Original article: Coordinated Movements Prevent Jamming in an Emperor Penguin Huddle
Standing in the center of a crowded bus on your way to class, you might think: “why don’t these people just move? It’s hot and I can’t breathe!” Male penguins huddling to keep their eggs warm in the Antarctic winter have the opposite problem – no penguin wants to be at the cold edge of the huddle. A penguin in the huddle wants to stay in the warm center, since the outside temperature can reach -45 oC. However, penguins on the edge of the huddle are trying to push through the crowd to reach the center. Through the independent motion of each penguin, the huddle stays tight enough for the center to remain warm but loose enough to keep moving.
In today’s study, Zitterbart and colleagues investigate how huddling Emperor penguins can keep moving, as in this video. They find that individuals take small, coordinated steps that reorganize the huddle on a time scale of hours.
Zitterbard and colleagues film a 2000-penguin colony (shown in Figure 1a) for 4 hours and track the positions of the penguins, with an x-coordinate corresponding to horizontal position on the camera image and y-coordinate corresponding to the vertical position on the camera image. When huddling, the penguins all face in the same direction and are arranged roughly in a hexagonal grid. Every 30 to 60 seconds, the penguins take small steps, 5-10 cm long. Figure 1b shows the track of one penguin, with a point every 1.3 seconds. There are clusters of points where the penguin is standing still (with no significant change in position), and then a straight line when the penguin takes a step.
Since penguins at one spot in the huddle don’t know that another part of the huddle is moving, they don’t all step at once. Instead, there is a wave of moving penguins that moves through the huddle at a speed of 12 cm/s, like a sound wave traveling through a material. Figure 1c shows the tracks of several penguins at the same y-position and different x-positions in the huddle. Their horizontal motion is correlated – the penguin at the top track moves first, and then the motion propagates to the neighboring penguins as a wave.
An unusual aspect of this study is that the results section is short – the authors only report the traveling wave of penguins through the huddle. However, they then move to an explanation of penguin motion using very interesting analogies to granular materials (such as sand or coffee beans).
There are three effects of the small steps:
- They allow the penguins to reach the best density for warmth.
- They move the entire huddle forward, and merge small huddles into big ones.
- They reorganize the huddle, allowing penguins to leave the huddle at the front and join it at the back.
The combination of small penguin movements and organized huddling is similar to the way colloids  solidify when the particles in the colloid attract one another. Penguins huddle when they are “attracted” to each other by cold temperatures; colloids are attracted by electrostatic or intermolecular van der Waals forces. Thinking of a group of organisms as a fluid, such as smoothly flowing fish schools and turbulent bacteria, is a well known method for understanding their behavior. In contrast, the small steps in a dense group of penguins is reminiscent of a material going from a fluid to a solid state. The waves in the huddle are similar to waves in other groups of animals, like human crowds rushing to escape a room. Luckily, the penguin waves do not result in injury. (Usually.)
Through small, careful steps, penguins are able to create a solid cluster of warmth in the Antarctic winter. If we took a hint from the penguins and were more careful about our motion when on a crowded subway, maybe our commutes would be much more pleasant experiences. Of course, the huddling penguins are not bounded by the walls of a bus – how they would move if they didn’t have an open boundary is still a mystery!
 Mixtures of small particles dispersed throughout another substance, such as the fat suspended in a water solution to form milk.