In the 2005 documentary March of the Penguins, there’s a scene that shows a group of male emperor penguins hunkered down through an Antarctic storm. The birds are incubating eggs, balanced on their feet, while the females have gone off to feed. To stay warm, the penguins huddle together, rotating from the inside to the outside of the huddle, and back again, to make sure that no one gets too cold — a seemingly simple solution to keep everyone cozy.
It turns out, huddles are far more complicated than that.
Or at least, the huddles that form among Antarctica’s Pointe Géologie Archipelago colony are. André Ancel of the University of Strasbourg in France and colleagues studied the 3,000 or so breeding pairs that live in this colony during the 2005, 2006 and 2008 breeding seasons, counting birds and recording their actions with pictures and video. Huddles, they found, are only temporary arrangements lasting a few hours at most. And a single penguin can break up the group in less than two minutes, the researchers report in the December Animal Behaviour.
Emperor penguins aren’t the only animals that huddle, but they may be the best at it. Densities can reach as high as eight to 10 birds per square meter, and this behavior helps them survive the extreme cold and wind found in Antarctica. Jamming together lets the birds conserve heat, but they can easily generate too much. As the birds breathe out, the air around them can reach temperatures as high as 37.5° Celsius, well above the 20° C upper limit for the birds’ comfort. The growth and decay of huddles occurs because of the need to manage this heat, Ancel and colleagues contend.
As the air temperature decreases, birds gather together in small huddles, looking for the nearest source of warmth. If there is less solar radiation or winds kick up, the birds take their time to find a big huddle and join the group. But huddles may not last long. The researchers recorded some huddles that lasted several hours, but others broke up after only a dozen minutes. On average, they found, huddles broke up after only 50 minutes.
Ancel and his team had hypothesized that huddle breakups would most often be initiated from the center, where penguins were the warmest. But that only happened once. More often, it was an individual near the edge that initiated the breakup — usually as he departed, but in one case two penguins started it by getting into a fight. Within two minutes of initiation, the breakup was complete, and the huddle dispersed.
The researchers think that the huddle breakups help to dissipate heat. “The breakup of huddles is sometimes accompanied by a haze of warm air rising over the colony, which indicates a significant dissipation of heat,” they note. And some birds that leave a huddle have been seen eating snow, possibly because they needed to cool down after being in the group.
Scientists once thought that the penguin huddle was simply a way to conserve energy, but it may be that it’s more complex, a way to thermoregulate through social interaction. Ancel and colleagues caution, though, other factors may be involved for emperor penguins elsewhere in Antarctica.