Anyone who's watched "The March of the Penguins" knows that Emperor penguins huddle together to cope with the harsh temperatures and winds of the Antarctic winter. It's a great deal for the birds inside the tightly packed scrum, but how do the penguins on the periphery get their turn?
Researchers spent a whole winter in 2008 tracking the movements of an Emperor penguin colony at Dronning Maud Land in Antarctica, and they present their answer this week in the open-access journal PLoS ONE. It turns out that the penguins engage in a series of continuous, coordinated shuffles that cause the birds on the outside to shift toward the interior, and push other birds toward the outside.
"Every 30-60 seconds, all penguins make small steps that travel as a wave through the entire huddle," the researchers write. "Over time, these small movements lead to large-scale reorganization of the huddle."
That's right: Penguins know how to do the wave.
The dynamics are so subtle that they're hard to interpret just by looking at the huddle. But when they're recorded on high-definition time-lapse video, the scientists say they can see a "striking analogy" to the movement of fluids such as soft glasses and colloids.
The challenge for the penguins is to huddle together closely enough to conserve body heat, but keep loose enough to avoid "colloidal jamming." That's the physical phenomenon you encounter when you try to pour ketchup out of a jammed-up bottle. (Ketchup is just one of the tasty colloids you might find in your kitchen; others include pudding and ice cream, peanut butter and jelly.)
The researchers say that penguins avoid getting permanently jammed up or squished by doing those little shuffle steps every 30 to 60 seconds. The penguins on the periphery push inward, which is like tapping the side of the ketchup bottle. Inside the huddle, the steps cause the birds to shift around, and the mass starts moving forward. Some penguins who join the huddle at the trailing edge. Others leave the huddle at the leading edge. Separate bunches of the birds flow together into bigger bunches, creating a critical mass. In a news release, the process is compared to kneading dough.
"This is an essential process in condensed matter physics, penguins included," the researchers write. "In further support of the phase transition analogy, we note that when the huddle breaks up, it occurs very rapidly, similar to the sharp jump in densities between ... a gas and liquid state."
Studying the dynamics of the Antarctic huddle could conceivably help scientists develop better models for other types of mass behavior, ranging from fish schools to traffic jams. The researchers note that the physics of traveling waves can be applied to the pushy interactions of panicky humans as well as the subtler shuffles of Emperor penguins.
"Why these waves are uncoordinated, turbulent and dangerous in a human crowd but not in a penguin huddle remain an open question but may possibly depend on the shape and magnitude of the interaction potential, and on the distance of the system from an effective temperature characterizing a critical point," they write. Here's another issue yet to be studied: whether the actions of particular penguins trigger the emergence of the wave, similar to the collective behavior observed in flocks of pigeons.
You just might hear more about huddling penguins in the years to come: The leader of the research team, physicist Daniel Zitterbart from the University of Erlangen-Nuremberg in Germany, is setting up a remote-controlled observatory in Antarctica to study penguins all year round. Who knows? Maybe one of these days we'll be watching a sequel to the earlier documentary, titled "The Wave of the Penguins."
More about penguins:
- Naked penguins baffle scientists
- Gay penguin pair raising chick
- How the penguin changed its feathers
- How penguins survive those deep dives
In addition to Zitterbart, co-authors of "Coordinated Movements Prevent Jamming in an Emperor Penguin Huddle" include Ben Fabry from the University of Erlangen-Nuremberg, James Butler from Harvard University and Barbara Wienecke from the Australian Antarctic Division.
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