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First published online October 21, 2004
Journal of Experimental Biology 207, 4057-4065 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01265
Why do macaroni penguins choose shallow body angles that result in longer descent and ascent durations?
t Charrassin2
1 National Institute of Polar Research, 1-9-10 Kaga, Itabashi, Tokyo
173-8515, Japan
2 Département Milieux et Peuplements Aquatiques, Muséum
National d'Histoire Naturelle, 43 rue Cuvier, 75231 Paris Cedex 05,
France
3 Centre d'Ecologie et de Physiologie Energétiques, Centre National
de la Recherche Scientifique, 23 rue Becquerel, F-67087 Strasbourg,
France
* Author for correspondence at present address: International Coastal Research Center, Ocean Research Institute, University of Tokyo, 2-106-1 Akahama, Otsuchi, Iwate 028-1102, Japan (e-mail: katsu{at}wakame.ori.u-tokyo.ac.jp)
Accepted 27 August 2004
It is generally assumed that air-breathing aquatic animals always choose the shortest route to minimize duration for transit between the surface and foraging depth in order to maximize the proportion of time spent foraging. However, empirical data indicate that the body angles of some diving animals are rarely vertical during descent and ascent. Why do they choose shallower body angles that result in longer descent and ascent durations? To investigate this question, we attached acceleration data loggers to eight female macaroni penguins, breeding on the Kerguelen Islands (48°45'–50°00'S, 68°45'–70°58'E; South Indian Ocean), to record depth, two-dimensional acceleration (stroke cycle frequency and body angle) and temperature. We investigated how they controlled body angle and allocated their submerged time. The instrumented females performed multiple dives (N=6952) with a mean dive depth for each bird ranging from 24.5±28.5 m to 56.4±75.1 m. Mean body angles during descent and ascent were not vertical. There was large variation in mean descent and ascent angles for a given dive depth, which, in turn, caused large variation in descent and ascent duration. Body angles were significantly correlated with time spent at the bottom-phase of the dive. Birds that spent long periods at the bottom exhibited steep body angles during ascent and subsequent descent. By contrast, they adopted shallow body angles after they had short or no bottom phases. Our results suggest that macaroni penguins stay at the bottom longer after encountering a good prey patch and then travel to the surface at steep body angles. If they do not encounter prey, they discontinue the dive, without staying at the bottom, ascend at shallow body angles and descend at shallow body angles in a subsequent dive. A shallow body angle can increase the horizontal distance covered during a dive, contributing to the move into a more profitable area in the following dive. During the ascent, in particular, macaroni penguins stopped beating their flippers. The buoyantly gliding penguins can move horizontally with minimum stroking effort before reaching the surface.
Key words: dive, acceleration, data logger, stroke, buoyancy, gliding, horizontal transit, penguin, Eudyptes chrysolophus
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