Penguin huddles move like traffic jams

Emperor penguins, the largest of all the penguin species, have a unique method of ensuring a successful breeding season despite the harsh Antarctic winter: The males incubate the eggs, tucking them in an abdominal pouch just above their feet. The conditions these birds endure are not for the faint of heart, with temperatures as low as −50° Celsius and hurricane-strength winds of up to 200 kilometers per hour. To stay warm and conserve energy during the breeding season and, well, any time of year, emperor penguins form huddles, with thousands of birds packing in close together.

While balancing an on egg on its feet, a male bird can’t move fast, but it can take small, careful steps. And just one step can set off a wave of motion that passes through the huddle. This wave moves like cars in a traffic jam, say Daniel Zitterbart of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research in Bremerhaven, Germany, and colleagues in a study published December 16 in the New Journal of Physics.

In 2011, Zitterbart and another group of colleagues reported in PLOS ONE that every 30 to 60 seconds, emperor penguins in a huddle take small steps that travel through the group like a wave. “Over time, these small movements lead to large-scale reorganization of the huddle,” they noted. To figure out what initiated those waves, the researchers built a computer simulation of penguin movement.

In their model, penguins acted like cars in a traffic jam, though instead of only moving forward, each bird could move in one of six directions. When a bird takes a single step, that motion results in a cascade of birds each taking a step, with a small time lag between one bird’s action and the next’s. “After the wave has traveled through the whole huddle, the conformation of the huddle is the same as before the wave, but the whole huddle
has now moved one step forward,” the researchers write.

They confirmed their model by comparing their virtual penguins to real ones the researchers had videotaped in Antarctica — one colony near the French research base Dumont d’Urville and another near the German research station Neumayer III.

“Our model exhibits similarities with jammed traffic systems such as the stop-and-go waves and intermittent behavior, but also differences, such as that the waves can travel in all directions through a penguin huddle,” the researchers write.

The research also revealed that, like humans, penguins may have a sense of personal space, or at least an optimal distance for huddling together. They may look like they’re jam packed against each other, but they may only be touching slightly. 

That would prevent the feather layer from being compressed and its powers of insulation from being compromised. But a movement of as little as 2 centimeters is enough for a neighboring penguin to think “too close” and start to move over, and the wave begins.

Any penguin in the group can set off a wave, but what causes that first penguin to move isn’t known. I have a couple ideas, though: Maybe the birds like to keep moving in the cold, and such movement helps to keep them warmer (it works for me). Or perhaps balancing an egg on your feet is even more difficult than it looks.