Stroke frequencies of emperor penguins diving under sea ice
2002
SUMMARY During diving, intermittent swim stroke patterns, ranging from burst/coast
locomotion to prolonged gliding, represent potential energy conservation
mechanisms that could extend the duration of aerobic metabolism and, hence,
increase the aerobic dive limit (ADL, dive duration associated with onset of
lactate accumulation). A 5.6 min ADL for emperor penguins had been previously
determined with lactate measurements after dives of <50 m depth. In order
to assess locomotory patterns during such dives, longitudinal acceleration was
measured with an attached accelerometer in 44 dives of seven adult birds
diving from an isolated dive hole in the sea ice of McMurdo Sound, Antarctica.
Detection of wing strokes in processed accelerometer data was verified in
selected birds with analysis of simultaneous Crittercam underwater video
footage. Mean dive duration of birds equipped with the accelerometer and a
time-depth recorder (TDR) was 5.7±2.2 min; 48% of these dives were
greater than the measured 5.6 min ADL (ADL M ). Highest stroke
frequencies (0.92±0.31 Hz, N =981) occurred during the initial
descent to 12 m depth. Swimming effort was reduced to a mean stroke frequency<0.70 Hz during other phases of the dive (while traveling below 12 m depth,
during foraging ascents/descents to and from the sub-ice surface, and during
final ascents to exit). The longest stroke interval (8.6 s) occurred during a
feeding excursion to the undersurface of the ice. In dives>
ADL M , mean stroke frequency during travel segments was
significantly less than that in dives M
( P <0.05). Mean stroke frequency of the entire dive correlated
inversely ( P r =-0.67) and
with mean dive depth ( r =-0.43). Emperor penguins did not exhibit any
significant (>10 s) periods of prolonged gliding during these shallow
(<60 m) foraging dives. However, a stroke/glide pattern was evident with
more than 50% of strokes associated with a stroke interval >1.6 s, and with
lower stroke frequency associated with increased dive duration.
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