1. How a Colony of 2,000 Emperor Penguins Moves Together in Waves
A German research team filmed a colony of about 2,000 emperor penguins near the Neumayer Station in Antarctica. The wave moved through the huddle, where penguins press close together. On August 3, 2008, the temperature was between -33°C and -43°C (-27°F to -45°F), and the density was about 21 penguins per square meter [1]. However, this density was from one specific observation, so it may not be the usual number.
In this huddle, small steps of 5 to 10 cm (2 to 4 inches) crossed the group like a wave every 30 to 60 seconds. The wave traveled at a speed of about 12 cm per second (4.7 inches per second), which is much slower than a person walking normally [1]. This does not mean each penguin walks every 30 to 60 seconds; rather, it is the time interval for the wave to pass through the whole group. There is no leader among the 2,000 penguins, yet their movements align. Why do they move together?
2. How a Tiny 2 cm Gap Starts a Wave in a Penguin Huddle
In 2013, Gerum and colleagues created a model where penguins react only to their immediate neighbors. If one penguin moves forward, creating a gap of about 2 cm or more behind it, the neighbor moves forward to close that gap. This creates a new gap behind the neighbor, and the process repeats, forming a wave [2]. The model considers neighbors in six directions, and its predictions matched the analysis of the observed video footage [2].
The 2 cm distance is not how far a penguin walks, but the threshold at which a neighbor starts to react. One explanation is that this distance is about twice the thickness of the penguin’s feather layer when compressed [3]. It is a simple reaction: if the neighbor moves, I move. Is this mechanism unique to penguins?
3. How Traffic Jams and Pedestrian Lines Also Form Waves
Traffic jams can form even without a collision or obstacle. In 2008, Sugiyama and colleagues at Nagoya University conducted an experiment with cars driving on a circular track. They started with cars spaced evenly at the same speed. Eventually, a traffic jam formed naturally, even without any obstruction. This jam wave traveled backward, opposite to the direction the cars were moving [7].
Waves also appear in lines of people. In an experiment in Julich, Germany, researchers found a mix of moving and stationary people in a crowded state. The difference in individual walking styles was involved in this separation [5]. The stop-and-go wave traveled backward, opposite to the direction people were walking [5]. In another experiment where people walked in a circular corridor, individual differences in walking speed were a key cause of flow disturbances [6]. This "matching the movement of the person ahead" point seems similar to the penguin’s reaction to neighbors. But are there differences?
4. How Penguin Huddle Waves and Traffic Jam Waves Differ in Direction
Penguin researchers have compared the movement of the crowded huddle to the movement of cars in a traffic jam (stop-and-go). The approach was described as drawing on models used to analyze groups and fish schools [4].
However, the direction of the wave is different. In cars and pedestrians, the stop-and-go wave travels backward, opposite to the direction of movement [7][5]. In contrast, the penguin wave starts where the gap appears and is described as traveling in any direction, including sideways [3]. It is an overstatement to say they use the exact same mechanism. The similarity is that movement passes from one individual to the next, creating a wave. The direction of that wave is not the same.
5. Why a Crowded Penguin Huddle Does Not Jam
Penguin huddles are so crowded that individual penguins can barely move. The paper’s abstract describes this state as close to "jamming," where particles are too packed to move [1]. By everyone taking small steps, the group can move without jamming, maintaining its shape and density [1].
Waves that start at similar times merge rather than crossing, keeping the group cohesive [3]. Additionally, penguins tend to join the huddle from the back edge and leave from the front edge, and the group was reported to move slowly like a conveyor belt [1]. This is not a strict rotation system. In contrast to traffic jams where the stop wave travels backward, the penguin wave seems to help prevent the group from jamming.
6. How You Can Observe Waves Yourself with Friends
You can observe these waves with family or friends. In a large room or schoolyard, have 4 to 6 people stand in a line. When the person in front takes a step, you take a step too. You can observe how the wave travels backward. Please go slowly to avoid collisions.
You can also watch lines at a supermarket checkout or at the bottom of a train station staircase. Observe how the movement starts and travels backward through the people waiting. Please watch from a place where you are not in the way.
To learn more, the starting points are the 2011 paper by Zitterbart et al. and the 2013 paper by Gerum et al. [1][2]. The experiments on cars and pedestrians are found in the sources cited above [7][5][6].