Read with me, if you will, the opening paragraphs of a swell new paper in PLoS One (access free; reference at bottom) about how emperor penguins, Aptenodytes forsteri, keep warm in the Antarctic winters. Just by filming the birds for 4 hours on a single day (August 3, 2008), the four researchers found an amazing group behavior. (I see no sense in rewriting the authors’ perfectly clear scientific prose):
Emperor penguins are the only vertebrates that breed during the austral winter where they have to endure temperatures below -45o C and winds of up to 50 m/s while fasting. From their arrival at the colony until the eggs hatch and the return of their mates, the males, who solely incubate the eggs, fast for about 110–120 days. To conserve energy and to maintain their body temperature, the penguins aggregate in huddles where ambient temperatures are above 0o C and can reach up to 37o C.
(37o C is, of course, human body temperature, so it’s nice and warm in the groups.)
Each colony consists of a group of huddles, and in each one the penguins are tightly packed, and all facing in the same direction. The density of penguins can reach—get this—21 animals per square meter! (And these are not small birds: they weigh between 50 and 100 pounds and can be up to 4 feet tall.) The picture below shows several huddles within a larger group:
Several emperor penguin huddles. Photo by Robyn Mundy.
More from the paper:
Huddling poses an interesting physical problem. If the huddle density is too low, the penguins lose too much energy. If the huddle density is too high, internal rearrangement becomes impossible, and peripheral penguins are prevented to reach the warmer huddle center. This problem is reminiscent of colloidal jamming during a fluid-to-solid transition. In this paper we show that Emperor penguins prevent jamming by a recurring short-term coordination of their movements.
The authors filmed the penguins on a single day, using time-lapse photography at a rate of one image every 1.3 seconds. As the figure from the paper (below) shows, individuals were tracked using the characteristic yellow and white face patch of the breed. Note that the males in these huddles are incubating eggs (nearly all of them had one), and when they move they do so by waddling with the egg balanced on their feet.
Here’s the amazing thing the authors found: penguins keep the huddle “fair”, and move from the periphery to the interior (and vice versa), by episodic but coordinated waves of penguin shuffling:
The jammed state of the huddle is interrupted every 30–60 s by small 5–10 cm steps of the penguins (Fig 1C,1E, Movie S2, S3), reminiscent of a temporary fluidization. These steps are also spatially coordinated and travel as a directed wave with a speed of about 12 cm/s through the entire huddle (Fig. 1E). After the wave has reached the end of it, the huddle re-enters the jammed state. Interestingly the propagation speed of the traveling wave is comparable to the speed of the individual penguins during the step. This is analogous to the propagation of sound waves in an elastic entropic medium (gas or fluid) where typical molecular velocities are comparable to the velocity of pressure waves.
You will of course want to see what this looks like. The links below go to the three movies from the paper (each between 1.5 and 4 minutes long), along with the descriptions. DO NOT MISS THESE STUNNING MOVIES.
Movie S1. Huddle formation and occurrence of coordinated traveling waves. Time lapse recordings (full field of view) over 2 h (resolution reduced from 10 MP to 480 p), showing about half of the penguin colony during the aggregation and huddling process. At the beginning of the movie (~12 p.m. with temperatures above −35°C), only few penguins aggregated in smaller huddles. As the temperatures gradually fell, larger and more stable huddles formed until nearly all the penguins aggregated in one large huddle.
To see the Sphenisciform Shuffle in the next two videos, keep your eye on the penguins’ white face patches. You’ll see them advancing in a jerky but coordinated way as the penguins step forward.
Movie S2. Huddle formation and occurrence of coordinated traveling waves (detail). Time-lapse recordings (detail of S2 over 1 h) showing multiple huddles. The penguins in a huddle mostly face in the same direction which defines a rear end and a front end of the huddle. When a penguin joins the huddle, it does so by aligning itself first in the direction in which the other penguins are facing, and then moving closer to the huddle. As a result, penguins tend to join a huddle at its rear (trailing) end and leave it at the front (leading) end. During the periodic traveling wave, the huddles move in the forward direction (in the direction in which the majority of the penguins are facing).
The shuffling is most evident in this video:
Movie S3. Coordinated traveling waves in a densely packed huddle. 21 min sequence from S2 (detail corresponding to Fig. 1B) at reduced speed. The movie shows the travelling wave of small steps every 30–60 sec.
The authors show that this type of movement is not unique to penguins, but has been seen in locusts and fish schools, as well as in tissue-cultured cells. They also show that it resembles “fluid-to-solid gelation of short-ranged attractive colloids.” But returning to the biology, the behavior raises two interesting questions:
- Is this behavior evolved or learned, or a combination of the two? Natural selection could of course favor this “altruistic” behavior since it’s to each penguin’s advantage to participate in a shuffle. The time you lose being cold on the periphery is more than compensated for by the larger amount of time you spend inside the warm huddle. (The behavior is not pure altruism, of course, for individuals gain rather than lose fitness by participating in the shuffle). But what about cheaters, who don’t move along, or didn’t when the behavior evolved? They would benefit by never having to be on the periphery, but they could of course have been punished for such cheating by the other penguins. It would be very hard to test whether this behavior was hard-wired, since it would involve creating large huddles of naive, hand-reared penguins under artificial conditions, and then subjecting them to an artificial winter.
- Mechanically, how does it work? The authors note, “It is also unclear whether the traveling wave in a huddle is triggered by a single or few leading penguins and follows a well-defined hierarchy among group members, similar to the collective behavior in pigeon flocks. Modeling attempts with self-driven agents have explained collective behavior such as temporal and long-range spatial synchronization in bird flocks, fish schools or traffic congestion by evolutionary strategies and a small set of simple interaction rules between neighboring agents. Similar mechanisms may also apply to the collective behavior of penguins in a huddle.
You don’t need fancy machinery or DNA sequencers to discover amazing things about our world. This senational behavior was revealed by four researchers armed only with a question and a video camera.
Zitterbart D.P., B. Wienecke, J. P. Butler JP, and B. Fabry. 2011. Coordinated movements prevent jamming in an emperor penguin huddle. PLoS ONE 6(6): e20260. doi:10.1371/journal.pone.0020260