QUICK SEARCH:   [advanced] Author: Keyword(s):
Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents

First published online May 13, 2004
Journal of Experimental Biology 207, 2101-2114 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00997

This Article Summary Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Ribak, G. Articles by Arad, Z. Search for Related Content PubMed PubMed Citation Articles by Ribak, G. Articles by Arad, Z.

How do cormorants counter buoyancy during submerged swimming?

Gal Ribak¹, Daniel Weihs² and Zeev Arad^1,*

¹ Faculty of Biology, Technion – Israel Institute of Technology, Israel
² Faculty of Aerospace Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel

View larger version (21K): [in a new window]   Fig. 1. (A) Planar view of the left foot of the great cormorant showing the webbed area of the foot (gray) and its span, measured from feet of carcasses. The feet and tarsusmetatarsus (TMT) are rotated 16° clockwise in the lateral (XZ) plane during the stroke. Also shown are points 4 and 5, which were used for kinematic analysis of the foot motion in the trained birds (see text). Point 4 is the joint connecting the tarsusmetatarsus (TMT) with the foot, and point 5 is the tip of digit IV. (B) Planar and (C) lateral views of the great cormorant during swimming (not to scale), showing the digitized points used for the kinematic analysis. Black dots are actual points on the body, red and yellow circles are the glued tags, and the empty circles are points calculated trigonometrically using two other points. Points visible in lateral view are: tip of the bill (1), base of the neck (2), mid-body (3), base of the foot (4), tip of digit IV (5), base of the tail (6), tip of the tail (7) and the center of mass (8). Points visible in the planar view are: tip of the bill (9), base of the neck (10), mid-body (11), tip of digit IV (12), base of the tail (13), tip of the tail (14) and the center of mass (15). Points 16 and 17 in the lateral view are the points of maximum curvature of the ventral and dorsal sides of the body. With point 6, they are used to define the general slope angle of the curves of the back of the body. Also shown are the division of the body's long axis into three subunits (analyzed separately in the kinematic analysis), and the definition of the tilt angle of the tail (T), body (B) and neck (N), relative to the mean swimming direction (red, dotted arrows).

View larger version (22K): [in a new window]   Fig. 2. The instantaneous horizontal speed (U; calculated from the displacement of point 11 in Fig. 1B) of the great cormorant during an average paddling cycle. Each symbol represents the instantaneous speed of one bird (mean of five runs). The solid line (black squares) is the mean of all birds (N=9). The acceleration of the body (power phase) occurs during most of the stroke phase (first six fields) followed by a deceleration until the next stroke phase. The X axis is the proportion of the cycle duration normalized by dividing the period of each phase (stroke, glide and recovery) by the mean duration of that phase (see text).

View larger version (30K): [in a new window]   Fig. 3. Tilting angles of the body, tail and neck (B, T and N, respectively) during the paddling cycle of a swimming great cormorant. Angles are relative to the mean swimming direction (see text). The angles are 0 throughout the paddling cycle. B and T are in anti-phase during the stroke phase, and T remains at a steeper angle than B during the glide and recovery phases. Each data point on the curve is the mean ± S.E.M. of all birds (N=9). The X axis is the same as described in Fig. 2 and in the text.

View larger version (35K): [in a new window]   Fig. 4. The position of the digitized points representing the body of the great cormorant, relative to the mean swimming direction. (A) The center of mass of the bird demonstrated no significant deviations from the swimming direction in the Z direction (right/left). (B) Deviations in the Y direction (up/down) of three points on the body. Positive values are upward movements, and negative values are downward movements. The points are at anterior (base of neck), center (center of mass) and posterior (base of tail) locations on the body unit (points 2, 8 and 6 in Fig. 1C). All points descended during the stroke phase and ascended during the glide and recovery phases. The base of the neck (most anterior point) follows the pattern of the center of mass, indicating that the entire body is descending and ascending. The larger amplitude of the base of the tail can be attributed to the tilt of the body (rotation). The deviations are relative to the mean direction of the birds, calculated as the straight line connecting the position of point 3 in Fig. 1C (for the XY plane) or point 11 in Fig. 1B (for the XZ plane) at the beginning and end of the paddling cycle. The red broken arrows mark these directions. Each data point is the mean ± S.E.M. of all birds (N=9). The X axis is the same as described in Fig. 2.

View larger version (25K): [in a new window]   Fig. 5. Deviations in the Y direction (up/down) of points on the neck. The deviations are relative to the mean swimming direction of the birds (red broken arrow). The Y axis is the same as described for Fig. 4B, and the X axis is the same as described for Fig. 2. The points are the tip of the bill and the base of the neck. Several points are missing at the end of the cycle, as the birds exited the view of the camera. It is clear, however, that the tip of the bill moves differently from the body (represented by the base of the neck), ascending when the body descends during the stroke phase and vice versa during the end of the glide and the recovery phase.

