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Stroke frequencies of emperor penguins diving under sea ice

R. P. van Dam1, P. J. Ponganis1,*, K. V. Ponganis1, D. H. Levenson1 and G. Marshall2

1 Center for Marine Biotechnology and Biomedicine, Scripps Institution of Oceanography, University of California — San Diego, La Jolla, CA 92093-0204, USA
2 Natural History Unit, National Geographic Television, 1145 17th Street NW, Washington, DC 20036, USA



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  		Fig. 1. Swim strokes of an emperor penguin approaching two through-ice dive holes (bright spots in the video frames). In this view, the bird is swimming toward the dive holes. A and B indicate the horizontal line with greatest luminosity for the video frames shown, as an example of the frame luminosity time-series (upper graph). Open circles show that peaks in the curves generated by video analysis correspond with peaks in the measured longitudinal acceleration (lower graph), allowing for calculation of stroke frequencies from either source.

 


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  		Fig. 2. Depth profile (filled circles) and measured stroke frequencies (open circles) of an emperor penguin. Identified segments of the dive are: A, initial descent; B, travel; C, foraging ascent; D, foraging descent, E, final ascent.

 


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  		Fig. 3. Histograms of swim stroke frequencies for emperor penguins diving with each of the two instrument configurations used in this study.

 


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  		Fig. 4. Correlation of mean stroke frequencies during travel segments to total dive durations for 44 emperor penguin dives. Open circles, dives by birds carrying the Crittercam system (broken line, r=-0.58). Filled circles, dives made with the smaller accelerometer and time-depth recorder instruments (solid line, r=-0.57).

 




© The Company of Biologists Ltd 2002