluation.

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