For an Emperor penguin looking to survive the worst of an Antarctic winter, there is no such thing as personal space. Colonies of the tubby birds are known to huddle up close, feather to feather, in groups of thousands to keep out the cold.
For the first time, researchers show that these gatherings are carefully organized structures, in which single penguins can trigger mass movements within the group that resemble the movements of cars in a traffic jam.
"If one penguin starts a wave," perhaps shuffling too close to its neighbor, "it travels in both directions, like a Mexican wave in a football stadium," said Daniel Zitterbart, a physicist at the University of Erlangen-Nuremberg in Germany who filmed and then analyzed the movements of the penguin herds.
Colonies of Emperor penguins assemble in formation when the temperatures drop below zero degrees Fahrenheit (-18 degrees Celsius), using each other’s bodies to keep warm and avoid the wind. The parents have to worry not only about themselves, but also about the eggs that they incubate with their feet.
"From a penguin perspective, you are as near [a neighbor] as possible, but not too near that you compress your feathers," Zitterbart said. At temperatures nearing -60 degrees F (-50 degrees C), Zitterbart has seen penguins stay in formation for up to nine hours at a time.
Every time Zitterbart watched the penguins, they arrived at an arrangement that physicists recognize as a "triangular lattice structure," with a penguin at each point of the triangle. They gather by gut instinct, but this also happens to be the most compact arrangement for "round-shaped" objects such as the well-layered birds, Zitterbart said.
In the New Journal of Physics, Zitterbart and his colleagues explain that the movements of a single bird within the huddle can set off a ripple effect that travels through the rest of the group.
That movement is similar to what happens when a single car in a traffic jam inches forward, setting off a chain reaction of tiny movements by all of the cars behind it. But there's a difference: Only the first car in the jam-up can start off the group movement. On the ice, any penguin in the huddle can trigger a wave of small movements.
"This is one case where a traffic jam is very useful," Zitterbart said.