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MACARONIS STAY WITHIN THE LIMIT
kathryn{at}biologists.com
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The only thing that penguins haven't learned to do since they returned to the sea is breathe under the icy water; they still have to come back to the surface after a foraging dive to breath. Unable to replenish their oxygen supplies while submerged, penguins have adapted to conserve the oxygen from their last breath, to last the duration of a dive, which for some species can be over 20 minutes. Pat Butler is fascinated by the energetics of penguin diving, and he wonders `how they manage to stay under water so long?' Over the years, he has found that many species drop their heart rate and take advantage of the cold to conserve energy and their limited oxygen supply during long dives in their natural environment. So in 1999, his team returned to the Antarctic to see how macaroni penguins deal with the problem, and discovered that unusually, these birds don't appear to use a drop in their body temperature to extend the length of a dive (p. 43).
Jonathon Green worked with the macaroni penguins at the British Antarctic Survey Base on Bird Island. He explains that the birds are `feisty, argumentative and bad tempered, but have a great affection for their [life long] partner'. He adds that they are only easy to approach when they are incubating or brooding their young. Green collected 15 females, and carefully fitted each bird with one of Tony Woakes' tiny data loggers. He says that the birds recovered quickly from their surgery, and soon returned to the water, where the data loggers began recording their diving behaviour, as well as their heart rate and body temperature.
Knowing how the birds' heart rates varied over the duration of each dive, Green estimated the amount of oxygen that the penguin consumed while diving, and found that most of the birds returned to the surface within two minutes, before they exhausted their oxygen stores. Butler explains that when a bird extends the duration of a dive, the penguin reduces its heart rate and body temperature to conserve oxygen until it returns to the surface. When Green looked at the bird's heart rate, it had dropped slightly while the birds were diving. But when he analysed the bird's body temperature in relation to the dive's duration, the birds weren't making use of the drop in their body temperature to extend the length of their dives! Butler admits that this surprised him. The birds get colder over periods when they dived repeatedly, but their ability to stay submerged longer was not improved by cooling.
Butler explains that understanding the energetics of these fascinating birds is a crucial component of the Antarctic's complex ecology. Knowing how much energy an animal uses in the course of a day can be translated into the amount of food that it consumes. With many species already competing to survive in the Antarctic's finely balanced ecosystem, understanding how each creature relates to its environment is essential if we are to minimise man's increasing impact on one of the planet's greatest wildlife sanctuaries.
References
Green, J. A., Butler, P. J., Woakes, A. J. and Boyd, I. L.
(2003). Energetics of diving in macaroni penguins. J.
Exp. Biol. 206, 43
-57.
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