View larger version (25K): [in a new window]   Fig. 6. Foot kinematics (left foot) of the great cormorant during the stroke phase, relative to still water. Points shown are the base of the foot and the tip of digit IV (points 4 and 5 in Fig. 1) and a reference point on the body (filled circle). Swimming direction is left to right. (A) The motion in the XZ plane during an average stroke. The position of the body is represented by a point on the ventral body mid-line. (B) The motion in the XY plane of points 4 and 5 of Fig. 1 and the center of hydrodynamic forces of the foot, located at two-thirds of the distance between points 4 and 5 (see text). The body is represented by the center of mass (point 8 in Fig. 2). Numbers adjacent to data points denote field numbers (fields are separated by 0.02 s intervals, starting at 1 as the first field of the stroke phase). The two views (XZ, XY planes) are averages analyzed from different paddling sequences due to limitations of the setup (see text). The two sequences were synchronized by matching the first fields of the stroke phase and resetting the values on the X axis in the first fields of the stroke phase.

View larger version (26K): [in a new window]   Fig. 7. The back-sweep arch of the foot of the great cormorant, relative to the body, in the XY plane (red symbols). The surface of the foot is represented by the center of hydrodynamic forces (see text). Each point is the mean from all birds (N=9). The points shown belong to the stroke phase and are spaced by 0.02 s intervals (the time frame between fields) and sequenced by the adjacent numbers. The position of the foot is presented relative to the position of the center of mass located at point (0,0) and marked by `x'. The bird swimming direction is from left to right. During the stroke phase, the body is at maximal tilt, as indicated by the orientation of the red figure. The arch of the foot trajectory is vertical, partly due to the fact that the body is tilted. The vertical motion relative to the center of mass is in the first 0.12 s of the stroke phase (fields 1–6). The white open symbols are the same data as the red but rotated at 15° counter-clockwise to demonstrate a hypothetical foot trajectory when the cormorant body aligns with the swimming direction (orientation of the white figure).

View larger version (33K): [in a new window]   Fig. 8. An analysis of the average trajectory of the foot of the great cormorant relative to still water and the resulting hydrodynamic forces (the same data as in Figs 6, 7). Black squares mark the position of the center of hydrodynamic forces of the foot in the XY plane. The positions of this point were used to calculate the trajectory, speed and angle of attack (AoA) of the foot. Red lines connect between points 4 and 5 in Fig. 1 and represent the webbed surface of the foot in the XY plane, demonstrating the AoA between this surface and the foot trajectory. The motion relative to still water, calculated for the center of hydrodynamic forces, fitted a circle (black line) with a radius of 6.6 cm. The speed (red) and the AoA (black) are aligned with each relevant point in the table to the left. Motion starts from the bottom and follows the arch of the circle up. The values of AoA and speed are organized in bottom-to-top order as well. Lift forces (green arrows) will be directed at a perpendicular angle to the foot trajectory and, in this case, toward the center of the circle. Drag (blue) is directed at the opposite direction to the motion of the feet, and in this case the inertial force (black) is in the same direction as drag. Forces smaller than 0.5 N were omitted from the figure.

View larger version (20K): [in a new window]   Fig. 9. The propulsive forces (drag, lift and inertia combined) generated by the feet of the great cormorant during the stroke phase (fields 1–6). Each blue line is the mean force vector from all the birds for a specific field. Fields are separated by 0.02 s time intervals and sequenced by adjacent numbers. Forces' magnitude and direction are estimated according to the model (see text) using low aspect ratio wing calculations. Values include the forces of both feet. The overall propulsive force (the resultant vector of fields 1–6; in red) is directed at 44° below the actual swimming direction of the birds, thus contributing a large vertical component to aid in resisting buoyancy. The model accounts for motion in the XY plane and disregards the contribution of lateral motion. The red arrow is the swimming direction of the birds. Variation (S.E.M.) of the vectors' direction and magnitude is mentioned in the table.

View larger version (24K): [in a new window]   Fig. 10. The contribution of hydrodynamic forces to buoyancy offset during the entire paddling cycle. Presented are the vertical components of hydrodynamic lift from the body (blue line) and from the tail (black line) and the vertical forces generated by the feet (green line), as calculated by the model described in the text. Values are the means ± S.E.M. of all birds (N=9). Also shown is the average buoyancy estimated from measurements on the carcasses (broken line). The net vertical force (red line) is calculated by subtracting the value of the vertical hydrodynamic forces from the positive buoyancy of the carcasses. The X axis is as described in Fig. 2.