Why Penguins Do the Wave, As Explained by Traffic Jams

Emperor penguins stay warm in brutal Antarctic winters by huddling together in huge packs, a fact so well-known it’s been the subject of more than a few movies. Yet as cool as that is, it gets cooler: Because penguins get so tightly packed together, they can appear to move as rippling waves, in what is one of the strangest animal phenomena you can see. Seriously, look at the video above.

Such movement has been thought to be organizational; as a huddle forms, penguins periodically inch forward to help each other cycle through the warm middle and colder edges. Research published in the New Journal of Physics this week demonstrates how such behavior works, using a rather unexpected corollary: traffic jams.

Emperor penguins are the only vertebrate known to breed during the Antarctic winter, where temperatures can hit -50°F, and as such, their movement through warm packs is crucial to the survival of adults and young alike. Previous work has shown that huddling can increase ambient temperature to as much as 37.5°C, or close to adults’ body temperature. 

Later work, published in 2011, first began to explain the source of penguin waves. That paper, which led by Daniel Zitterbart, who’s a co-author on the newest report, showed that “every 30–60 seconds, all penguins make small steps that travel as a wave through the entire huddle. Over time, these small movements lead to large-scale reorganization of the huddle.” In other words, because they’re too tightly packed to move together—in a bid to conserve heat, of course—penguin packs can only move as one. Cool, right?

This video from the authors of the latest paper covers a lot of material. I’d key in on the models shown at the 0:49 mark, which shows how wave propagation is important to maintaining order in the group.

If you’ve ever been stuck in a huge crowd, you’d know just how hard moving anywhere with any sort of organization can be. So why are penguins so good at it? The New Journal of Physics study, led by Richard Gerum of the University of Erlangen-Nuremberg, investigates the movement through fluid dynamics.

Curiously enough, the tendency for groups of individual objects to cluster up has been shown in many systems, and not just ones involving penguins. Because penguins can respond to their surroundings, they’re best described as “self-driven agents.” The authors explain that means they can “actively perceive the positions of their neighbors, process this information and then activate their locomotory system to move into an appropriate direction.”

With that in mind, the team treated the four-foot-tall penguins as individual agents that tend to group up in similar patterns. The reasoning is that they want to be as close to their neighbors as possible to conserve warmth. In fact, in testing of a mathematical model of a huddle, the authors found that any time penguins were irregularly positioned, they were able to quickly reassemble into their optimal, packed huddle shape where heat benefits are largest. And rather than move in unison, they moved in waves.

This figure shows the results of Gerum et. al‘s penguin model. B shows various levels of disorder assigned at the start of the model, and A shows how over time, penguins tend to realign to specific patterns, no matter how disordered their starting point. The authors argue that wave motion is key to maintaining this order.

In broad terms, the authors argue that sounds like another system of tightly-controlled movement we’re all familiar with: the traffic jam. The physics of traffic has been heavily studied, and the authors looked to apply similar models in their quest to explain Emperor penguins’ peculiar waves.

In traffic, movement occurs in spurts because you have to wait for the person in front of you to move before you can, and the delays add up down the line. It’s also exacerbated by drivers’ tendency to pull up as close to the person in front of them as possible, requiring sudden stops and starts, rather than keeping a distance to smooth things out.

Therefore, because of penguins’ tendency to stick to a specific, tight spacing, any disturbance will ripple through the group. 

Imagine yourself as an Emperor penguin for a moment. (Yes, you’re bitchin’.) You’re boxed in by a grid of fellow penguins all around you. The guy behind you, with his egg tucked between his feet, takes a short step to the right, which opens up a gap for cold air to get in, so everyone else around you also moves to the right. Once the now-cold penguins to your right figure out what’s going on, they take a step to warm up again, and so on.

As the authors write, “the resulting cascade of the reorganizations spreads out from the initiating penguin as a wave with a fixed speed and a rectangular-shaped front.”

Any penguin within the huddle occasionally performs a step. This locally disturbs the triangular configuration of the huddle and triggers each of the neighboring penguins to also perform—after a small time-delay—a single step. This delay depends on the actual distances between the penguins, the threshold distance and the speed of the movements. 

It’s fairly intuitive when you really run through it in your brain box: Large groups of objects, like cars on the freeway or penguins in Antarctica, can’t move as one because each object is an individual. Because of that, they can only respond to stimuli from their neighbors, which ripples on through the system.

One the traffic side, that’s one reason autonomous cars may be able to slow congestion; a network of computer-controlled vehicles could work more in unison, and thus more smoothly, than human drivers. Autonomous penguins don’t exist, and hopefully won’t ever replace their natural counterparts, which means Emperor penguins are pretty much stuck with their traffic jams. Thankfully for the frosty birds, traffic jams are where they want to be.

@derektmead