Incubating a clutch of eggs can be an arduous task, even in the spring. But male emperor penguins probably endure the world's most gruelling brooding conditions, huddled together on the Antarctic ice shelf. How the males survive the extreme conditions has long intrigued biologists. In the 1950s several teams monitored the bird's mass and rectal temperature as they huddled, and in the 1990s André Ancel and Yvon Le Maho estimated their metabolic rate and suggested that `huddling is the key factor for emperor penguins to protect themselves from the cold' says Caroline Gilbert. At the time it seemed that the males benefited from the mild temperatures at the huddle's heart, but more recent studies by Le Maho and Ancel's group have suggested that the birds may benefit in other ways. Together, Gilbert, Ancel, Le Maho and their colleague Stéphane Blanc discuss the current view of the metabolic benefits of huddling (p. 1).

Gilbert describes her experience of working with the penguins at the Dumont d'Urville base in Antarctica as `very special'. `There is still light as the base is close to the Antarctic circle' she explains, but admits that the environment is `very challenging'; the high winds frequently lower the temperature to –40°C. Fitting temperature and light data loggers to huddling birds, Gilbert and her colleagues found that the birds spend 90% of their time either packed deep inside the huddle or shuffling around the edge,at relatively balmy temperatures above –10°C. The team also found previously that the birds benefit further from the huddle's warmth by lowering their body temperature and metabolic rate, saving precious fuel during their 4-month fast.

In this review, the team have calculated the huddler's metabolic savings. Knowing the mass loss pattern of birds huddling in large groups, and comparing it to the mass loss of birds in small groups (10 penguins) and isolated individuals, the team was able to calculate that huddling penguins save 26%more energy than penguins in smaller groups. Curious to know how much of the huddler's energetic savings were due to protection from the cutting wind, the team calculated the energy costs when the birds were exposed to winds at 4.9 m s–1 and found that gathered birds saved 32% more energy than isolated individuals. And when the team calculated the reduction in the birds'metabolic rate, they found that lowering the emperors' body temperature by 1°C contributed between 7% and 17% to their energy savings. Finally, the team considered the effect of reducing the birds' exposed surface area and found that huddling shielded a massive 74% of their surface from the elements,reducing the huddled birds' wind chill metabolic costs to 26% of an unprotected penguin.

Gilbert hopes that this new analysis of the metabolic benefits of huddling`gives a better overview of energy savings' she says. `In the 1950s and 70s,scientists hypothesized intuitively that warmth created inside the groups was the explanation of their energy savings' she says, but adds `we demonstrate here that it is only one component'.

Gilbert, C., Blanc, S., Le Maho, Y. and Ancel, A.(
). Energy saving processes in huddling emperor penguins:from experiments to theory.
J. Exp. Biol.